Insulation for pitched roof

aFlabbyPanda

We've a dormer build in the late 80s and we are trying to improve the insulation/heating. The upstairs rooms are much colder than the rest of the house and in particular the west facing attic has a constant breeze blowing through it.

I was going to install insulation board in the attic but I was unsure if I need to leave a 50mm gap against the felt or can I push the board flush against it?

Pic of the pitched attic attached.

pmcb

I would recommend Icynene spray foam insulation
https://www.icynene.ie/attic-insulation/ & thermal board (37mm)

listermint

aFlabbyPanda wrote: »

We've a dormer build in the late 80s and we are trying to improve the insulation/heating. The upstairs rooms are much colder than the rest of the house and in particular the west facing attic has a constant breeze blowing through it.

I was going to install insulation board in the attic but I was unsure if I need to leave a 50mm gap against the felt or can I push the board flush against it?

Pic of the pitched attic attached.

You need the gap . The attic has a breeze because it's designed they way. It removes moisture from the area preventing rot on your roof rafters and purlons.

Your issue is insulation on the ceiling it's not rafter insulation. You could concentrate efforts on the ceiling joists.

aFlabbyPanda

I'm waiting on a quote for the foam spray but I don't think I can afford it right now.

The ceiling joists already have the wool/fibre glass insulation in them, are you saying this should be replaced? I'm no expert but it looks OK and seems to be laid to a sufficient depth (its covers the joints etc). The downstairs is fine but the issue is the rooms upstairs (in the dormer section). I've already replaced the attic hatch doors so there is no draft in the rooms but the heat just seems to disappear quickly. There is no access to the attic above the dormer section unless we start removing the slates from the outside.

The east attic is warm and has no wind by comparison. I get the point about ventilation but you can feel the wind on your face in the west attic (and when compared to other homes we've owned just seems excessive).

The Continental Op

Hasn't any air movement been already lost by having the insulation all the way over the eves?

I'd pull back the current insulation put trays at the end then use a product like Xtratherm Rafterloc Insulation between the rafters. That assumes there is enough room to leave a 50mm air gap under the sarking. If there isn't enough depth to the rafters to do that then I'd consider making the rafters thicker by adding some battens down them but you could alway not push the Rafterloc Insulation up tight but that may not work very well.

MicktheMan

aFlabbyPanda wrote: »

We've a dormer build in the late 80s and we are trying to improve the insulation/heating. The upstairs rooms are much colder than the rest of the house and in particular the west facing attic has a constant breeze blowing through it.

I was going to install insulation board in the attic but I was unsure if I need to leave a 50mm gap against the felt or can I push the board flush against it?

Pic of the pitched attic attached.

Dormers are notorious for heat loss by air leakage. In all likelihood, insulating the pitched roof will do nothing for this because it is outside the thermal envelope and also insulation on its own does not deal with air leakage.
In general you need to airtighten on the warm side of the insulation thermal envelope.

aFlabbyPanda

The Continental Op wrote: »

Hasn't any air movement been already lost by having the insulation all the way over the eves?

I'd pull back the current insulation put trays at the end then use a product like Xtratherm Rafterloc Insulation between the rafters. That assumes there is enough room to leave a 50mm air gap under the sarking. If there isn't enough depth to the rafters to do that then I'd consider making the rafters thicker by adding some battens down them but you could alway not push the Rafterloc Insulation up tight but that may not work very well.

The current rafters are 100mm so I was either going to use 50mm PIR or extend the rafters and use 100mm to satisfay the 50mm gap.

The Continental Op

aFlabbyPanda wrote: »

The current rafters are 100mm so I was either going to use 50mm PIR or extend the rafters and use 100mm to satisfay the 50mm gap.

If you are I'd definitely consider one of the products thats cut specially so it can be compressed to fit between rafters it make the whole job much much easier.

Calahonda52

aFlabbyPanda wrote: »

The current rafters are 100mm so I was either going to use 50mm PIR or extend the rafters and use 100mm to satisfay the 50mm gap.

Not clear what you have here, is it like either of the attached

aFlabbyPanda

Calahonda52 wrote: »

Not clear what you have here, is it like either of the attached

Mine is the second one, the image I posted is the attic behind the kneel wall (if that's the correct name)

The joists above the ground floor are insulated as well as the vertical walls upstairs.

MicktheMan

aFlabbyPanda wrote: »

Mine is the second one, the image I posted is the attic behind the kneel wall (if that's the correct name)

The joists above the ground floor are insulated as well as the vertical walls upstairs.

in that case:

MicktheMan wrote: »

Dormers are notorious for heat loss by air leakage. In all likelihood, insulating the pitched roof will do nothing for this because it is outside the thermal envelope and also insulation on its own does not deal with air leakage.
In general you need to airtighten on the warm side of the insulation thermal envelope.

Irish Steve

If your dormer is anything like ours, which was late 80's build, and built by the people that trained the celtic cowboys, there are plenty of issues with draughts in your dormer.

Are the vertical walls upstairs insulated? When the downstairs ceilings were insulated, were the gaps between the upstairs floor and the downstairs ceiling filled with something like expanded polysyrene to stop the draught blowing through that area, as I can almost guarantee that there's no insulation under the upstairs floor, so the heat loss there will be only massive, even with a relatively light breeze blowing.

There will also be an issue with the section above the upper ceiling, that will need insulation, and if you have any downlighters in the ceilings, they will need to be dealt with by either replacing them with a more modern LED light that will seal the hole, or putting a heat seal sock over the light above the ceiling. A long time ago, the only solution back then was to get hold of some catering size tins after they were empty, and put them over the light, with a small nick in the edge for the wiring, and then put the transformer on top of the tin to keep it from moving, and that made a huge difference to the draught below.

If there are any sloping ceilings in the dormer section, they will need some attention, and it could be tricky getting the insulation in.

Any access hatches into the outer crawl spaces have to be draught proofed, as do any holes through the walls for radiator pipes.

aFlabbyPanda

You're spot on with the floor cavity not having insulation, but the walls are insulated. We did seal the floor and skirting when laying the new floor. No down lighters upstairs and the hatch was sealed already. There is some insulation that we can see in the pitched ceiling upstairs.

I'll start by closing off the space under the floor but anything else probably isn't an option for now.

Calahonda52

Sealing the space under the floor needs to be done properly, I have posted relatively recently on my preferred method.

https://www.boards.ie/vbulletin/showthread.php?p=112432397
is one place

The eaves will need ventilation baffles and you can deal with section 1 in my sketch by either taking off the slates or adding more inside the room, bearing in mind that additional insulation on existing plasterboard is not recommended due to the existing being on the cold side = condensation risk = mould risk.

Have you access to section 2, the top piece?

aFlabbyPanda

OK that now is more along the lines I had in my head. Something like below was what I working towards.