What is interstitial condensation and how can you prevent it?

woman checking walls for damp
Interstitial condensation can be causing damage to your home without you even realising it (Image credit: Getty Images)

Interstitial condensation may well be something you haven't heard of before, yet it is a problem that affects many homes and can, if left unchecked, cause damage to the structure of a building. 

While you may well have read up on how to stop condensation, where interstitial condensation differs from other types of condensation is the way in which it often remains hidden from the view of homeowners, lurking unseen until it is too late to prevent it from causing costly issues. 

In this guide, we explain what interstitial condensation is, how to spot it before it begins to cause problems for you, how to prevent it and the ways in which you can put it right.

What is interstitial condensation?

Interstitial condensation is quite different from 'surface condensation' which is a far more obvious issue, causing droplets of water to appear on the surfaces of your home — window condensation is a good example of surface condensation. 

“Surface condensation happens when the humidity in your home changes — such as when cooking or taking a shower," explains Ian Kernaghan, head of product, design and development at Eurocell. Think of the condensation that forms on your toilet cistern, mirror or windows — all of these are good examples.

"Interstitial condensation, on the other hand, penetrates deep into your walls, floors and roof and can seriously damage your property," continues Ian. "It happens when warm air vapour travels through your walls or surfaces and cools down inside it, depositing moisture." 

condensation inside window

Unlike surface condensation, as can be seen on this window, interstitial condensation is hard to spot.  (Image credit: Getty Images)

What causes interstitial condensation?

Let's begin by asking 'what is condensation?' In short, it is useful to know that condensation occurs when warm, moist air comes into contact with a surface that is colder than it is — such as the inside of a roof or the glass in a window. When the temperature of this air suddenly drops, it releases the water it is holding in the form of water droplets — or condensation. 

"The word ‘interstitial’ means small places, so the condensation occurs in floors, walls and roof spaces, out of view," explains Ian Kernaghan. When warm, moist air within the house moves into a wall, floor or ceiling and condenses within it, this is interstitial condensation.

The causes of interstitial condensation are fairly similar to the causes of other types of condensation — too much humidity, poor ventilation, fluctuations in air temperature and inadequate insulation. 

"Modern buildings, because of their increased air tightness efficiency, may be more prone to condensation, but being aware of it and ensuring proper ventilation means you can benefit from the better energy efficiency of modern building materials without risking moisture build-up," advises Ian. 

Why is interstitial condensation a problem?

Just as with other types of condensation, there are several problems that can arise if interstitial condensation is not dealt with.

“This type of condensation can lead to structural damage, mould growth, staining and discolouration to your interior walls and even your furniture," warns Ian. "It can also lead to poor air quality which can exacerbate respiratory health conditions like asthma."

Spotting interstitial condensation

“Interstitial condensation, in particular, is difficult to diagnose because you can’t see it happening," says Ian. "Once you see the signs, the damage might already be done, which can result in costly repairs." 

So just how can you tell if this is a problem affecting your home? Sadly, most of the signs to look out for will mean that some degree of damage has already been done. 

Be on the lookout for damaged areas of external masonry in cold weather in particular. This indicates a problem as it will mean that water droplets formed by this type of  condensation have frozen, expanded and then caused the masonry to blow.  

Check for condensation in loft spaces too. Increased surface condensation – usually on the underside of roof felt, ceilings and inside walls – can also hint at a deeper problem. 

loft insulation

If you spot condensation on the underside of roof felt, or inside walls, it could indicate interstitial condensation is lying somewhere within.  (Image credit: Getty Images)

How can you prevent interstitial condensation?

Thankfully, there are several steps you can take to prevent interstitial condensation in your home — and if you can take measures to stop the problem from ever occurring you will certainly avoid costly repairs later down the line.

"To prevent it you need to control the moisture levels in your home," explains Ian. "You can install vapour barriers to stop moisture from reaching cold surfaces inside your walls or roof and ensure all parts of the building are well-ventilated.”

You can also use similar preventative measures to those used to deal with surface condensation, such as ensuring good ventilation and keeping the temperature within your home stable. If you are struggling to regulate the humidity in your home, it will be well worth investing in one of the best dehumidifiers you can get your hands on. 

Ian Kernaghan
Ian Kernaghan

Ian Kernaghan is head of product, design and development at Eurocell and has been in the window industry for more than 35 years.


What causes condensation on the inside of roof felt?

Many people become very concerned when they find that the underside of their roof felt is wet, but the causes of this are generally the same as for any other kind of surface condensation. 

Condensation in roof spaces occurs in cases where there is either too much warm air travelling upwards from living spaces, or when there is poor roofing ventilation in place. Again, this tends to be more common a problem in modern houses, with better thermal performance as the roof spaces are usually colder. 

In most cases this is only a temporary issue and will resolve itself once the weather warms up, but if it is allowed to build up, it could potentially cause structural damage to the roof. 

Once again, good ventilation is key to preventing the issue. Ensuring the ceiling is well sealed is also important as it will mean the warm air won't be able to get up there in the first place — insulating your loft hatch and checking seals around light fittings can help. 

Natasha Brinsmead

Natasha is Homebuilding & Renovating’s Associate Content Editor and has been a member of the team for over two decades. An experienced journalist and renovation expert, she has written for a number of homes titles. Over the years Natasha has renovated and carried out a side extension to a Victorian terrace. She is currently living in the rural Edwardian cottage she renovated and extended on a largely DIY basis, living on site for the duration of the project. She is now looking for her next project — something which is proving far harder than she thought it would be.