| Question | Answer |
|---|---|
| Has the Better Building HAMT Solver been verified? | 100%. As part of our development, we have tested our solver against HAMSTAD and BS EN 15026:2023 - Hygrothermal Performance Benchmark Tests. Access our testing results. |
| Is the Better Building HAMT Solver a transient solver or a steady-state method? | Transient. The steady-state Glaser method still appears in some compliance documents, but it cannot model seasonal drying, year-over-year moisture accumulation, or dynamic rain and solar loads. Those things matter. The Better Building HAMT Solver runs fully transient simulations, tracking temperature and moisture conditions through the assembly at each time step across the full simulation period. |
| Can Better Building be used for ASHRAE 160 or DA07 compliance? | Absolutely. The Better Building HAMT Solver has been developed under the primary assumption that ASHRAE 160 or DA07 form the basis of the modelling requirements. All inputs and outputs pertaining to these standards are available on the platform. |
| Is Better Building a replacement for WUFI? | Yes, it provides the mostly the same functionality, meets ASHRAE 160 or DA07 requirements and has been verified against HAMSTAD and BS EN 15026:2023 |
| Does Better Building meet NCC 2022 and 2025 Condensation Management Requirements? | The Better Building HAMT solver enables all inputs, calculations and outputs as per DA07 and therefore meets the requirements of NCC 2022 and 2025. |
| How long can a HAMT simulation run? | 10 years max. Single-year simulations tell you whether an assembly survives a representative climate year. They do not tell you whether moisture is accumulating year on year. Most credible assessments run five years minimum to verify cyclic steady-state behaviour. |
| How are initial conditions set? | Temperature and moisture content can be set per layer at the start of the simulation. This is important as starting an assembly dry in a humid climate, or saturated in an arid one, can distort first-year results significantly and enable a 'stress testing' of the wall or roof system being modelled. |
| Has the Better Building HAMT Solver been tested against WUFI? | WUFI Pro 6.6 has been used as a benchmark to assess the capabilities of the Better Building HAMT solver. The primary focus of this testing has been to review features that are outside the verification enabled through HAMSTAD and BS EN 15026: 2023. These include a review of moisture sources, rain exposure, rain deposition, Sd values, short and long wave radiation, explicit radiation and adhering fraction of rain. A consistent, strong relationship was found between the two software outputs in all cases. Read More. |
| Does Better Building account for wind-driven rain? | Yes. Wind-driven rain is included in the Better Building HAMT simultations, an important boundary condition for HAMT modelling and the main moisture source that affects the hygrothermal performance and durability of building envelopes. Note, you should always review you climate file for precipitation as it must be present to enable wind-driven. Hygrothermal Convergence Errors A convergence error in the context of hygrothermal HAMT simulations, refers to the discrepancy that occurs when iterative numerical methods fail to reach a stable solution within a predefined tolerance level. These error indicate that the simulation's equations (governing heat and moisture transfer) are not balanced properly, leading to unstable or non-physical results. |
| How is an assembly defined? | As an ordered sequence of material layers, each with a specified thickness and material assignment. A practical limit on the number of layers is set at 10 layers or materials. |
| Can it model vapour control layers and membranes? | Yes. Vapour barriers, vapour retarders and vapour-permeable membranes are represented by their water vapour diffusion resistance factors or Sd values. |
| Does it support moisture-adaptive vapour retarders? | Yes. Moisture-adaptive membranes, sometimes called smart vapour retarders or air and vapour control layers (AVCL's), change their water vapour diffusion resistance with ambient relative humidity. They are increasingly common in timber-frame construction and high-performance wall assemblies. |
| Can air layers be included in the assembly? | Yes. Sealed air layers and ventilated cavities are both modelled. A ventilated cavity exchanges heat and moisture with either the exterior or the interior depending on configuration. This is relevant for rainscreen cladding, ventilated roof assemblies and lightweight wall systems with service cavities. |
| Can the surface orientation and inclination be specified? | Yes. Both the compass orientation and the inclination from horizontal are defined for each assessment. These determine how much solar radiation and driving rain reach the surface. They are not cosmetic inputs. |
| Can moisture sources be placed within the assembly? | Yes. Internal moisture loads can be introduced at defined locations within the assembly, useful for modelling construction moisture, embedded framing or other moisture-sensitive inclusions. |
