# Insulating from inside: condensation and mould risk, and when to insulate outside

> What LNEC and a UK technical guide say on internal insulation, vapour barriers, ETICS and thermal paints, and why the cause of the damp comes first.

- Page: https://paredecerta.pt/en/guides/internal-insulation-condensation-risk
- Language: English
- Updated: 2026-11-04

## Summary

- LNEC writes that external insulation corrects thermal bridges and reduces indoor condensation.
- Careless internal insulation can cause condensation inside the wall and mould, according to the 2021 UK guide.
- A vapour barrier by default is no longer a rule: on many old walls it may do more harm than good.
- LNEC warns that ETICS is not suitable for buildings from before about 1945. Thermal paints do not replace insulation: the study we read points to under 1% energy saving.

Thinking of insulating a cold wall from the inside to stop mould? It can work, but it can also make things worse, because the insulation changes where water vapour condenses. This guide explains the risk, when insulating on the outside (ETICS) is the better route, and why "thermal" paints are not a substitute.

*Updated on 3 November 2026. General information, not a substitute for assessment by a technician.*

## The essentials

- LNEC (Portugal's national civil engineering laboratory) writes that external insulation systems **correct thermal bridges and reduce the problem of condensation indoors**. In the same text they are the most effective option.
- The UK's internal wall insulation guide (2021) says external insulation is preferable for protecting the wall and reducing moisture risk. Done from the inside without the right precautions, there is a **high risk of condensation inside the wall, and of mould**.
- A vapour barrier "to be safe" is no longer a rule: the same guide says that on many older walls it may do more harm than good.
- Before insulating, the cause of the damp must be fixed and the wall dry.
- We found no independent evidence that "thermal" paints replace insulation.

## Why it can make things worse

Indoor air carries water vapour. An uninsulated outer wall is cold on the inside, but heat from the house still warms it a little. Put insulation on the inner face and the wall gets **even colder**, because it stops receiving heat from the room. Vapour that passes through or around the insulation can reach the cold wall and condense behind it, where you cannot see it. This is "interstitial condensation" (inside the construction).

The UK guide (Enhabit, September 2021, published on gov.uk) says warm air leaking through gaps in the insulation carries a lot of moisture and **greatly increases** that risk. So it requires an air barrier, which is not the same as a vapour barrier. It names "dot and dab" bonding as the highest-risk method, because it leaves air gaps behind the insulation. It asks for continuous adhesive, correct fixings and a smooth wall.

To tell condensation from a leak, see [damp walls: the three types](https://paredecerta.pt/en/guides/damp-walls-three-types). On mould itself, see [mould on walls](https://paredecerta.pt/en/guides/mould-on-walls).

## Vapour barrier: yes or no?

| Situation | What the UK guide says |
|---|---|
| Old solid wall that can dry in both directions | "Moisture open" build-up, no vapour barrier, with an air barrier. A vapour barrier "used to be regarded as essential", but it is now agreed that in many cases it may do more harm than good |
| "Moisture closed" system | Only with a ventilated cavity of at least 25 mm on the cold side |
| Wall with impervious outer cladding, or a cavity wall in good order | A moisture closed system may be considered |
| Calculation | The simple Glaser method suits only closed, weathertight constructions. For open ones, dynamic hygrothermal simulation |

Note that this guide is mainly about British solid-wall houses. Many Portuguese homes have hollow-brick or double walls. Practical conclusion: **ask the designer for a condensation study**, and do not accept the rule "always a vapour barrier" or "never one".

The guide adds two useful rules. Ventilation must be assessed before insulating and must work afterwards. And ventilation that pushes humid air into the wall (PIV) should not be installed with internal insulation. See [bedroom condensation and ventilation](https://paredecerta.pt/en/guides/bedroom-condensation-ventilation) and [bathroom extractor fans](https://paredecerta.pt/en/guides/bathroom-extractor-fan-ventilation).

## Insulating outside: when it is better

LNEC (Veiga and Pina dos Santos, undated text that cites Decreto-Lei 80/2006) says external insulation is the most effective: it corrects thermal bridges, improves summer comfort (the wall's mass stays inside) and protects the masonry. It takes no floor area and causes little disruption. The drawback is the look of the façade, especially in refurbishment.

The industry body APFAC (2024 manual) says the same about thermal bridges and mould. It represents the sector that sells the system, so it carries less weight than LNEC.

**Important exception.** LNEC warns that ETICS is **not suitable for buildings from before about 1945**, before reinforced concrete, because thick porous walls struggle to let moisture evaporate. There the choice is more delicate: get an opinion from a technician experienced in refurbishment.

A data sheet (webertherm EPS 100, Saint-Gobain, version of 6 January 2025) gives the EPS board a water-vapour diffusion resistance factor of 30 to 70 and water absorption by immersion below 2%. These are the manufacturer's figures.

In a block of flats the façade is a common part. See [painting the façade: licence and condominium](https://paredecerta.pt/en/guides/painting-the-facade-licence-condominium) and [damp in the building](https://paredecerta.pt/en/guides/condominium-common-parts-damp). Article 1422(3) of the Código Civil (Civil Code) requires approval by the owners' meeting, by two thirds of the building's value, for works that change the building's architectural line or aesthetic appearance.

