Insulating Sloping Room Edges

[^woody^]

If you are minded to insulate from below, then you are better off fixing Celotex (50mm or more if you can) and then plasterboard. This is more efficient and cheaper than the equivalent thickness of insulated PB.

You can overboard. Fix with long enough black phosphate plasterboard screws max 200mm spacing, you only need a couple of screws to hold the Celotex first before PB. Foil tape all insulation joints.

Yes, if you can slide some insulation down the slope in the loft it would help, but may not necessarily be essential. It depends on what you want to achieve, how much you want to spend and how much messing about you want to do

 

[robinbanks]

This is more efficient and cheaper than the equivalent thickness of insulated PB.

I think that might depend on the builders merchant the materials are coming from. In the one I use for price compare 40mm PIR sheet + 12.5mm plasterboard is only 30pence/sqm cheaper than a 12.5/40mm bonded board, and when you consider the bonded board being faster to fit..

..but go to wickes and the bonded board 50% more and it's polystyrene too, so thermally much worse.

Definitely pays to shop around!

 

[woodpeckerx]

Definitely pays to shop around!

@robinbanks I'm just buying from Wickes and B&Q etc, I know its not the cheapest and I buy the smaller bits as I don't have a van.

What I've decided to do as Mrs wants insulation both sides is carry on with my 50mm PIR from loft side then I'll go 25mm PIR on the room side and board over with tapered edge board, tape joints and whack in some easifill 60.

The master bedroom and ensuite is an extension with a different roof construction, its got a purlin about 12 inches tall near where the vaulted edge starts so can't get anything down past it, for these rooms I'm ripping off the old board and same 50mm PIR between rafters then will 25mm PIR and reboard room side. Taken some off already to check I can do it, looks doable but I'll be busy for weeks at my pace

Another couple of questions, at the angled board edges should I use angled board tape? and should I use 9.5mm or 12.5mm board?

 

[^woody^]

I think that might depend on the builders merchant the materials are coming from. In the one I use for price compare 40mm PIR sheet + 12.5mm plasterboard is only 30pence/sqm cheaper than a 12.5/40mm bonded board, and when you consider the bonded board being faster to fit..

..but go to wickes and the bonded board 50% more and it's polystyrene too, so thermally much worse.

Definitely pays to shop around!

Foil PIR such as Celotex is almost twice as efficient as the other types of foam insulation including jablite or styrofoam typically bonded to plasterboard

 

[JohnD]

If you're going to fit insulated plasterboard, you may as well pull down the old ceiling, then you have easy access to insulate the void and can do an all-round better job.

It will be a bit messy and dirty.

 

[robinbanks]

Foil PIR such as Celotex is almost twice as efficient as the other types of foam insulation including jablite or styrofoam typically bonded to plasterboard

I was doing a like for like Xtratherm PIR (0.022 W/mK) bonded board, but I get your drift

 

[robinbanks]

I know its not the cheapest

You can probably buy a roof rack for the premium Wickes shaft you on a couple of sheets of insulation compared to specialists or friendly local free-delivery builder merchants. Have a ring around and ask for prices and delivery. Bulky stuff like sheet materials defo better off on a Hiab lorry

Could also ask on here for BM recommendations in Beds

 

[robinbanks]

is carry on with my 50mm PIR from loft side

But, I thought this whole exercise was to save the aeons of time it will take to do 60 bays of that. Note the earlier advisory on installing rigid board in ventilated hard-to-reach places; you might as well stand the board up in the garden if a misalignment means the wind can get behind it and cool the plasterboard anyway

If you're over boarding no matter what, then I'd either use a looser fill/woolly type if you're leaving the existing PB up or celotex between the rafters if you're taking it down

 

[robinbanks]

should I use 9.5mm or 12.5mm board?

What is your rafter spacing and are you using bonded boards or separate sheets?