Author Topic: Dormer roof insulation  (Read 895 times)

Dormer roof insulation
« on: January 25, 2021, 06:43:50 pm »
In our house, the part that is two storeys is a big dormer bedroom and bathroom, built about 20 years ago.

It has about 50 mm of polyurethane insulation on the sloping walls, between the rafters, but it has been so badly fitted as to be next to useless.

Here is what the top edge typically looks like:-



The sheets of PU foam are loose. They were only held in by balancing them on a couple of nails each. There is a gap all the way down the sides of them. There are two sheets in each space, and some have moved so much that the top bits have fallen down over the bottom bits.

There was no attempt to join the tops of the PU foam to the fibreglass insulation that is on the ceiling. The difficult bits above the windows have no insulation at all, but don't feel any colder because the rest is virtually useless.

Where I can get to the tops of the foam sheets, they can't easily be lifted out because each rafter has a ceiling joist bolted to it, making the gap 50 mm narrower.

In one part of upstairs, there is a cupboard so that there is large enough vertical wall for a shower. In that cupboard there's no plasterboard. In there, I've been able to use expanding foam to seal the gaps and hold the sheets of foam in place, and all the drafts in here vanished when the foam was held in place.

The only access to most of this is from where I took the photo, in the tiny loft.

Any suggestions as to how I should improve the insulation? Would it be easier in the long run to just take the plasterboard off and do the insulation properly? It's one room where we haven't decorated yet.
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Mrs Pingu

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Re: Dormer roof insulation
« Reply #1 on: January 25, 2021, 07:03:15 pm »
Any suggestions as to how I should improve the insulation? Would it be easier in the long run to just take the plasterboard off and do the insulation properly? It's one room where we haven't decorated yet.

Being as it's 'modern' plasterboard and not lath and plaster I suspect the answer is yes if you want to do the best job.
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Re: Dormer roof insulation
« Reply #2 on: January 25, 2021, 08:40:21 pm »
Assuming insufficient headroom to add internal insulation then it sounds like ripping off the plasterboard and doing a proper job is the way to go.  Point noted about vapour barrier - you definitely don't want condensation forming behind the insulation. Why do some people just do a crap job in the first place eh?   It'd be a good opportunity to increase the insulation thickness too.

Re: Dormer roof insulation
« Reply #3 on: January 25, 2021, 08:48:55 pm »
Why do some people just do a crap job in the first place eh?
To meet contractual obligation with the least possible effort.

It'd be a good opportunity to increase the insulation thickness too.
Yes, I would probably join the insulation up across the ceiling in PU foam as well.
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MikeFromLFE

  • Previously known as Millimole
Re: Dormer roof insulation
« Reply #4 on: January 25, 2021, 10:39:36 pm »
A lot of years ago we got a contractor who quoted us to insulate the space between our kitchen ceiling and the sloping roof which is maybe 1m at its highest point.
They were going to blow loose insulating material into the space.
I never went ahead with it due to concerns about access to the recessed light fittings in the ceiling. .
I don't know if this type of approach meets current regulations.
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Re: Dormer roof insulation
« Reply #5 on: January 25, 2021, 10:52:14 pm »
A lot of years ago we got a contractor who quoted us to insulate the space between our kitchen ceiling and the sloping roof which is maybe 1m at its highest point.
They were going to blow loose insulating material into the space.
I never went ahead with it due to concerns about access to the recessed light fittings in the ceiling. .
I don't know if this type of approach meets current regulations.
Although I've got recessed light fittings, they are on the flat bit, and they are now LEDs so barely get warm. I would be quite happy to change them to surface ones if needed.

My concern on blown insulation would be that it could just fall out of the bottom and fill the eaves.
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Re: Dormer roof insulation
« Reply #6 on: January 27, 2021, 07:40:48 am »
My concern about 'sealing' the kingspan to the rafters would be condensation.

There will always be a condensation point in an exterior boundary. The question is; where?

Insulation can be designed so that the condensation point is in the thickness of the insulation. It might be designed so that it is on the exterior of the insulation (if that is the case, you need a vapour barrier between the room and inside of the insulation.

Regardless, what you don't want is condensation forming against rafters or joists.

The traditional solution to this was a draughty roof void - a vented uninsulated space - that worked well for preserving timber but was lousy for reducing heat loss.

If you were to rip off the plasterboard, then I think placing tyvek (tape the seams) on the inner side (i.e. just underneath the plasterboard) might be a good idea.