## "Thermal" paints: be careful

Some products promise insulation or condensation control from paint alone. We found two sources:

- Buckmaster and Abramson (Journal of Thermal Engineering, 2015) simulated a room and concluded that even under very favourable conditions energy use falls by **less than 1%**.
- A NOVA University Lisbon dissertation (Botelho, 2024), using tests and modelling, according to the summary we obtained **did not confirm** manufacturers' claims that these paints prevent condensation on interior surfaces. We could not open the full text (the repository blocked automated access).

See also [anti-damp and anti-mould paint: does it work?](https://paredecerta.pt/en/guides/anti-damp-anti-mould-paint-does-it-work).

## What the regulation says

Portaria 138-I/2021 (the ministerial order on minimum energy performance) requires new elements of the opaque envelope to have a technical sheet, CE marking and a declaration of conformity, and installation to follow the manufacturer's instructions. We saw no condensation-risk test for your wall in it.

## A step list before you decide

1. **Find the cause.** Condensation, leaks and rising damp need different fixes ([rising damp: treatment](https://paredecerta.pt/en/guides/rising-damp-treatment)).
2. **Repair and dry.** The UK guide says to remove the water source and let the wall dry before insulating.
3. **Look at the thermal bridges** in corners: [thermal bridges and mould in corners](https://paredecerta.pt/en/guides/thermal-bridges-mould-in-corners).
4. **Compare outside and inside** with a technician. Ask for the condensation-risk calculation in writing.
5. **Plan ventilation** before and after.
6. **Paint last**, with [the right paint](https://paredecerta.pt/en/guides/which-paint-for-each-room). For quantities, use the [calculator](https://paredecerta.pt/en#calc).
7. Check [grants for home works](https://paredecerta.pt/en/guides/grants-home-renovation-painting): painting alone is usually not eligible.

## What we could not confirm

- **A Portuguese guide** from ADENE or LNEC on internal insulation and condensation: not found.
- **The date of the LNEC text:** the file does not give it.
- **Botelho thesis (2024):** we saw only the summary, through a search.
- **Plasterboard with insulation** (Knauf): the page would not open, so no figures.
- **Prices and timescales** for insulation: no reliable source.

## Sources (read on 3 November 2026)

- Enhabit, for the UK government: *Retrofit Internal Wall Insulation: Guide to Best Practice* (September 2021), [gov.uk (PDF)](https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/1019707/iwi-guidance.pdf). Read: introduction and sections 4, 6 and 7.
- LNEC: M. R. Veiga and C. Pina dos Santos, *Revestimentos de isolamento térmico de fachada: eficiência, durabilidade e comprovação de qualidade*, [LNEC repository (PDF)](https://repositorio.lnec.pt/bitstream/123456789/16962/2/Revestimentos%20de%20isolamento%20t%c3%a9rmico%20de%20fachada_%20efici%c3%aancia%2c%20durabilidade%20e%20comprova%c3%a7%c3%a3o%20de%20qualidade.pdf). Read: introduction.
- APFAC: *Manual ETICS* (January 2024), [apfac.pt (PDF)](https://www.apfac.pt/wp-content/uploads/2025/03/MANUAL_ETICS_16.01.20241.pdf). Read: chapter 2 and the section on condensation.
- Portaria n.º 138-I/2021 of 1 July, [Diário da República (PDF)](https://files.dre.pt/1s/2021/07/12602/0001200053.pdf). Read: point 1 of the annex.
- Código Civil, article 1422, [pgdlisboa.pt](https://www.pgdlisboa.pt/leis/lei_mostra_articulado.php?nid=775&tabela=leis).
- Saint-Gobain Portugal: data sheet *webertherm EPS 100*, [saint-gobain.pt (PDF)](https://www.saint-gobain.pt/Documentos/ficha-tecnica/ficha-tecnica-webertherm-eps-100.pdf), version of 6 January 2025.
- D. Buckmaster and A. Abramson, *Characterization of the effects of insulating wall paint on space conditioning in a room*, Journal of Thermal Engineering, vol. 1, no. 1, 2015, [dergipark.org.tr (PDF)](https://dergipark.org.tr/en/download/article-file/199352). Read: abstract and conclusion.
- Botelho (2024), dissertation, NOVA University Lisbon, [run.unl.pt](https://run.unl.pt/handle/10362/177253). Cited through a search summary, not read directly.

## FAQ

### Does internal insulation cure mould?

It may help with thermal bridges, but without a condensation study it can cause mould behind the insulation. The UK guide (2021) says the risk is high if its recommendations are not followed.

### Do I always need a vapour barrier?

No. The UK guide says on many solid walls it may do more harm than good. Ask the designer for a condensation study.

### Do thermal paints insulate?

We found no independent proof. Buckmaster and Abramson (2015) calculated under 1% energy reduction even in favourable conditions.

### Is ETICS right for any house?

According to LNEC, it is not suitable for buildings from before about 1945, because moisture struggles to evaporate from the walls.

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Source: https://paredecerta.pt/en/guides/internal-insulation-condensation-risk. Independent information, not legal or tax advice.