ASHRAE 90.1-2022 Thermal Bridging: What You Need to Know

ASHRAE 90.1-2022 now requires you to account for thermal bridges, the weak spots in your building envelope where heat bypasses insulation. This isn't optional anymore. Whether you're using the prescriptive path, trading off envelope components, or running a full energy model, thermal bridging has to be part of the conversation.

This guide gives you the big picture. For calculation details, see our Technical Reference.

What Changed?

Before 2022, energy models assumed your wall insulation connected perfectly to your roof insulation, which connected perfectly to your floor — no gaps, no shortcuts, no steel beams conducting heat straight through your carefully specified R-values.

That was never true, and everyone knew it. But the standard didn't require you to account for it.

Now it does. Section 5.5.5 introduced requirements for common thermal bridges like:

• Shelf angles supporting brick cladding

• Floor slab edges at building perimeters

• Balcony connections penetrating the envelope

• Parapets and roof-wall intersections

• Window perimeters where frames meet walls

These details have always mattered for actual building performance. The difference is that compliance now depends on addressing them.

Why This Matters for Your Project

Thermal bridges can account for significant heat loss through an otherwise well-insulated envelope. That's not a rounding error, it's the difference between a building that performs as designed and one that doesn't.

Getting thermal bridging wrong affects:

• Energy costs — Higher heating and cooling loads than predicted

• Comfort — Cold spots, condensation risk, occupant complaints

• Compliance — Failed inspections or performance shortfalls

• Liability — Gaps between promised and delivered performance

The good news: addressing thermal bridging early in design is straightforward. Addressing it late or discovering it during commissioning is expensive.

A Word of Caution on Table Values

The standard provides default Psi and Chi values in Table A10.1. These are useful for getting started, but they come with limitations you should understand.

Default values are conservative, sometimes very conservative. They're designed to cover a wide range of construction types, which means they may significantly overestimate heat loss for your specific detail. This can push your envelope design toward more insulation than actually needed, adding cost without proportional benefit.

When table values work well:

• Early feasibility studies

• Simple buildings with standard details

• Projects where envelope isn't a critical path item

When table values fall short:

• Complex building geometries

• High-performance or passive house targets

• Details that differ from standard assumptions

• Projects where every R-value point matters for compliance

If you're relying heavily on table values, you may be leaving performance (or budget) on the table.

The Case for Thermal Bridging Modelling

Detailed thermal modelling, using 2D or 3D finite element analysis, gives you actual Psi and Chi values for your specific details. This takes more effort upfront but pays off in several ways:

More accurate compliance. You're demonstrating performance with your actual design, not a generic assumption. This matters when you're close to code thresholds.

Design optimisation. Modelling lets you compare options: Does a thermal break at the shelf angle save enough to justify the cost? Is continuous insulation more effective than thicker cavity insulation? You can answer these questions with numbers instead of guesses.

Reduced risk. When your energy model uses real thermal bridge values, the gap between predicted and actual performance shrinks. Fewer surprises at commissioning.

Documentation. A thermal model creates a record of your design decisions and their justification. This is valuable for certifications, disputes, and future renovations.

Consider this: The cost of thermal modelling a handful of key details is typically less than the cost of one change order. If it prevents even one late-stage redesign, it's paid for itself.

Set a Thermal Bridging Budget Early

Here's a practice that separates smooth projects from painful ones: establish a thermal bridging budget during schematic design.

This means deciding, before you've detailed every connection, how much heat loss you're willing to allocate to thermal bridges. It's similar to setting a fenestration budget or an energy use intensity target.

How to set a budget

1. Start with your envelope U-factor targets. What does code require? What are you aiming for?

2. Estimate your thermal bridge inventory. How many linear metres of slab edge? How many balcony connections? How many shelf angle supports? You don't need exact numbers yet — order of magnitude is fine.

3. Allocate a percentage. A common starting point is 10–15% of total envelope heat loss for thermal bridges. High-performance projects might target 5–10%.

4. Check feasibility. Do your preliminary details fit within the budget? If not, you know early that something needs to change — the details, the targets, or both.

5. Track it through design development. As details get resolved, update your thermal bridge inventory with real values. Are you still within budget?

Why this works

Setting a budget forces the conversation early, when changes are cheap. It gives the design team a clear target. And it prevents the common scenario where thermal bridging gets ignored until someone runs the energy model and discovers a problem.

Pro tip: Include your thermal bridging budget in the basis of design document. This creates accountability and makes it harder for value engineering to quietly erode envelope performance later.

Practical Steps for Project Teams

For Architects

• Flag thermal bridge locations early. Mark shelf angles, slab edges, and penetrations on your envelope diagrams during schematic design.

• Coordinate with structure. Steel connections through insulation are thermal bridges. The earlier structural knows this matters, the more options you have.

• Request thermal details from product reps. Many cladding and curtain wall suppliers have pre-calculated Psi values for their systems.

For Mechanical Engineers

• Don't ignore envelope in load calcs. Thermal bridges affect peak loads, not just annual energy. A slab edge with a high Psi value can create localised cold spots that drive perimeter heating requirements.

• Push back on unrealistic U-factors. If the architect's envelope spec assumes zero thermal bridging, the loads you calculate won't match reality.

For Energy Modellers

• Use actual Psi/Chi values when available. Default tables are a fallback, not a best practice.

• Document your assumptions. If you're using table values, note which details they apply to and flag any that might warrant detailed analysis.

• Sensitivity check thermal bridges. Run your model with conservative and optimistic thermal bridge assumptions. If the results vary significantly, that's a sign detailed modelling is worth the investment.

For Owners and Developers

• Include thermal bridging in RFPs. If you expect the design team to address thermal bridging properly, say so. Specify whether you want detailed modelling or will accept table values.

• Budget for it. Thermal modelling and thermal breaks cost money. Projects that plan for this deliver better buildings than projects that treat it as an afterthought.

What Happens If You Ignore This?

Thermal bridging requirements are now part of code compliance. Ignoring them means:

• Prescriptive path: You haven't demonstrated compliance with Section 5.5.5.

• Trade-off path: Your Appendix C calculation is incomplete.

• Performance path: Your energy model doesn't reflect reality.

Beyond compliance, buildings with unaddressed thermal bridges underperform. They cost more to heat and cool. They have comfort problems. They may have condensation and moisture issues at thermal bridge locations.

None of this is new information, the physics hasn't changed. What's changed is that the standard now requires you to deal with it.

Next Steps

1. Review our Technical Reference for calculation methods and code references.

2. Identify thermal bridges in your current project. Where are the slab edges, shelf angles, balconies, and penetrations?

3. Decide on your approach. Table values for simple projects, detailed modelling for complex or high-performance ones.

4. Set a thermal bridging budget during schematic design.

5. Document your assumptions so future phases (and future teams) understand what was decided and why.

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