Envelope heat loss: U-values, thermal bridging and infiltration

The building envelope separates conditioned indoor space from the Canadian winter. Heat escapes through it by conduction across walls, roofs, floors and windows, and by air leaking through gaps and cracks. Together, these losses set much of the heating load and a significant part of annual energy use.

For many years, envelope performance was described simply by the insulation value in the wall cavity. We now know that this can badly overstate actual performance. Thermal bridges and air leakage frequently dominate. This guide explains how to account for all three mechanisms and why they matter for HVAC design.

Conduction basics

Steady-state conduction through an assembly is:

Q = U x A x delta t

where U is the heat transfer coefficient in W/(m²·K), A is area in m² and delta t is the indoor to outdoor temperature difference in K.

U is the inverse of the total thermal resistance. In Canada, resistance is expressed in SI as RSI (m²·K/W) and in imperial as R (ft²·°F·h/Btu).

Resistances of layers in series add together, including the inside and outside air films. That gives the centre-of-assembly or clear field value, which assumes uniform layers with no interruptions.

Nominal, clear field and effective values

Three levels of accuracy are commonly used:

1. Nominal: the rated value of the insulation alone, for example R-20 batts. Useful for product comparison only.
2. Clear field: the assembly including framing members repeated uniformly, such as studs at regular spacing. Steel studs reduce performance much more than wood studs because steel conducts heat so well.
3. Effective or overall: the clear field plus all the linear and point thermal bridges at interfaces, such as slab edges, window perimeters, parapets and penetrations.

For load calculations and energy models, the effective value is what counts. In many commercial assemblies with steel framing, concrete balconies or poorly detailed window interfaces, the effective value is substantially worse than the nominal value.

Thermal bridging

A thermal bridge is a path of higher conductivity that bypasses the insulation. Common examples:

• Concrete slab edges and balconies extending through the wall.
• Steel shelf angles supporting brick veneer.
• Window and door perimeters where the frame does not line up with the insulation.
• Parapets and roof-to-wall junctions.
• Steel girts, clips and fasteners through exterior insulation.
• Structural steel beams or columns penetrating the envelope.

Psi and chi values

Modern practice quantifies thermal bridges as:

• Linear transmittance (psi, W/(m·K)): heat loss per metre of a linear bridge, such as a slab edge.
• Point transmittance (chi, W/K): heat loss at a discrete point, such as a beam penetration.

The overall heat loss of a wall is then the clear field loss plus the sum of each psi value times its length, plus each chi value. The BC Hydro Building Envelope Thermal Bridging Guide is a widely used Canadian source of psi and chi values for common details. NECB 2017 and NECB 2020 include provisions for accounting for thermal bridging in compliance calculations, with details depending on the compliance path and the adopted edition.

Why it matters beyond energy

Thermal bridges create cold interior surfaces. When a surface falls below the room dew point, condensation forms, which can lead to mould and material damage. This is a common problem at slab edges and window perimeters in humidified buildings. See condensation and mould prevention. Infrared thermography during cold weather is an effective way to locate thermal bridges in existing buildings.

Infiltration and air leakage

Infiltration is uncontrolled outdoor air entering through the envelope. Exfiltration is indoor air leaving. Both carry heat and moisture.

Driving forces

• Stack effect: warm indoor air rises and escapes at the top of the building, pulling cold air in at the bottom. It is strongest in tall buildings during cold weather, which describes Canadian winters exactly.
• Wind: creates positive pressure on the windward side and suction on leeward sides and the roof.
• Mechanical systems: unbalanced supply and exhaust can pressurize or depressurize the building.

Measuring airtightness

Airtightness is measured by a whole-building pressure test, with fans pressurizing or depressurizing the building to a reference pressure difference, often 75 Pa for commercial buildings. Results are expressed as leakage per unit of envelope area at the test pressure. Recognized test methods include ASTM E779, ASTM E3158 and CAN/CGSB-149.15. Some Canadian jurisdictions, such as those using the BC Energy Step Code or Toronto Green Standard, now require or encourage airtightness testing for certain buildings.

Estimating infiltration for design

For heating load calculations, infiltration is often estimated from an assumed leakage rate per unit envelope area, adjusted for building height, exposure and the operating pressure. NECB energy modelling rules prescribe a default infiltration rate for the reference building. Where a test result is available, convert from test pressure to typical operating pressure using a flow exponent.

Exfiltration and moisture

In winter, warm, moist indoor air leaking outward through the envelope can condense inside wall and roof assemblies. This is often more damaging than the energy loss. Keeping humidified buildings at a slight positive pressure must be balanced against this risk. See building pressurization and infiltration control.

Putting it together: a heat loss procedure

1. Identify each opaque assembly and calculate its clear field U-value.
2. Identify all linear and point thermal bridges and their lengths or counts.
3. Apply psi and chi values to calculate the effective U-value for each assembly.
4. Use whole-window U-values for fenestration.
5. Calculate conduction loss for each assembly at design temperature difference.
6. Estimate infiltration based on test data or a reasonable assumed leakage rate.
7. Combine with ventilation and other loads for the total; see heating load calculations for Canadian climates.

Implications for HVAC design

Accurate envelope data allows right-sized heating equipment and perimeter systems. As envelopes improve, perimeter heating may be reduced or eliminated, and the remaining load is often dominated by ventilation. That shifts design priorities toward heat recovery and toward systems suited to low loads, a theme in net zero ready building HVAC strategies.

Frequently asked questions

Why is my wall's actual performance lower than the insulation rating?

Framing, fasteners, slab edges and window interfaces all conduct heat around the insulation. Steel framing and concrete balconies have an especially large effect.

Is airtightness more important than insulation?

Both matter. In many buildings, improving airtightness gives significant energy and comfort benefits at modest cost, and it also reduces moisture risk in assemblies.

Can thermography measure U-values?

Thermography shows relative surface temperature differences and is excellent for finding thermal bridges, missing insulation and air leaks. It does not directly measure U-values.

Key takeaways

• Use effective U-values that include thermal bridging, not nominal insulation values.
• Psi and chi values quantify linear and point thermal bridges.
• Stack effect makes infiltration significant in tall Canadian buildings in winter.
• Airtightness testing provides real data for heat loss calculations.
• Cold surfaces at thermal bridges are a condensation and mould risk, not just an energy loss.