Start with leaks and water damage. Then compare the whole assembly, including its junctions, before buying insulation.
1. Find where the heat is going
A cold room does not identify its own cause. A draught around a hatch suggests air movement; a uniformly cold surface suggests heat transmission. A damp corner may involve a thermal bridge, a leak or high indoor humidity. Photograph symptoms and note outdoor temperature, wind and indoor humidity so comparisons mean something.
Inspect accessible areas without disturbing unknown materials: loft hatches, pipe penetrations, window seals and the junction between walls and floors. A home energy assessment can add a blower-door test and thermal imaging. Air sealing and insulation work together; plan ventilation and combustion-appliance checks as part of the work.
DOE · Air sealing guide ↗2. Understand U, R and thickness
U-value describes heat flow through an assembly: lower is better. Thermal resistance R describes opposition to heat flow: higher is better. In SI units, an ideal homogeneous layer has R = thickness / conductivity. The whole assembly also contains other layers, surface resistances and repeating bridges such as timber or metal studs.
Use metres for thickness and W/(m·K) for conductivity. A 100 mm layer with conductivity 0.040 gives R = 0.10 / 0.040 = 2.5 m²K/W. American R-values use different units: do not paste them directly into an SI calculation. Our thickness tool estimates one extra continuous layer, not a complete certified assembly.
DOE · Types of insulation ↗3. Decide where the wall can dry
Adding insulation changes temperatures inside the wall. That can change where moisture accumulates and how easily it dries. Fix rain entry and rising or penetrating damp before covering surfaces. Specify the water-control layer, air barrier and vapour-control strategy as a connected assembly.
Interior insulation of existing masonry deserves particular care because the original wall becomes colder. A generic rule such as “plastic goes on the inside” is not suitable for every climate or wall. Ask the designer to explain the drying path and the details at floors, reveals and roof junctions.
DOE · Moisture performance of high-R walls ↗4. Compare complete proposals
Ask every contractor to price the same scope. A cheaper material price may omit reveals, flashing, ventilation work or repairs. Keep product conductivity, fire classification, installed thickness and junction details with the quote.
- Record the existing construction and condition.
- Fix moisture sources and define ventilation.
- Choose the target assembly performance for your climate.
- Check continuity at corners, openings and penetrations.
- Inspect the finished installation before it is concealed.
Match the symptom to the next check
| What you notice | What to check | Useful next step |
|---|---|---|
| Draughts around an opening | Seals and air-barrier continuity | Inspect and repair the leak path |
| Cold strip at a slab edge | Thermal bridge at the junction | Review a continuous insulation detail |
| Damp patch after rain | Flashing, drainage and rain entry | Repair the water source first |
| High heating use, no clear symptom | Whole-building heat balance | Get an energy assessment |
Worked example
What changing a wall U-value actually changes
Compare 80 m² of wall at a 25 K indoor–outdoor temperature difference.
Before: 80 × 1.20 × 25 = 2,400 W After: 80 × 0.30 × 25 = 600 W
The wall loses 1.8 kW less under those conditions. This is not annual energy savings: weather, ventilation, heating efficiency and other surfaces still matter.
What to have ready
- Assembly layers and thicknesses
- Declared conductivity or tested U-value
- Moisture and ventilation plan
- Window, floor and roof junction details
Common mistakes
- Treating insulation as an air seal.
- Using centre-of-panel performance for an entire framed wall.
- Turning a single cold-day calculation into a yearly savings claim.
Reference check: 5 September 2026. Sources include US public agencies. Principles are general; regulations and design values must be checked where you live.Worked examples use stated hypothetical inputs.