| What material properties does the solver use? | A full hygrothermal simulation calls on: dry bulk density, specific heat capacity, porosity, thermal conductivity (dry value plus moisture dependence), vapour diffusion resistance factor (dry cup and wet cup values), moisture storage function (sorption isotherm up to free water saturation), liquid transport coefficients for the suction and redistribution phases separately, and water absorption coefficient. Not every material has all of these measured from test data. But the more complete the description, the more the simulation reflects actual behaviour rather than a best guess. Better Building includes a database of common building materials with calibrated hygrothermal properties. |
| Can custom material properties be entered? | Yes. All parameters can be entered manually. This is necessary when working with proprietary products, or when using manufacturer-supplied test data directly rather than generic database values. |
| Are material properties treated as moisture-dependent? | Yes. Thermal conductivity, vapour diffusion resistance and liquid transport coefficients are functions of local moisture content. Most building materials behave differently wet than dry. A solver that ignores this is simplifying something that is not actually simple. |
| Is explicit radiation included in the Better Building HAMT Solver? | Yes. Explicit radiation is included to account for radiative cooling. |
| Why does the vapour diffusion resistance factor of wood refuse to hold still? | The vapour diffusion resistance factor (the metric µ-value) says how much harder vapour moves through a material than through still air. For wood it will not sit still: measured dry cup it reads higher, wet cup it reads lower. Some of that drop is real diffusion; some is liquid water sneaking through the pores during the test. So either carry a moisture-dependent factor, or fix it at the dry cup value and let the liquid transport coefficients handle the rest. |
| What moisture problems should I actually check for in a wall? | Two scenarios catch most trouble. Winter interstitial condensation: face the wall north, start in autumn, run at least two cold years, and watch the cold-side sheathing. Summer inward vapour drive, the one that bites brick and stucco: wettest sunlit orientation, hot year, start in spring, check behind the cladding. Judge the trend, not one dramatic peak. A layer that dries back each year is fine; one that ratchets up is not. |
| How much moisture is too much in wood-based sheathing like OSB? | Moisture content is a percentage of dry mass, so you need the dry bulk density to convert. Wood-protection standards cap wood-based materials around 15 to 20 mass-percent. Keep decay and mould apart: decay needs sustained wetness, mould stirs near 80 percent surface humidity held for weeks. Judge running 24-hour, 7-day and 30-day averages, not the worst single spike. |
| Which material properties matter most when the one I need isn't in the database? | A full simulation calls on dry bulk density, specific heat capacity, porosity, thermal conductivity (dry plus moisture dependence), the vapour diffusion resistance factor (dry and wet cup), the moisture storage function, liquid transport coefficients for suction and redistribution, and the water absorption coefficient. You rarely get the full set, since datasheets sell R-value, not sorption isotherms. Get conductivity and the resistance factor right first, then invest in whichever layer actually stores water. |
| How do I read condensation risk off temperature and dew-point curves? | The crossing rule is exact: at or below the dew point, humidity is 100 percent and condensation can form. But staying above it does not mean safe. A hygroscopic material holds a rising water load well before saturation, and 90 percent humidity in timber already invites mould. Keep the dew-point test for air gaps and non-hygroscopic layers; elsewhere, read moisture content against the sorption isotherm. |
| How is winter precipitation like snow handled at the exterior surface? | The usual simplification: precipitation falling while the air is below 0 °C (32 °F) counts as snow, and snow does not wet the surface, so the water absorption coefficient sits the cold spell out. Fine for vertical walls, where snow rarely clings. Shakier on low-slope roofs, where snow piles up and melts later; if that melt loading matters, model it separately. |
| What's the difference between permeability and permeance, and how do layers add up? | Permeability ignores thickness; permeance describes one specific layer. In metric the pair is the dimensionless µ-value and the sd-value in metres, where sd = µ times thickness. (Imperial: perm-inch and perm, divide permeability by thickness, so 0.392 perm-inch at 0.2 mm / 0.008 in is near 50 perm.) Layers in series add their sd-values directly, like resistances (imperial: add the reciprocals of the perms and invert). That is why a hollow block behaves as its two face shells in series. |
| How do I represent a low-permeance interior finish like vinyl wall covering? | Two equivalent moves: an explicit thin room-side layer with its own resistance, or that resistance applied straight to the interior surface. The number that matters is the sd-value (permeance in perms). A low-permeance finish is a vapour retarder whether intended or not, and can decide a summer inward-drive case. When it is uncertain, sweep a range and watch the moisture behind it. |
