Insulation Condensation Risk Calculator

Summary

Every time insulation is added to a wall, roof or floor, the temperature profile through the construction changes. The inner layers get warmer, and the outer layers get colder. Moisture-laden indoor air diffusing outward meets those cold layers, and if the vapour pressure is higher than the saturation pressure at that temperature, it condenses inside the construction, out of sight. That is interstitial condensation, and it is behind rotten joist ends, mouldy battens behind internal wall insulation, and saturated mineral wool in poorly detailed cold roofs.

This calculator walks through the Glaser method with worked numbers, so you can screen a build-up before quoting, decide whether a vapour control layer (VCL) is needed and how strong it must be, and check the surface temperature factor for mould risk. For the theory, see interstitial condensation. This page is the hands-on calculation.

Be clear about the method's limits. Glaser is a steady-state vapour diffusion model. It ignores air leakage (often the biggest moisture route), rain penetration, built-in construction moisture, and moisture stored in hygroscopic materials. For internal wall insulation on solid masonry, especially exposed walls or historic buildings, BS 5250:2021 points to dynamic hygrothermal simulation (BS EN 15026, using software such as WUFI) instead. Use this calculator to screen. Use a specialist assessment to sign off risky builds.

Key Facts

• BS EN ISO 13788:2012 — the calculation method for internal surface temperature (to avoid mould) and interstitial condensation (Glaser monthly method).
• BS 5250:2021 — management of moisture in buildings. It sets the assessment framework and when to use dynamic simulation.
• BS EN 15026:2023 — hygrothermal performance by numerical simulation (for example WUFI), used for solid-wall IWI and complex cases.
• Layer temperature — T_x = T_in − (ΣR to point x ÷ R_total) × (T_in − T_out).
• Surface resistances — Rsi 0.13 m²K/W and Rse 0.04 m²K/W for U-value and interstitial calculations. Use Rsi 0.25 for the surface-temperature (mould) check per BS EN ISO 13788.
• Saturation vapour pressure (T ≥ 0°C) — Psat = 610.5 × e^(17.269T ÷ (237.3 + T)) Pa.
• Saturation vapour pressure (T < 0°C) — Psat = 610.5 × e^(21.875T ÷ (265.5 + T)) Pa.
• Actual vapour pressure — Pv = RH × Psat(T_air).
• sd value — water-vapour diffusion-equivalent air layer thickness (m) = μ × thickness (m). Higher sd means more vapour resistance. Roughly, 1m sd ≈ 5 MN·s/g.
• Vapour control layer — typically sd ≥ 10m, often far higher [check product data]. Foil-faced PIR board with taped joints acts as a VCL.
• Breather membrane — low sd (typically ≤ 0.1–0.2m) on the cold side, letting vapour out.
• "5:1 rule" — common guidance that the vapour resistance on the warm side should be about 5 times that on the cold side. It's a rule of thumb, not a substitute for the calculation.
• Temperature factor fRsi = (T_si − T_out) ÷ (T_in − T_out). For dwellings, keep fRsi ≥ 0.75 to control mould risk (BRE IP 1/06).

Quick Reference Table

Saturation vapour pressure and dew points:

Typical layer properties (indicative; use manufacturer data where available):

Detailed Guidance

The calculation step by step

1. List layers from inside to outside, with thickness, λ and μ.
2. Thermal resistance of each layer: R = thickness ÷ λ. Add Rsi and Rse.
3. Temperature at each interface using the cumulative R.
4. Saturation pressure (Psat) at each interface temperature.
5. sd of each layer = μ × thickness. Sum them.
6. Actual vapour pressure at each interface, falling linearly with cumulative sd from Pv,in to Pv,out.
7. Compare. Where Pv > Psat, condensation is predicted. The strict Glaser method draws tangents from Pv,in and Pv,out to the Psat curve to locate condensation planes. The simple comparison is a reliable screening check.
8. For compliance, repeat monthly with BS EN ISO 13788 climate data, checking the condensate evaporates over the year and doesn't exceed the material's tolerance.

Worked example A: internal wall insulation, no VCL

Construction (inside to outside): 12.5mm plasterboard → 60mm mineral wool (battens ignored for simplicity) → 215mm solid brick. Conditions: inside 20°C, 60% RH (Pv,in = 0.60 × 2,339 = 1,403 Pa); outside 0°C, 90% RH (Pv,out = 0.90 × 611 = 549 Pa).

The insulation keeps the brick cold, but the vapour passes straight through the plasterboard and wool. At the wool/brick interface the actual pressure (1,335 Pa) is almost double the saturation pressure (750 Pa). Expect wet wool, mould on battens and damp at the inner face of the brick.

Worked example B: how strong a VCL is needed?

Add a VCL behind the plasterboard. For no condensation at the wool/brick interface, we need:

Pv(interface) = 1,403 − 854 × (s ÷ (s + 2.15)) ≤ 750 Pa

where s is the total sd inboard of the interface (plasterboard + wool + VCL). Solving gives s ≥ 6.98m, so the VCL alone needs sd ≈ 6.8m or more.

