Hi there!

I have a question about vapour barriers, energy efficiency and avoiding problems with interstitial or room side condensation. I would really appreciate any helpful comments regarding how to approach our project to get good value, design and low risk with regard to damp and good energy saving. I know that our situation is not ideal design wise because it is a conversion. I’m hoping to do the best in the circumstances.

Our barn footprint is approx. 22m by 11m. It’s being split into two homes, with bedrooms being created in the roof space, like a dormer bungalow, with velux windows.

The original wall construction is cavity wall 1980’s vintage, with reconstituted stone mock stone effect on the outside and breeze blocks on the inside. There are internal breeze block piers along the interior of outside wall at intervals for support ( no original internal walls for support ) Design is to be open plan interior.

So far we have had a new internal cavity wall dividing the two dwellings, new attic roof trusses installed with 22mm chipboard floor deck. Each bedroom has a velux triple glazed window. The roof has a permeable membrane felt, with reused concrete pantiles

Insulation: We were not keen to use cavity wall insulation because of possible damp problems sometimes reported. The plan is to use 100mm celotex to internally insulate the outside walls. The surface of the celotex would then be in line with the breeze block piers. We are then thinking of putting either foil backed plaster board or insulated plaster board over the breeze block piers and the celotex so that the wall appears smooth without piers sticking out into the room.

We will be insulating between the roof trusses with 150mm celotex equivalent wedged in.

We have discussed with the builder about whether having a complete continuous vapour barrier connecting the celotex in the rafters with the celotex lining the masonry walls is a good idea or not. Obviously the celotex has foil backing, but how important is it, for example, that the breeze block piers should have some kind of membrane over them which is then taped to the adjacent celeotex foil so the membrane is continuous from floor, around the walls to roof ridge?

One reason not to be obsessive about taping is that I am concerned about interstitial damp and room side damp. We do not plan to have a heat recovery unit, although I might consider individual extractor fans in bathroom etc to have heat recovery in them.

Also do you think the breeze block pier with less insulation than the adjacent wall might be problematic for interior condensation (thermal bridging) ? I know it’s not ideal but it’s a trade off of looks and less cost against any effects of thermal bridging.

I would like to know how some experienced people on this forum would approach vapour proof barriers etc in this instance. i.e. what needs to be membraned and taped or is that kind of fussiness more relevant to single skinned walls?

It would be really helpful if you can be specific as possible about what you would tape( if at all) and what membrane you would use to connect up to the celotex foil, and what extra insulation measure you might think wise. Somebody said that foil is better membrane than plastic as plastic can encourage condensation ?

Hi Daryl

Thanks for response.
Apologies for the ramblings of a newbie.

So if i get you right you support having a cloak of VCL membrane ( ie plastic sheet ?? ) being taped between any bits of foil on the celotex ? eg around the piers, eg to join the wall celotex and the rafter celotex at the wall plate area

In the rafters is the celotex taped to the wooden rafters or is just wedged in ok ?

Regarding plastic/polythene being prone to condensation, than say foil ...I am only quoting what my builder said to me; is this rubbish? ; if you cover stuff with plastic/polythene outside to protect from rain, like i did with some wooden planks, there is always moisture underneath ( from condensation) The polythene seems to attract water condensation. perhaps this is what he means.

Where you say "I'm unsure of the connection between taping obsessiveness and interstitial damp tho'...? "

As i tried to allude to in my post, I wondered of the reported problem of interstitial damp (see gbezine.greenbuilding.co.uk/the-risks-of-retrofit ) is probably more relevant to single skinned walls where moisture ingress from driving rain outside means that more damp is coming in from the outside and has no where to go because of the impermable VCL because someone has obsesively closed every gap and is no longer breathable in the old fashioned sense. I guess i would like confirmation that interstitial condensation is not likely to be a problem with internal insulation in a cavity wall situation.

Posted By: kayseraseraOur barn footprint is approx. 22m by 11m. It’s being split into two homes, with bedrooms being created in the roof space, like a dormer bungalow, with velux windows.

The original wall construction is cavity wall 1980’s vintage, with reconstituted stone mock stone effect on the outside and breeze blocks on the inside. There are internal breeze block piers along the interior of outside wall at intervals for support

Hello, and welcome to the forum.

I'm curious why a barn would have a cavity wall?

One reason not to be obsessive about taping is that I am concerned about interstitial damp and room side damp.

You're aware that internal insulation can cause condensation problems just as much, probably more so, than cavity wall insulation? If you leave gaps in the taping you're guaranteed to have condensation on the inside of your blockwork.

Also do you think the breeze block pier with less insulation than the adjacent wall might be problematic for interior condensation (thermal bridging) ?

I would suppose so.

Who is designing the project? It seems to me that you would be well advised to seek professional advice, from an architect or similar. I'm also curious what your goals are, in terms of insulation and comfort, that led you to join this forum?

Posted By: kayseraseraI don't know that cavity wall can be more problematic than internal insulation; is that your personal experience ? I've not heard that generalisation before is there anything viewable online that illustates this so i check it out?

I don't think cavity wall can be more problematic than internal insulation, I think and said the reverse. I had cavity fill on my previous property without any problem. There's no way I personally would consider buying a property with the intention of using internal insulation. I expect there are articles online but I'm afraid I can't point to one myself, perhaps somebody else can.

On the celotex website it says small amount of vapour getting through VCL will pass through the block layer. It's breeze block therefore more permeable than regular concrete block ? the breeze block will be warmer than a single skin wall so I am asuming that condensation is not as likely as if it were single skinned, and probably a very wet wall.

If the gaps aren't sealed then (a) all the vapour that gets through will be concentrated at those gaps and (b) there will also be air movement, resulting in a lot more vapour than you might expect. Remember that breeze block is not airtight. The outside of the breeze block won't be a lot warmer than the outside if it is facing into an uninsulated ventilated cavity. You shouldn't be assuming condensation or not; your architect should be running a calculation to predict it for you.

You don't mention any mechanical ventilation to help keep the RH levels down. Are you planning to not have any?