Best 50mm insulation material

[ZACCCC]

Take down all the PB on the sloping part.
Insert 50mm insulation between joists.
Then re-board with insulation backed PB (30 or 40mm insulation) and long enough screws to screw into joists. 30mm insulation + 12.5mm PB, use 60-70mm screws.
Use scrim tape along edges. Fill edges and screw heads.
Where new PB meets wall and exisitng ceiling, you'll need to cut it at an angle.

Thanks but I don't really understand what you mean. You say 'take down all the PB on the sloping part' and then 'insert 50mm between joists'. But if I've taken the PB down, what's the 50mm insulation going to rest on? Or am I missing something here?

 

[freddiemercurystwin]

In an ideal world the insulation will be cut just right so that it will friction fit into the right place and stay there under its own steam, at least until you board it. Any gaps being filled with expanding foam. You can also tap a few panel pins into the bottom of the rafters and tap them up to hold your boards in place, or a bit of expanding foam in the gaps will hold there long enough for you to board it.

 

[ZACCCC]

In an ideal world the insulation will be cut just right so that it will friction fit into the right place and stay there under its own steam, at least until you board it. Any gaps being filled with expanding foam. You can also tap a few panel pins into the bottom of the rafters and tap them up to hold your boards in place, or a bit of expanding foam in the gaps will hold there long enough for you to board it.

Thanks for that - I get it now. But rather than take down the existing ceiling and PB I think I'll go with the earlier suggestion ie. put insulation in through the section I've already cut out of the ceiling and then put up a new ceiling of insulation backed PB under the existing ceiling. Does that sound reasonable?

 

[RogueHanger]

Ensure your screws are long enough to give at least 20-25mm in the timber joists. The pros might suggest a little more.

Also ensure that the edges of your new insulation lap the edges of your old, so there's no gaps.
To get the right angle on the edges I use an old circular saw with the right angle set. I don't suppose it does the blade any good.
It's also worth wearing a dust mask.

 

[ZACCCC]

Ensure your screws are long enough to give at least 20-25mm in the timber joists. The pros might suggest a little more.

Also ensure that the edges of your new insulation lap the edges of your old, so there's no gaps.
To get the right angle on the edges I use an old circular saw with the right angle set. I don't suppose it does the blade any good.
It's also worth wearing a dust mask.

Thanks for that advice. Mind, with my DIY skills I don't think the finessing of the angles at the edges will be my main worry!

 

[seasickstevie]

If your DIY skills lack finesse, you've got no chance of sliding the PIR down or up through the end aperture like you suggest, with anything like an acceptable tolerance. you'll have gaps all over the place.

 

[ZACCCC]

If your DIY skills lack finesse, you've got no chance of sliding the PIR down or up through the end aperture like you suggest, with anything like an acceptable tolerance. you'll have gaps all over the place.

What I meant was that as I've never put up any PB before, the angles at the edges might well be not the best. But I don't see why I can't cut some PIR insulation to the width of my joists and slide it in - how difficult is that? As far as I'm concerned that's the easy bit.

 

[seasickstevie]

Let's know how you get on ...
Don't forget, the last thing you want when insulating for reasons of heat loss, is air gaps.

 

[ban-all-sheds]

You even more don't want them for reasons of condensation and mould and rot on the roof timbers.

 

[ZACCCC]

The insulation boards I've ordered are Xtratherm (I only need two). Should I be putting a vapour barrier underneath them? I'm hoping that that's not necessary because it would mean I'd have to take the whole ceiling down, whereas all I need to do now is slide the boards down between the joists, working from the loft.

 

[ban-all-sheds]

Should I be putting a vapour barrier underneath them?

Yes.

The insulation will stop the roof from getting warm, i.e. it will stay cold. Any warm, moist air (quite a lot from a bedroom) which gets into the cold space will condense.

I'm hoping that that's not necessary because it would mean I'd have to take the whole ceiling down

 

[jeds]

Should I be putting a vapour barrier underneath them?

No. As long as the 50mm above the insulation is ventilated - as you've previously described.

 

[ZACCCC]

Conflicting advice there but thanks for the input fellahs.

Bits that I've read on the internet and things that I've seen on youtube don't give me a clear answer either, but I think I'm going to just put the insulation boards in and patch the ceiling up and get it replastered. From what I've read, the foil on the underside of the boards is impermeable to condensation and in any case I've read that the plasterboard and skim also acts as a vapour barrier.

 

[noseall]

Only foil backed plasterboard is an effective V.C.L.

You would be foolish not to incorporate a vcl at this stage for the effort it takes.

 

[ZACCCC]

Only foil backed plasterboard is an effective V.C.L.

You would be foolish not to incorporate a vcl at this stage for the effort it takes.

Noseall, thanks for that.

Since the insulation boards are only 50mm and the recommended depth for full insulation is much more, I was thinking of putting up another ceiling under the present one using insulation backed PB. I presume that that this would have its own VCL as you call it. Do you know if that's the case?