| How should I model foil-faced insulation? | Model the foil as its own thin, vapour-tight layer (a high sd-value) against the insulation, not smeared into the insulation's properties. Match the permeance; if the exact facing is not listed, borrow another membrane with the same value. A 1D model will not resolve radiant exchange inside solid layers, so the low-emissivity benefit only shows where the foil faces an air cavity. Handle that through the cavity's surface properties. |
| Can a one-dimensional simulation account for air leakage or pressure differences? | No. A 1D model moves moisture by diffusion and capillary flow, not air streaming through gaps, laps, or the hole around a late recessed light. That convective load is often the biggest real risk, and it is a 2D or 3D problem. For a truly airtight build the omission barely costs you. The catch is "truly": a clean result assumes airtightness, so earn it with detailing and a blower-door test. |
| Why does interior insulation make a solid masonry wall wetter? | Insulating solid masonry on the inside makes it colder, and colder means wetter. Old brick has such poor thermal resistance that even a thin interior layer drops the brick temperature sharply, so adding thickness past the first 25 mm (1 in) barely changes the moisture picture. The dominant water source is driving rain on the outer face, not interior vapour. So the real levers are shedding rain outside and drying inward. Expect higher masonry moisture, and check freeze-thaw. |
| Does vapour move according to relative humidity or vapour pressure? | Vapour diffusion chases vapour pressure, not relative humidity: it follows Fick's law, so vapour flows high-to-low vapour pressure whatever the percentages read. Relative humidity drives the other mechanism, liquid capillary transport, through the moisture storage function. Both are live, each on its own process. Confusing them is why "but it is more humid inside" sometimes predicts flow the wrong way. |
| How do I model paint or a thin surface coating? | A thin film can dominate how a wall dries. Model it as an explicit thin layer or as an added surface resistance; both agree if the vapour resistance matches. What you need is the film's sd-value (permeance in perms), and units bite: makers quote a finished-film value, so do not divide by thickness again. Oil-based paints are far less permeable than latex, which can flip a wall from drying to trapping. |
| What's the difference between the rain absorption factor and the water absorption coefficient? | The rain water absorption factor (often near 0.7) is only splash reduction: the fraction of driving rain that stays on the wall after the rest bounces off. It is dimensionless, not the share absorbed. How much actually soaks in is the material's water absorption coefficient, the A-value, in kg/(m² √s). So model a water-repellent surface with a low A-value, not by slashing the 0.7, which is dwarfed by rain-load uncertainty anyway. |
| Why do initial moisture conditions matter, and how do I set them? | A wall that begins soaked behaves nothing like one that begins dry for the first year or two. New construction traps build moisture: wet plaster, fresh concrete, rained-on timber. If you are chasing a known problem, set the initial water content to match, then give the run lead time to shed it before reading conclusions. Note it is spread evenly through a layer, so localised wetting is only approximate. |
| What is porosity used for, and how precise does it need to be? | Porosity is the pore fraction, estimated from the true and dry bulk densities. Its main job is modest: it caps water content, with the maximum roughly porosity times 1000 kg/m³ (about 62 lb/ft³), the density of water. So the exact value rarely needs agonising. Any plausible number works, as long as the maximum sits above free water saturation. If you measured open porosity, use it; otherwise pick a number that gives a sensible maximum. |
| How do I approximate a ventilated cavity behind a rainscreen? | A vented cavity is not captured by diffusion, because outdoor air is flushing through it. Represent that with an air change source feeding the cavity outdoor air at some rate (1/h), which dries it and weakens the cladding's grip on the layers behind. The honest difficulty is the number: cavity air change swings with vent area, height and wind, from a few to several hundred per hour. Sweep a range rather than trusting one value. |
| How do I model non-porous materials like glass, steel or metal panels? | Vapour-tight, non-hygroscopic materials (glass, steel, metal) want stripping down: drop the moisture storage function and liquid transport coefficients, and set a very low porosity. Get the thermal properties right, since those still drive the result. Do not read these layers' moisture content; it is meaningless. What matters is their surfaces, where a cold impermeable face is a prime condensation site, so watch temperature and humidity at the interface. |
