Condensation barrier in loft extension


5 Apr 2010
Reaction score
United Kingdom
Standard pitched roof construction, slates nailed to pine sarking boards which are nailed to some fairly hefty (50mm x 250mm approx) rafters.

Roof space above existing ceiling is currently unused with loft insulation laid over the ceiling between the joists. It's a fairly cold and drafty space up there with inadequate space to stand except right at the apex of the roof.

We want to remove the ceiling of the room below and open up the full height to create a vaulted ceiling with velux windows.

Question is how to insulate the roof once the ceiling is removed and, critically, where to add the condensation barrier?

My guess would be to add insulation board (appox 100mm thick) between the rafters leaving an air gap for ventilation between the insulation board and sarking boards above. Then I'd add a waterproof membrane below the insulation board and then plasterboard below that.

Does this sound about right or have I created a condensation or moisture trap somewhere?


Sponsored Links
Typically to comply with Building Regs you'll typically need about 110mm Celotex between the rafters and a continuous layer of about 30mm beneath depending on rafter centres although these are variable depending on how much insulation you fit between and/or bellow ie it may be more cost effective to just buy a job lot of 70mm and have some between and some beneath for example if you get my gist. Under that you can use a foil backed plasterboard which works as your vapour barrier. A minimum 50mm ventilated gap between the insulation and underside of your sarking to be maintained although no problem achieving that with the size of your rafters, Apart from your eaves ventilation you will need continuous ridge vents to enable the ventilation to work.

Note the vapour barrier prevents moisture entering the insulation it will not prevent rain penetrating the ceiling if you get a leaky roof.
  • Thanks
Reactions: iep
Great advice, thanks. That all makes perfect sense to me. Didn't realise I'd need the continuous layer of insulation board but I don't think that will be too hard to accommodate. BTW, how do I attach the plasterboard if I go with the 70mm continuous insulation board? Presumably very long screws all the way through to the rafters?

I guess then that the biggest change to the roof will be having to install the ridge vents? Do I also need vents at the bottom of the roof (I would suspect yes)?

Understood that the moisture barrier will not protect against a leaky roof. TBH, I'd rather know if I have a leak so the timber doesn't go rotten without me knowing about it.


Yes long screws, this can be a bit of PITA, something you will need to assess depending on who's doing it and budgets etc etc. Especially awkward just holding the plasterboard in position before you fit the boards. As mentioned depends on how much hassle you can cope with. If getting someone to do this they probably won't be keen! There was a thread on here a while back where a DIYer did this though so it is possible.

Yes vents required at the eaves too but you probably already have some ventilation at the eaves.
Sponsored Links
Thanks again, that all makes perfect sense.

Sadly, this is a very old house with no (deliberate) eaves ventilation at all. Ventilation in the loft is achieved by having no membrane below the slates. It's a fairly breezy place up there.

However, to do this properly, I'll obviously need to add eaves and ridge vents between every pair of rafters. with approx 20 rafters along the length of the room this may be the unexpected cost that I had been looking for.


I might have missed something but if you're taking the ceiling out to open up the roof void, will there be any tendency for the rafters to spread. Building Control might want a word.
Hi Tony, every second joist will be left in (to mount uplighters on) with purlins at the base of the rafters to tie it all together and spread the load. Already run by a structural engineer and signed off.



thank goodness!;
(follow the previous post on insulation/ventilation and you won't go wrong).

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Sponsored Links