How do I install an insulation vapour barrier?

This is a highly technical subject, but lack of an insulation vapour barrier can lead to serious damp problems. It applies particularly to internal insulation.If you want to skip the technical details just remember that with internal wall insulation it is essential to seal the room-facing surface of the insulation to stop water vapour getting into the back of the insulation or into the wall and causing damp. Any small amount of vapour which still manages to enter the wall will evaporate to the outside as brick is quite porous.


Why install an insulation vapour barrier?

Internal wall insulation

A vapour barrier reduces the risk of interstitial condensation

When you apply internal insulation to an outside wall the wall becomes colder (for the 11 months of non-summer in the UK). This creates a danger that warm water vapour from inside the home will get behind the insulation, and as it cools it can condense on and inside the wall fabric in the same way that you get condensation on the surface of single glazed windows. This invisible condensation inside a wall is called interstitial condensation.

To prevent this condensation from remaining in the wall it is essential that water vapour can pass out through the wall more easily than it can enter from inside. This is particularly important, to prevent wet or dry rot, in timber frame construction and where timber battens are attached to the wall to secure the insulation.

The better your insulation the colder the wall behind will be, so the more important it is to prevent excessive moisture entering it.

There is a school of thought which says that walls should be allowed to breathe as uninsulated walls do, particularly those built with lime cement and plaster, i.e. most homes built before 1914.

Modern Portland cement and render have very low permeability; most bricks are fairly permeable but different types vary quite a lot. This Breathing Construction is OK as long as the balance of being substantially more breathable outside than inside is maintained.

In practice when internally insulating an existing building the safe route is to install a vapour barrier on the inside, as you don’t know how permeable the walls are. If your house is rendered lime render is quite permeable but cement render (which came in after 1919 mostly) has very low permeability to water vapour, so in this case you should pay special attention to the vapour barrier on the inside, ensuring that there are no gaps in it .

If you still have interstitial condensation a last resort is to remove cement render and replace it with a breathing render of lime or some modern renders which are breathable.


Insulation vapour barrier instructions

If you use permeable insulation material such as sheep’s wool, rockwool or fibreglass, then on the warm side of the insulation and its accompanying timber frame you should install a vapour barrier, commonly a plastic sheet of sufficient density. Expanded polystyrene (EPS) sheets are semi-permeable, so for safety should also have one. See Wikipedia for a brief discussion.

Extruded polystyrene (XPS) has good vapour resistance, while most other insulation boards (polyurethane et al) have aluminium facings which make them impermeable. However, joints between insulation boards as well as joints between floor etc and the boards must be sealed.

I use Celotex or Kingspan insulation boards which are foil faced, and then I cover them with separate plasterboard. The insulation therefore forms the vapour barrier, but it is essential to seal all joins and gaps; I use Aluminium tape (sold for this purpose) to seal joints between boards as it sticks well to the clean new Aluminium foil on the boards.

However, its adhesion to old walls, floors etc. is suspect, so here I leave a deliberate 15mm gap which I fill with expanding polyurethane foam; the expanding foam is semi-permeable, but this is better than impermeable tape which might in time come off and leave gaps. A separate foam gun (£10 to 20 from Screwfix) is invaluable for this; the foam canisters for it are slightly different from the regular ones.

If you use the insulated plasterboard this should come with an Aluminium foil vapour barrier between the plasterboard and the insulation, but check a corner to make sure. The cheapest insulated plasterboard which uses expanded polystyrene does NOT have a vapour barrier, so don’t use it!

The skim coat of plaster will give some degree of vapour resistance at joints, but make sure that the joint between the bottom of the insulated plasterboard and the floor is sealed, as the plasterers bible seems to say that they must leave a gap there. I recommend asking the plasterer to leave a 20mm gap which one of you can then fill with expanding foam before the skirting board goes back on.


Allow any vapour to escape

You should ensure that the odd bit of vapour which still gets through can escape easily to the outside and not be trapped on the wall surface; this means removing anything impermeable, such as vinyl wallpaper or GLOSS paint from the inside of the plaster. It’s a good idea to remove all wallpaper as the paper could grow mould in the cold and somewhat damp conditions behind the insulation. Removing the plaster is optional.


Managing your builder

Many builders and plasterers have no understanding of vapour barriers, so supervision is down to you. If your contractor does not agree, contact the manufacturer of the insulation who should confirm what this article says. The really important thing is to ensure that the vapour barrier has no gaps in it and that it is secured to the side walls, floor and ceiling in a manner that is thorough and permanent.

Some builders want to seal the wall, i.e. the wrong side of the insulation; the wall must be able to conduct water vapour outwards, while stopping rain and leaks from coming in; brick does a pretty good job of this except in conditions of particularly intense and prolonged driving rain or major leaks.


Condensation in loft

Similar condensation can occur in insulated lofts which have non-permeable roofing felt under the slates and here you can go into the loft and see the condensation or the resulting white fluffy mould on the underside of the roofing felt. When you put insulation on the floor of the loft, the loft space gets colder and warm water vapour entering the loft can form condensation on the cold roofing felt. However if water vapour can pass out through the roof more easily than it can enter from below then there will not be a problem.

So good practice, since about 1980, is to use breathing roofing felt or fit ventilators in the roof, and on the warm side to draughtstrip the loft hatch and to seal holes where light cables and any pipes (in the cylinder cupboard) go into the attic. The ceiling below forms the vapour barrier which is usually adequate and the problem comes mostly from gaps and holes in the ceiling.

If your roof has no felt it will usually have sufficient ventilation round the slates to prevent any significant condensation, i.e. early morning condensation will soon evaporate. However it is a good idea to draughtproof the hatch and pipes and light fittings etc anyhow to reduce heat loss by excessive airflow. If you have modern central heating there may still be pipes from the old system left passing through the ceiling in the (former) cylinder cupboard through large holes.

Where external wall insulation is used the house wall is the vapour barrier (to a degree) and any condensation will be outside the structure of the house, in the insulation or render, so at least the house itself should be dry. Preferably the render used should be permeable to avoid this outer condensation.

© Martin Normanton, Walsall Ecohouse, Jan 2013.


Find out more – visit a refurbished home

You can find out more about external and internal wall insulation at green open house events in September. Speak to real homeowners as they share their personal experience of refurbishing their homes as part of SuperHome Open Days. SuperHomes are older homes refurbished by their owners for greater comfort, lower bills and far fewer carbon emissions – at least 60% less! Entry is free. Book now.

Also see:
Draught-proofing- a good thing?
interstitial condensation
internal wall insulation
insulating a solid wall

Further Information:
Visit Martin’s SuperHome for more.

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