| How do R-value and thermal conductivity relate? | R-value and conductivity are two views of the same thing: R equals thickness divided by conductivity. To hit a rated R, back out the conductivity and enter that, watching units (conductivity W/(m K), resistance m² K/W; imperial R in ft² h °F/BTU, about 5.68 times the metric). A meaningful in-service R uses the conductivity at realistic moisture, often near 80 percent humidity, not the bone-dry lab value, because damp insulation conducts more heat. |
| How is a still, unventilated air gap represented? | An enclosed, unventilated gap is modelled as an equivalent solid: a thermal conductivity back-calculated from the air space's thermal resistance (m² K/W; imperial ft² h °F/BTU), bundling conduction and radiation into one number, so gap width and surface reflectivity both change it. On the moisture side it barely resists vapour (µ near one), carries no liquid transport, and stores almost nothing. Once the gap is actually ventilated, this still-air stand-in understates reality and you want an air change source. |
| What does a "smart" or variable-sd vapour retarder do, and when does it help? | A variable-sd retarder changes its resistance with humidity: tight in dry winter to block inward vapour, open in humid summer to let the wall dry inward. That two-way behaviour is exactly what fixed polyethylene cannot do. Model it with its full humidity-dependent sd curve, not one number, or you throw away the point. If your product is not tabulated, start from a similar membrane and adjust, and check the summer drying. |
| Which side of the assembly should the vapour retarder go on? | The rule of thumb is "retarder on the warm side", the interior in a heating climate, because it keeps indoor vapour off the cold surface. It is a fine starting point, not a law. In a cooling climate, or a sun-driven wall, a tight interior layer traps inward-drying moisture and a vapour-open interior is safer. This is what simulation settles: try each side and compare the moisture behind it. |
| What's the difference between a vapour barrier and an air barrier, and do I need both? | They do different jobs. A vapour barrier slows diffusion, the slow molecular drift driven by vapour pressure. An air barrier stops bulk airflow, which carries far more moisture through any gap it finds. A material can be either, both or neither: polyethylene is both, painted plasterboard is a fair air barrier but a modest vapour retarder. Most failures are air-transported, so if you fix only one, fix the air barrier. |
| What is an air change source and when do I add one to a layer? | An air change source injects outdoor (or other) air into a chosen layer at a set rate (1/h), to represent ventilation or leakage that diffusion cannot. Use it for a vented cavity, a breathing attic, or infiltration reaching a plane inside the assembly. It swaps both heat and moisture with the source air. Real rates are uncertain and wind-dependent, so treat the number as a sensitivity variable and report the range. |
| What air change rate should I assume for a ventilated cavity or roof space? | There is no single right number. A barely vented cavity might sit at a few air changes per hour; a well-vented rainscreen or breezy roof space can reach tens or hundreds, depending on vent area, height and wind. So bracket it: run a low and a high rate and see whether your conclusion depends on the choice. If it does, you have found something worth measuring rather than assuming. |
| Is exterior or interior insulation safer for moisture? | Exterior insulation is usually safer: it keeps the structure warm and dry and buries most thermal bridges. Interior insulation is often the only retrofit option, and it leaves the wall behind it colder and wetter, with less inward drying. Neither is disqualifying, but interior insulation demands more care: driving-rain exposure, freeze-thaw, embedded beam ends. Simulate before committing to a thickness. |
| How do I model an EIFS or external render-on-insulation system? | An external render-on-insulation system builds from standard layers: EPS or mineral wool over the wall, then a lime-cement base coat and a thin acrylic or mineral finish. Generic library entries usually get you close. What decides the result is the finish's vapour resistance (a tight acrylic limits outward drying) and how well the render sheds water, since rain behind the insulation is slow to leave. Focus on the outer coat. |
| Should a roof or wall assembly be vented or unvented? | Venting gives a drying path, which is forgiving; unvented removes it and asks the materials to manage moisture alone. Both work by design. A vented roof or rainscreen flushes cavity moisture outside. An unvented build-up leans entirely on getting vapour control and drying direction right, with no cavity to bail it out. Simulation shows whether it dries year on year or accumulates. |
| How does a capillary-active interior insulation change the moisture risk? | Calcium silicate and wood fibre take a different bet from sealed foam: instead of a vapour barrier, they stay vapour-open and use their suction and redistribution coefficients to wick interstitial moisture back to the room to evaporate. On solid masonry that can be safer, because it tolerates wetting instead of trapping it. But it only works if those liquid transport properties are real and the contact with the masonry is tight, with no air gap. |