With a VCL of sd = 20m: s = 20.185m, so Pv = 1,403 − 854 × (20.185 ÷ 22.335) = 631 Pa, below 750 Pa. Pass, with margin.

This is why the details matter. A VCL with sd 20m that is punctured by sockets and unsealed at junctions performs like one with a fraction of that resistance, because air leakage carries far more moisture than diffusion. Tape every joint, seal service penetrations, and use a service void where possible: see vapour control layers.

Worked example C: surface temperature and mould (fRsi)

Use Rsi = 0.25 for this check.

Uninsulated 215mm brick (with 13mm plaster ignored): R_total = 0.25 + 0.279 + 0.04 = 0.569. T_si = 20 − (0.25 ÷ 0.569) × 20 = 11.2°C, so fRsi = 11.2 ÷ 20 = 0.56. That is below 0.75, so mould risk is high, especially in corners and behind furniture. With room air at 20°C / 60% RH (dew point 12.0°C), surface condensation is likely on cold days.

With the insulation from example A: R_total = 0.25 + 0.06 + 1.714 + 0.279 + 0.04 = 2.343. T_si = 20 − (0.25 ÷ 2.343) × 20 = 17.9°C, so fRsi = 0.89. Pass.

The surface problem is solved, but example A showed the interstitial problem it can create. The two checks always go together. Also check junctions (window reveals, floor and ceiling junctions, party walls) where the insulation stops. These thermal bridges are where IWI jobs most often grow mould: see thermal bridging.

Choosing design conditions

When the simple method is not enough

Get a BS EN 15026 hygrothermal assessment, or follow BS 5250:2021 and specialist guidance, when:

• Insulating solid masonry internally, especially in exposed (driving-rain) locations, or on historic or listed buildings.
• Using vapour-open, capillary-active systems (wood fibre, calcium silicate, lime). Glaser undervalues their ability to redistribute moisture.
• Timber elements bear into the wall (joist ends in solid walls are a classic failure point).
• Flat roofs and room-in-roof conversions where warm and cold deck build-ups mix: see warm roof cold roof.

Frequently Asked Questions

Does foil-backed plasterboard count as a VCL?

The foil has a high vapour resistance, but the joints between boards don't, and nor do the screw holes and sockets. Foil-backed board helps, but as a VCL it is only as good as its taped joints and sealed penetrations. Many specifiers prefer a separate membrane with taped laps, or taped foil-faced PIR, for high-risk walls.

Why does the calculation say condensation, but the wall I insulated last year seems fine?

The steady-state Glaser check is deliberately pessimistic. Real walls store and redistribute moisture, and indoor humidity may be lower than assumed. But problems can take two or three winters to show, often behind finishes. A predicted condensation plane is a warning to design it out, not proof that it won't happen.

Should I put a VCL on solid-wall IWI with breathable insulation?

Not always. Vapour-open, capillary-active systems (wood fibre with lime plaster, calcium silicate) are designed to let moisture move both ways and dry inward. Adding polythene can trap moisture driven in from outside. These systems need a hygrothermal assessment rather than the simple check: see internal wall insulation damp risk.

What if condensation is predicted but very small?

Under BS EN ISO 13788's monthly method, a build-up is acceptable if all condensate evaporates during the summer months, the maximum amount doesn't damage the materials, and it doesn't wet sensitive materials like timber above about 20% moisture content. Screening calculations can't show this. The monthly method or simulation can.

Do I need this calculation for Building Regulations?

Approved Document C (Regulation 7 and Requirement C2) requires buildings to resist harmful interstitial and surface condensation, and refers to BS 5250 and BS EN ISO 13788. Building control or a warranty provider may ask for a condensation risk analysis for new build-ups, especially IWI, flat roofs and room-in-roof work. Insulation manufacturers often provide one free for their products.

Regulations & Standards

• BS EN ISO 13788:2012 — hygrothermal performance of building components: internal surface temperature to avoid critical surface humidity and interstitial condensation (calculation methods)

• BS 5250:2021 — management of moisture in buildings, code of practice

• BS EN 15026:2023 — hygrothermal performance by numerical simulation

• BS EN ISO 6946:2017 — thermal resistance and U-value calculation method

• BS EN ISO 12572:2016 — determination of water vapour transmission properties (μ and sd values)

• Approved Document C (2013 edition, incorporating amendments) — resistance to contaminants and moisture, Requirement C2 (condensation)

• Approved Document L (2021 edition) — thermal performance, requiring condensation risk to be considered when upgrading elements

• BRE Information Paper IP 1/06 — assessing the effects of thermal bridging at junctions, including the fRsi ≥ 0.75 criterion for dwellings

• BS 5250:2021 Management of moisture in buildings — BSI

• Approved Document C: site preparation and resistance to contaminants and moisture — GOV.UK

• Approved Document L: conservation of fuel and power — GOV.UK

• Moisture risk assessment and guidance — Historic England

• interstitial condensation — the physics of interstitial condensation and prevention

• vapour control layers — choosing and installing VCLs properly

• internal wall insulation — IWI systems and detailing

• internal wall insulation damp risk — damp risks specific to solid-wall IWI

• thermal bridging — junction losses and cold-spot mould risk