Thursday, 29 May 2014

Does insulation cause damp?

Some people will tell you that you should never insulate your home because it will cause damp. For example see 'Loft and cavity wall insulation causes damp' ( It only takes a relatively few bad cases to give insulation a bad name. However it is hard to know how often problems occurr. If damp was a ‘notifiable disease’ like Mumps or Legionnaire’s disease then we would have a better idea. The other problem is that even when there is a problem it can take a few years before it is visible. I expect to learn more about this topic at our forum on 17th June. In the meantime, here are some potential problems I already know about - and how to avoid them.

I should add that although it is very easy to find terrifying stories there are millions of homes with cavity wall insulation in the UK – 13 million as of the end of 2012 – and the vast majority have no problems. There are relatively few homes with solid wall insulation so far – only 150,000 as of the end of 2012.

Where there are cases of damp, sometimes it is due to water getting in from outside and sometimes it is due to condensation. Condensation occurs when moist air cools to the point where it can’t hold all the moisture any more – when the humidity goes above 100%. However, you can get mould growth even before that point – you should keep your home below 55% humidity to avoid problems.

Wind driven rain on cavity walls
Cavity walls were first invented to protect against wind driven rain. The idea is that only the external wall gets damp. The inner wall is protected from both the rain and the wind. However, if you fill a cavity wall then the filling can carry moisture through from the outer to the inner wall.

For this reason you should avoid filling cavity walls if you live in an area prone to wind driven rain. This map shows which areas of the UK are more or less vulnerable.

This problem is particularly bad if there are ‘snots’ in the wall. These are bits of mortar that have fallen down and caught on the ties that hold the two layers of the wall together. Snots can carry moisture from the wall onto the ties. While the cavity is open the ties will usually dry out quickly but after filling they can stay wet and eventually rust. Your installer should check the cavities are clear of snots before filling them.

If your cavity walls are unsuitable for filling then you should consider external insulation instead.

Cavity wall insulation installers are notorious for hard sell techniques – Which? Magazine asked eight assessors to visit a house that was unsuitable both because of location and because of cracks in the wall – all insisted the house was suitable. Hopefully the auditing that Green Deal installers have to undergo will improve this situation.

If you suffer damp in this way, then the best thing to do is to unfill the cavity wall but if that isn’t possible you could consider protecting the outer bricks. You could use an external render or an invisible, breathable coat such as Stormdry® Masonry Protection Cream.

Condensation in cold lofts

If you insulate your loft at the level of the joists then the loft space will be cooler than it was before. There is usually some warm air escaping from inside the house carrying moisture with it and when this air gets into the loft it cools so you can get condensation. To avoid this problem you need to make sure the loft is well ventilated so that the moist air disperses quickly. This is why you should never insulate tightly under the eaves – leave at least 50mm gap. Also if the roofing felt below the tiles is old it is a good idea to replace it with a modern breathable type.

Minimising air leakage into the loft will also help and is a good idea anyway. So make sure the loft hatch is draught proof and check for air gaps around pipes– for example if you have a cold water tank in your loft, it will have pipes going in and out through the ceiling somewhere.

Penetrating damp after internal insulation.
If you choose internal insulation your walls will be colder than they were before so when they get wet they don’t dry so quickly. Usually this is a problem with the mortar as most bricks don’t absorb much water, though some are more porous than others. If the weather is very cold then the moisture can freeze, ultimately causing the mortar to crumble and it will need repointing more often. If the damp penetrates all the way to the insulation it can make the insulation wet – and wet insulation doesn’t insulate very well.

This is mainly a problem in regions with wind driven rain that penetrates the walls – just like the cavity walls issue. So you should consider carefully before using internal insulation in an area prone to wind driven rain (see the map) and very cold weather.

This problem can be reduced if the internal insulation is breathable because the wet walls can then dry to the inside. However, there isn’t a great deal of research as to how much this helps.

You can also consider a protective coat on the outside of the walls, such as the invisible, breathable cream already mentioned for cavity walls - Stormdry® Masonry Protection Cream.

Condensation on cold surfaces due to decreased ventilation
After you have insulated your home everything inside will be warmer than before so other things being equal the humidity will have decreased and you are less likely to get condensation. However, adding insulation also tends to fix draughts. Cavity wall insulation will reduce draughts from badly fitted windows and pipes and external insulation is especially good at plugging holes but even if you insulate on the inside you may find that air tightness has improved significantly. This means that moisture accumulates in the air more than it did before, possibly leading to condensation on cold surfaces. There are always some places that are colder than others, usually where there is a route that bypasses the insulation (a thermal bridge). Some of these are easy to see – such as window sills that connect inside and out – and others aren’t – such as anywhere insulation hasn’t been fitted properly and is hidden behind plaster.

A catch-all way to avoid problems is to keep the humidity low – you should aim to keep it below 55%. Then even where there is a cold surface there won’t be enough moisture in the air to cause condensation. So you need ventilation – but too much ventilation is also a major heat loss. Ideally you should have ventilation that automatically adjusts to the humidity (see ‘How much ventilation do you need?’). You can also keep an eye on your humidity levels quite cheaply with a hygrometer. Mine is a TFA digital thermo-hygrometer and it costs about £16.

It is a good idea to take extra precautions when using internal insulation in kitchens or bathrooms where there is lots of hot water making the air steamy. In the first place you should make sure you have an effective extractor fan. Some experts recommend you should use a breathable insulation such as mineral wool or wood fibre. Also, some sorts of insulation buffer the moisture in the air by absorbing it when the air is humid and releasing it when the air is dry. Wood fibre based insulation is good at this. However, you should not expect the insulation to cope with a regularly steamy bathroom so you should have the extractor as well.

If you have cavity wall insulation installed it should come with a 25 year guarantee from the Cavity Wall Guarantee Agency (CIGA) If you have solid wall insulation installed through the Green Deal it should also come with a 25 year guarantee which includes ‘damage to structure and wall coverings including render and plaster and damage likely to risk health’.


In summary, to avoid all these damp problems:

  • Don’t fill cavity walls or insulate solid walls on the inside in areas prone to wind driven rain
  • Make sure your cavities are clear of snots before filling them
  • Make sure not to block ventilation in your loft and fit a breathable felt below the tiles
  • Keep an eye on your humidity and take action if it goes above 55%
  • For kitchens and bathrooms especially, consider humidity triggered extractors and/or breathable insulation.

And just in case:
  • Check that you have a 25 year guarantee that covers problems triggered by the insulation.

More information.
I have found these web pages and downloads helpful.

Is interstitial condensation a risk? (
Will cavity wall insulation prevent condensation? (yougen)
Does cavity wall insulation cause damp & condensation? (yougen)
Holes in insulation advice service, finds Which? Probe (Which? Magazine) March 2011
NHBC Guidance notes: condensation in roof spaces (pdf)
Masonry permeability moisture and internal insulation (bere;architects) Jan 2012
Stormdry damp cavity wall insulation (

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