There are professionals available who design these solutions.
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Re: Dormer roof insulation
« Reply #7 on: January 27, 2021, 09:05:00 am »
Tyvek (at least the sort sold by the acre over on the west side of the Atlantic) is not a vapor barrier.  It's an air and water barrier.  Water vapor can pass through it.  Not all that quickly, but it can.  That's by design, so that if moisture does get past an interior vapor barrier (holes, laps, etc.) it has a way to keep on going, instead of just staying and soaking the interior of the wall.

Present building codes here require either an inch clear between the underside of the roof, and the top of the insulation, or require enough vapor-impermeable insulation in contact with the bottom of the roof that the dew point would be inside the thickness of that insulation.  In that case, that water vapor won't condense.  However, water in the vapor form will pass into whatever it can get to, and if the upper side is sealed, you want the lower side to not have a second vapor barrier, so that eventually everything can dry toward one side.

"Building Science" and "Envelope Specialists" have become very important in architectural practice over here.  We architects, having messed things up by tossing more and more insulation at our problems, can't be trusted to get things designed properly, let alone design things so that they can be built.

If you have some time, you might see if Joe Lstiburek has any advice for your situation.  I do think that removing the plasterboard, ensuring that the framing up there is dry, filling the cavity with vapor-impermeable (foamed in place, likely) insulation, and then replacing the ceiling, may be the best way forward.  Might be a summer project so that the framing can dry out.  Get a moisture meter and be sure the wood is in the 12% or so range before you re-insulate.

Re: Dormer roof insulation
« Reply #8 on: January 27, 2021, 11:31:27 am »
Thank you for the advice. It seems that the plasterboard will have to come off at some point.

I will be leaving an inch or two gap between any insulation and the roofing felt. There is plenty of ventilation at the eaves and into the roof space as long as a gap is open the whole way up. I would have left a gap like that anyhow.

If I do seal in polyurethane blocks, they would be sealed on the front face near the plasterboard, and fit between the blocks and the rafters won't be perfect, so there will be some ventilation for the sides of the rafters, as well as the top inch or two under the roofing felt.

Advice from someone off this forum was to add a vapour barrier, and I know about taping the joins, so that'll get done as well.

I'll see what Joe Lstiburek has to say. Now I've searched for his name, I see that there is certainly quite a lot of information that he has written.
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Re: Dormer roof insulation
« Reply #9 on: January 27, 2021, 12:10:56 pm »
Looking at your pic I think the insulation maybe a PIR possibly Cellotex - I have noticed similar curling in some sheets previously, but yours are badly fitted.

You should keep a  gap of at least 50mm between the insulation and the felt and it should be ventilated. (Kingspan and others do lots of free tech sheets /advice lines etc)
The best option is then to strip off the plasterboard and overboard with another layer of insulated plasterboard which includes a vapour barrier.
The rule normally is that the vapour barrier goes on the warm side of the insulation (heated side underneath the plasterboard.
You may consider giving the timbers a brush applied preservative (allowing it to dryout) before reinsulating.

Tyvek that I have used is a breather membrane and not a vapour barrier - usually seen on timber frames cavity side or in conjunction with timber cladding.

Re: Dormer roof insulation
« Reply #10 on: January 27, 2021, 03:22:40 pm »
Since we have some actual experts on the channel, could you tell me if running the silver-bubble-wrap stuff (and taping it) over the underside of the rafters/joists, before re-plasterboarding, is a suitable exercise?
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Re: Dormer roof insulation
« Reply #11 on: January 27, 2021, 08:12:57 pm »
if you remove the plasterboard, then when you replace, use foil backed plasterboard, this is a vapour barrier.

Aunt Maud

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Re: Dormer roof insulation
« Reply #12 on: February 01, 2021, 08:22:41 pm »
Normally the insulation goes up against the sarking on a pitched roof and on top of the vapour barrier on a flat roof to create a warm roof, otherwise you're creating a cold roof if there's a gap between the insulation and the sarking on a pitched roof or insulation and roof covering on a flat roof.

That's a condensation nightmare and as aeolus recommends it should be ventilated.

Modern, highly insulated roofs have no gap between the two and you can create a hybrid of PIR and rockwool if you wish.

If it was mine, I'd rip out the old and re-insulate it properly.

Re: Dormer roof insulation
« Reply #13 on: February 13, 2021, 06:43:47 am »
Since we have some actual experts on the channel, could you tell me if running the silver-bubble-wrap stuff (and taping it) over the underside of the rafters/joists, before re-plasterboarding, is a suitable exercise?
I won't claim actual expertise status, but the use of silver, reflective layers requires an airspace on one side.  Presumably the bubble thickness would create that.  However the reflective insulation products are better for dealing with "attic is too hot, and heat is transmitted to living space below" than "attic is cold, heat is leaving living space too fast".