| What actually drives freeze-thaw risk in insulated masonry? | Freeze-thaw needs two things at once: masonry wet enough, and pore water actually freezing. It is not simply air touching 0 °C (32 °F): fine pores and dissolved salts depress the freezing point, and only larger-pore water does the damage. Interior insulation raises the risk by keeping the outer masonry colder and wetter. Assess it on the outer masonry's moisture content during freezing spells, not temperature alone. |
| What's different about modelling a flat or low-slope roof? | A flat roof is the demanding case: it is vapour-tight on top, so it can only dry downward, if at all. Set inclination near horizontal, keep the membrane's high resistance honest, and watch just beneath it, where sun drives moisture down and winter drives it up to the cold deck. Surface absorptivity matters a lot here. And ponding is a real load the rain model does not capture, so treat a genuinely flat, drainage-poor roof conservatively. |
| How do I model a pitched roof or ventilated attic space? | A pitched roof is friendlier, because it often has a ventilated space giving moisture a way out. Model the slope at its true inclination, and the vented space either as an air change source or, above a ceiling, as a separate space with its own climate. The recurring risks are condensation on the cold underside of the deck in winter, and warm humid indoor air leaking up. Get the ceiling airtightness and vent rate right. |
| How do I approach a green (vegetated) roof? | A green roof adds a wet living layer, modelled by a moisture source in the substrate for rain and irrigation, clipped so it never exceeds free water saturation. It shades and cools the membrane, but all that water makes the calculation cranky, so some convergence complaints are normal. Watch the moisture content just below the waterproofing. If it behaves sensibly across the year, a noisy solver is forgivable; if it jumps unphysically, be sceptical. |
| Why does surface colour (solar absorptivity) matter so much? | Surface colour matters more than newcomers expect: short-wave absorptivity sets how hot the surface gets in sun, and that drives both drying and summer inward vapour drive. A dark surface can run tens of degrees above air temperature, baking absorbed rain inward; a light one stays cooler. So it is a real design lever, not cosmetic. When it is unknown, test a range, because on a rain-loaded sunny wall it can move the result as much as the insulation. |
| How do I pick material data for historic or unknown brick and stone? | Old masonry is variable and undocumented, so no library entry is truly yours. Pick a database brick of similar density and water behaviour, then sanity-check the two properties that dominate: the water absorption coefficient and the vapour resistance. Handmade historic brick tends to be more absorptive and open than modern extruded brick, so lean that way. And since the material is uncertain, test a range and see whether your conclusion survives. |
| Can the simulation deal with salt and efflorescence? | Standard models simulate heat and moisture, not salt chemistry, so they will not directly predict efflorescence or salt decay. What they give you is where and when the masonry is wet, how often it cycles, and how deep the moisture runs, which is exactly what salt trouble tracks. Salt also lowers the freezing point and raises equilibrium moisture. So treat the simulation as a moisture map that flags salt-prone zones, and bring in salt-specific assessment separately. |
| How do I model a drained-and-ventilated rainscreen wall? | A rainscreen splits the job: the outer skin sheds most rain, and a drained, ventilated cavity handles the leakage that gets past. In the model the cavity carries an air change source fed by outdoor air, which dries it and decouples the cladding from the backup wall. The layers behind then see far gentler conditions than a face-sealed wall. The two steering inputs, cavity vent rate and assumed rain leakage past the screen, both deserve a sensitivity check. |
| How do I model a floor, slab or basement wall against the ground? | For anything touching the ground, the trick is the earth-facing side. A ground-bearing floor or basement wall has soil on one face and indoor conditions on the other, so you assign a ground boundary, not outdoor weather. A floor between heated storeys has indoor conditions on both faces. Orientation and inclination stop mattering once solar, wind and rain are off. The real below-grade questions are rising damp and the drying capacity of any interior finish. |
| What boundary condition goes on the ground-facing side? | The soil side is not the outdoor climate, and using the air weather file there is a common mistake. Deep ground holds a slow, damped temperature that barely moves day to day and stays effectively saturated, so use a ground temperature condition, often with high or fixed humidity. Near the surface the ground still feels the seasons; well below it does not. Get it wrong and you will flatter or exaggerate condensation on the inside face of a basement wall. |
