Why calculated and actual performance diverge
A wall’s insulation value describes heat moving through the wall by conduction, in the idealised case. Real buildings lose heat two further ways that the wall calculation does not capture:
- Air moving through and around the assembly, carrying heat with it
- Heat bypassing the insulation at fixings, junctions and structural elements
Both are detailing outcomes rather than specification outcomes, which is why two buildings with identical insulation schedules can perform very differently.
Air leakage
Air carries far more heat and moisture than diffusion does. A gap is not a small conductive path; it is an open route.
The leaks that matter are rarely the obvious ones. Typical paths:
- Junctions between elements — wall to floor, wall to roof, wall to window
- Service penetrations — pipes, cables, ducts, extract terminals
- Loft hatches, recessed light fittings, unsealed voids
- Behind dry lining, where the air barrier was assumed to be but is not
- Around window and door frames, behind the trim
Fixing them is a matter of designating an air barrier layer, drawing it continuously, and detailing every place it is interrupted — the pen test described in building envelope design.
Airtightness is not the same as ventilation. You seal the envelope so ventilation becomes controlled and heat recoverable, instead of happening randomly through gaps in weather you cannot choose. Sealing without providing ventilation raises humidity and creates condensation problems, particularly in retrofit.
Thermal bridging
A thermal bridge is any path where heat crosses the envelope while bypassing the insulation.
Swipe sideways to see the other columns. The first column stays visible.
| Bridge | Why it happens | Typical consequence |
|---|---|---|
| Slab edge | Floor structure passes through the wall line | Cold strip inside, condensation at floor junction |
| Balcony | Structure cantilevers straight through the envelope | Severe; often the worst on a building |
| Window perimeter | Frame and reveal interrupt the insulation | Cold reveals, condensation, mould at corners |
| Structural fixings | Metal penetrates the insulation layer | Cumulative; many small bridges add up |
| Studs and rafters | Framing bridges insulation between it | Reduces the assembly value substantially |
| Wall to roof junction | Two insulation lines meet | Gap unless deliberately detailed |
| Party wall junction | Insulation line stops at the boundary | Heat loss into the cavity |
Bridges do two things. They reduce the real performance of the assembly, and they create cold internal surfaces — which is the more immediate problem, because condensation and mould appear there long before anyone notices the energy cost.
Where insulation sits in the envelope
Insulation is the thermal control layer of the building envelope, and its position decides how many times structure crosses it.
Start with the test scope. For non-reflective home insulation covered by 16 CFR 460.5(a), R-value tests use the material alone, excluding airspace, at a mean temperature of 75°F and a temperature difference of 50°F ±10°F. This is not a universal rule for reflective systems: paragraphs (b)–(c) address reflective insulation with defined airspaces and heat-flow conditions; (d) covers reflective facings and permits a separate system-with-airspace value. The applicable product test does not establish the performance of a wall with studs, junctions and fixings.
Swipe sideways to see the other columns. The first column stays visible.
| Where the insulation sits | What interrupts it | What it means for the structure |
|---|---|---|
| Between studs or rafters | Every framing member, plus plates, headers and junctions | The framing bridges the layer at regular intervals |
| As a continuous layer outside the structure | Fasteners, openings and the junctions where it has to be detailed | During heating, the structure is on the warmer side; fasteners and junctions still need checking |
| As a continuous layer on the inside face | Floors, internal walls and party walls that meet the outer wall | During heating, the structure is colder; resolve floors and walls crossing the layer |
A widely taught building-science arrangement puts the thermal control layer outside the structure, over the water, air and vapour control layers, so the structure is protected from temperature extremes. Where that is not possible, as in many retrofits, the same principle still applies: count the places where structure passes through the insulation, and detail each one.
Why continuity matters more than thickness. Heat takes the easiest route out. Adding thickness to a layer that framing crosses at every stud improves only the insulated part of the wall; the bridges stay as they were, and so do the cold internal surfaces where condensation forms. A continuous layer removes the regular bridges instead of thickening the parts in between them.
Where to spend first
Editorial screening order, not a cost or payback ranking: review project heat loss, moisture, access and ventilation before choosing the sequence.
- Air barrier continuity. Locate accessible leaks and coordinate repairs with ventilation.
- The worst individual bridges — balconies, slab edges, window perimeters.
- Continuous external insulation, which removes framing bridges rather than mitigating them.
- Insulation thickness, compared alongside the remaining bridges and leaks.
Takeaway: Choose the next measure from project findings, rather than assuming the same return in every building.
Verifying rather than assuming
Blower-door testing assesses air leakage; thermal imaging needs suitable temperature differences and interpretation to distinguish possible bridges from leaks or missing insulation. Record defects while the work remains accessible, then verify repairs.
| Junction to record | Check method and timing; project entry |
|---|---|
| Window perimeter / service penetration | Air-barrier inspection and leakage investigation before concealment: ____ |
| Slab edge / balcony connection | Detail review before construction; temperature survey when conditions permit: ____ |
| Wall-to-roof transition | Continuity inspection while accessible; repair and retest record: ____ |
Takeaway: Record the junction, investigation conditions and repair verification, not just the final leakage number.
For how these decisions sit relative to form, shading and mass, see passive design basics and thermal mass vs insulation; the sustainable and climate-responsive design guide covers the order of leverage.
Frequently asked questions
What is a blower door test?
A fan mounted in a doorway pressurises or depressurises the building so the air leakage rate can be measured. Combined with smoke or thermal imaging it also locates the leaks, which is usually more useful than the number alone.
Can a building be too airtight?
A building cannot be too airtight, but it can be under-ventilated. Airtightness and ventilation are separate systems — you seal the envelope so that ventilation is controlled and heat can be recovered, rather than relying on uncontrolled leaks.
Where are the worst thermal bridges?
Typically slab edges, balconies, window and door perimeters, structural fixings passing through insulation, wall-to-roof junctions and party wall junctions. Anywhere structure crosses the insulation line is a candidate.
Is it worth improving airtightness in an old building?
It can be worthwhile, depending on the existing leakage, repair access and ventilation strategy. The caution is that sealing an old building without adding controlled ventilation can raise internal humidity and create condensation and mould where there was none.
Sources
- Opaque Envelope — Building Technologies Office, U.S. Department of Energy Accessed August 28, 2026.
- Building Envelope Design Guide — Whole Building Design Guide, National Institute of Building Sciences Accessed August 28, 2026.
- 16 CFR Part 460: Labeling and Advertising of Home Insulation — U.S. Federal Trade Commission, via the Electronic Code of Federal Regulations Accessed October 8, 2026.