Insulating Sloping Room Edges

[woodpeckerx]

All my upstairs rooms have sloping edges, or vaulted edges if that is the right term, checked with a FLIR and the edges are cold.

Checking the loft there is no insulation at all on the slopes, just single plasterboard. I've got enough room to get in a 50mm PIR board and leave 20mm clear above it for ventilation.

Armed with some 50mm board, saw and solvent-free grip filler, decided to try a couple, nightmare to get right into the corner. Due to the way the joists are bolted to the rafters the board has to be fitted in 2 pieces, first bit slid down then across behind the end of the joist etc

Is there an easier way to do this?

Certainly looks better on the FLIR but took all evening just to do 3 rafters (2 in pic below) and got about 60 to do

 

[robinbanks]

Possibly one of those situations where wool is easier because it can be ripped into smaller bits and stuffed in- it's "self healing" and having expanded slightly its better at gap filling than rigid board, which requires careful application of expanding foam.

If board shrinks or there's a gap in the foaming and the wind gets to the plasterboard side then the insulation is relatively useless; a problem that is less evident with wool. In some areas, despite being lower performance thermally, wool's deformability means it works as well.

If the compartment being insulated is fairly well sealed then tipping or blowing a loose fill insulation in could be an option there are other ways of ventilating the loft if you close off the eaves

More expensive but also an option is to put insulated plasterboard on the room side of the slope then skim and decorate it

 

[Diver Fred]

More expensive but also an option is to put insulated plasterboard on the room side of the slope then skim and decorate it

That's the way I'd go.

 

[woodpeckerx]

Thanks for the replies.

Possibly one of those situations where wool is easier because it can be ripped into smaller bits and stuffed in

I was worried wool would fluff up blocking the eaves ventilation, is there a product that doesn't fluff up? I only have 70mm between the plasterboard and sarking.

If board shrinks or there's a gap in the foaming and the wind gets to the plasterboard side then the insulation is relatively useless

I can understand that, no way I can get my foam gun in to try and seal any gaps either.

More expensive but also an option is to put insulated plasterboard on the room side of the slope

I would rather try and do it from the loft space, already got too many other "decorated" rooms taken apart

 

[cdbe]

I would use 50mm cavity batts - probably the best compromise between solid boards and "fluffy" mineral wool. Should be rigid enough to push and stay in place without foam.

 

[^woody^]

Does an air gap need to be maintained. Is there eaves and ridge ventilation? Otherwise fill the gaps with insulation, or fill and wedge open the felt laps further up the roof.

You need 50mm of rigid insulation such as Celotex to maintain an air gap and give meaningful insulation. But what that tends to do is create cold bridges of the rafters and you get lines of condensation stains. So, the best thing is to insulate between the rafters plus a layer below as thick as you can.

 

[woodpeckerx]

I would use 50mm cavity batts

Thanks, did see those but having never had my hands on one wasn't sure if they would fluff up.

 

[woodpeckerx]

Does an air gap need to be maintained. Is there eaves and ridge ventilation?

Vented soffit at eaves, no sign of venting at the ridge, house built circa 1960, did they ridge vent back then?

But what that tends to do is create cold bridges of the rafters and you get lines of condensation stains. So, the best thing is to insulate between the rafters plus a layer below as thick as you can.

Sorry you've lost me there, the section of rafter (about 450mm long) I'm insulating is what would have been the joist if the roof were flat to the edges, I can't get insulation below it as it has the angled plasterboard nailed to it.

 

[woodpeckerx]

I just had another idea, how about rockwool with hollow polycarbonite sheet wedged between the rockwool and the sarking, I found some 16mm sheet on Wickes that looks like it would let the air through above the rockwool?

 

[^woody^]

I can't get insulation below it as it has the angled plasterboard nailed to it

Yes, and that needs to come off or insulated over then reboarded.

 

[woodpeckerx]

Yes, and that needs to come off or insulated over then reboarded.

I see what you mean but I might have to just put up with the end of the rafters being a cold bridge for now.

What do you think of my idea of using rockwool with hollow polycarbonate sheet wedged above it, I've found they do 10mm one as well, thinking I could use 100mm rockwool with 10mm hollow sheet wedged on top, would compress the rockwool a bit but should fluff into the gaps?

Trying to do the best I can without ripping too much apart and it not taking me forever to get some improvement...

 

[^woody^]

The problem with Rockwool or any quilt in that location is that it will be subject to cold air movement not just above, but within it, so it may not perform very well, or as well as expected.

If you used cavity slabs as mentioned by cbde above, then that would be easier and no need for polycarbonate.

 

[woodpeckerx]

The problem with Rockwool or any quilt in that location is that it will be subject to cold air movement not just above

Thanks Woody, the comments have all been really helpful but has led me to a couple of questions.

I'm think to the best way to tackle this properly might be as @robinbanks and you suggested to insulate the room inside with an insulated plasterboard.

Can I overboard the existing angled board in the room with an insulated plasterboard?
If so how should I fix it to the existing plasterboard, adhesive or screw it on?
The board seems to come as 27mm and 55mm, would it be best to go 27mm and insulate the loft side as well?

 

[robinbanks]

I've always pondered this "got to have the loft ventilated with massive open holes at the eaves/ridge - every loft I've ever been in, felted or not has had a breeze blowing through it from everywhere, not just the eaves; I've never come across one where the laps of felt have been hermetically sealed together and to all the edges such that they make the loft space anywhere approaching a draught proof still-air space.

Then compare to the living area of the house where gaps are routinely sealed up and a few tiny tricklevents in window frames are deemed adequate to dispose of human generated moist air with some low ACH achievement, yet the loft space that has a bit of (moisture breathable, note) felt and tiles thrown on it is somehow airtight without the gale of open eaves blowing through it..

I'd fill those eaves pockets with insulation and vent elsewhere..

 

[robinbanks]

Can I overboard the existing angled board in the room with an insulated plasterboard?
If so how should I fix it to the existing plasterboard, adhesive or screw it on?
The board seems to come as 27mm and 55mm, would it be best to go 27mm and insulate the loft side as well?

Yea, you can, it makes a mess to rip the existing down but gives access to add further insulation - yoir call as to how much pain you want to cause yourself

I personally would use a mechanical fix (screw) rather than try to bond insulated plasterboard to wallpaper/paint/skim/whatever is on the existing ceiling

Be aware that fixings should really penetrate into the rafters by at least 25mm, so you might need longer than 75mm plasterboard screws if you're using 50mm insulated board screwed over the existing ~15mm boards.

Debate goes on as to whether to get boards with PIR bonded or buy PIR sheets separate. In some BMs the bonded sheets are considerably more expensive in others they're on par. If buying separate, choose a plasterboard thickness that suits your rafter spacing (eg 15mm board for 600 rafters) because the non bonded sheet lends no structural rigidity to the board. Bonded boards can have thinner plasterboard