| How do I judge mould and rot risk in a timber-framed or CLT wall? | Timber assemblies live or die by the wood's moisture content, so judge them there, not on condensation alone. Watch the coldest, outermost wood layer, the sheathing or outer CLT face, and read its moisture content and interface humidity across the year. Rot needs sustained wetness; mould stirs near 80 percent humidity held for weeks. Better still, feed the interface into a mould index model. A single winter peak tells you less than a rising trend. |
| Where should I place the monitoring point to assess the sheathing? | Put the monitoring point where trouble shows up, which is almost never mid-layer. The revealing spots are interfaces: the cold outer face of the sheathing, the back of an impermeable cladding, the underside of a roof deck, the plane behind an interior retarder. Read temperature and humidity there for condensation and mould, moisture content there for the material's limit. One point per suspected weak plane, and label them. |
| Which reference year should I run: cold, hot, or typical? | Do not settle for one "typical" year, because failure hides at the extremes. Run a cold reference year (harsh for winter condensation) and a warm one (harsh for summer inward drive), often the coldest and warmest in ten from a thirty-year record. Those bracket the realistic spread. Some standards ask for a deliberately moist year. The aim is to stress the assembly where it is vulnerable and confirm it survives. |
| How do orientation and inclination change the result? | These only bite once the weather loads are on, and then hard: they set how much solar and driving rain the surface catches. The wettest wall usually faces the prevailing driving rain; the north face dries slowest. So winter condensation checks often point north while rain-load checks face the storm. Inclination matters most toward horizontal. With solar, wind and rain off, orientation stops mattering. |
| How do I build climate data for a site that isn't in the library? | For an unlisted site you build a climate file from hourly data: temperature, relative humidity, solar (global and diffuse), driving rain, wind speed and direction, plus latitude, longitude, altitude and time zone. Public records and typical-year datasets supply most of it; driving rain and the direct/diffuse solar split are the usual gaps to derive. Garbage in, garbage out: the result is only as good as the rain and solar data. |
| What exterior surface transfer coefficients should I use? | The exterior side carries several: the heat transfer coefficient (rising with wind), short-wave absorptivity, long-wave emissivity, and the rain splash factor. Sensible defaults exist and are usually fine. The two worth thinking about are absorptivity, since colour swings surface temperature and drying, and the rain factor, which sets how much driving rain is offered to the wall. The interior side is simpler. |
| How do I set the indoor temperature and humidity? | The indoor climate is half the boundary condition. You can fix values, but real rooms swing, so better to vary temperature and humidity through the season, tied to outdoor temperature plus a moisture load for the occupancy. A humid, crowded, poorly ventilated interior pushes far more vapour into the walls than a dry one, and that often decides the result. Standards give moisture-load classes; set unusual uses like a pool or kitchen deliberately. |
| What do EN 15026, EN 13788 and ASHRAE 160 actually give you? | They do different things. EN 13788 is the old Glaser monthly steady-state check: quick, and blind to rain, storage and liquid transport. EN 15026 is the modern transient standard behind proper simulation. ASHRAE 160 pins down inputs (indoor humidity, rain deposition) and gives explicit mould criteria. Roughly: Glaser for a first sniff test, transient simulation to EN 15026 for anything real, and ASHRAE 160 when you need agreed inputs and thresholds. |
| How long should a run be, and how do I know it reached equilibrium? | A run must be long enough to forget where it started: at least two years, often three to five with heavy materials. You are waiting for dynamic equilibrium, each layer returning to the same moisture content at the same point every cycle. If it is still climbing at the end, extend it. Read conclusions from the settled cycles, not the opening year. An assembly that ratchets up indefinitely has told you it fails. |
| What counts as a "pass"? Which criteria do I judge against? | There is no universal green light. The model gives temperature and moisture over time; you judge it against a stated criterion: total moisture not trending up year on year, wood under its mass-percent limit, interface humidity not sitting in the mould band too long. Some standards hand you thresholds and a mould model; elsewhere you argue from material limits. State the criterion up front, because "it looks fine" is not an assessment. |
| What does a mould index or isopleth actually tell me? | A mould index turns "will it grow mould" into a running score, tracking how long and how favourably conditions at a point sit in the range mould likes. An isopleth is the graphical cousin, plotting conditions against the germination boundary. Both beat a single humidity snapshot, because mould responds to duration. Feed them the vulnerable interface, and remember the threshold depends on the material: timber is friendlier to mould than glass. |
| How do I use the water balance and convergence to trust a result? | Check the plumbing before trusting a result. The water balance should close: in, minus out, should match the change in stored moisture. A large mismatch means the numbers are not believable. Convergence failures are the other tell. A handful over a long run is cosmetic; clusters, especially with a broken balance or unphysical jumps, mean a shaky result, usually from a very wet material or too coarse a build-up. Fix the model. |
| Moisture content or relative humidity: which criterion for which material? | Match the criterion to the material. For something damaged by its own water load, wood rotting, masonry freezing, use its moisture content against that material's limit. For biological risks like mould, use the relative humidity and how long it stays high, because mould responds to humidity, not kilograms. Get the pairing wrong and you can pass a wall that actually fails. |
| What are moisture and heat sources and sinks for? | Sources and sinks inject or remove heat or moisture at a chosen plane, to represent something the physics does not carry on its own: rain penetration behind a cladding, green-roof water, a heating element, a deliberate load. They are a scalpel, not a fix-all. Using a source to paper over a boundary condition just hides the problem. Place it where the effect occurs, keep the magnitude defensible, and be ready to justify it. |
| How do I model a rain leak or defined water ingress? | Real walls leak, so a sober robustness test deposits a small fraction of the driving rain as a moisture source behind the cladding, then checks whether the assembly dries it away. Some standards suggest a set percentage. The question is never "is it perfectly sealed", but "can it recover from a realistic leak". Put the source where water would collect, size it from the rain load, and judge by whether the layer returns to a safe moisture content each year. |
| Can a 1D tool handle a thermal bridge, or do I need 2D? | A 1D model assumes flow straight through the layers, which is exactly what a thermal bridge violates, so it cannot represent a corner, balcony slab, lintel or stud. Bodging it with heat sources just launders a 2D effect into a confident wrong number. What 1D does well is the clear field of the wall, away from bridges. For the cold junction itself, where condensation and mould start, you need a 2D calculation. |
| When do I genuinely need a 2D or 3D model? | Reach for 2D or 3D when the geometry, not the layering, drives the moisture: corners, junctions, slab penetrations, window reveals, ties and framing, or lateral drainage. Heat and moisture curve around these in ways parallel layers cannot capture. For a plain field of wall, roof or floor, 1D is faster, better validated and adequate. A layer question stays 1D; a geometry question does not. |
| What is the moisture storage function (sorption isotherm), and why does it matter? | The sorption isotherm is the backbone of the method: it says how much water a hygroscopic material holds at each humidity, from dry to free saturation. It is why a material can be safely damp, and why hygroscopic materials buffer moisture instead of condensing at the first cold snap. Get it wrong and everything downstream drifts. It is also the bridge between your two criteria, converting a moisture content into the humidity that governs mould. |
| Free saturation versus maximum saturation: what's the difference? | Free saturation is how wet a material gets by ordinary capillary suction, with air still trapped in the larger pores. It is the practical ceiling in service and where the sorption isotherm tops out. Maximum saturation fills every pore, reached only under pressure or prolonged condensation past free saturation. Normal wetting stops at free saturation; pushing into that gap is what genuine condensation in a porous material means. Porosity sets the maximum; capillarity sets the free value. |
| What is capillary (liquid) transport, and how does it differ from diffusion? | Two mechanisms move moisture. Vapour diffusion is water as gas through the pore air, driven by vapour pressure, slow and always present. Capillary transport is liquid water through the pore network, driven by moisture content, and far faster once wet. The suction coefficient governs active wetting, redistribution the slower spreading after. This is why a wall sheds absorbed rain inward faster than diffusion alone predicts, and why a capillary-active layer redistributes water a vapour-only material would trap. |
| What are the main limitations I should keep in mind for any result? | Every result rests on assumptions worth saying out loud. It is 1D unless you go further, so it misses thermal bridges. It models diffusion and capillary flow but not air leaking through gaps, so it assumes airtightness. Its material data are generic unless measured, and its climate is a representative file, not next year's weather. It reports moisture and humidity over time; it does not declare pass or fail. None of this makes it untrustworthy: it is a well-behaved model of a messy reality, and its output is guidance, not prophecy. |