Celotex and condensation in a loft

[RobKett]

Hi all, first post here and I can't find any other threads so apologies if this has been done to death.

I want to put celotex in our loft to replace a very thin ancient layer of rockwool. Then I want to put chipboard boards over the central part, leaving the edges (where I won't be walking) with just loose bits of wood to store things on.

The beams are exactly 750mm deep - the question is, can I put 750mm celotex straight in there so there will be a sandwich of plasterboard (the ceiling - celotex - loft board) or do I need to leave any ventilation gap between the celotex and the loftboard? I can't see why I should but I know it's easy to overlook condensation issues so thought I'd ask (can't find guidance online anywhere).

In the longer run I want to take the boards up, put cross battens down and do a cross layer of celotex to cut out cold-bridging from the beams and generally thicken the insulation, then put the boards back down, and I have assumed that this would not have a gap between the two layers of celotex but again I can't find this confirmed anywhere.

Be grateful for any advice.

 

[JohnD]

The beams are exactly 750mm deep

surely not

 

[freddiemercurystwin]

No gaps required, sandwich away.

That said, the majority of posters in this situation will shove a layer of rockwool between the joists, stick some 100x38 joists on top of your existing (perpendicular) and then shove another layer of rockwool in between. Celotex is good but also expensive. Then chippy on top.

 

[RobKett]

The beams are exactly 750mm deep

surely not

75mm!

 

[RobKett]

No gaps required, sandwich away.

That said, the majority of posters in this situation will shove a layer of rockwool between the joists, stick some 100x38 joists on top of your existing (perpendicular) and then shove another layer of rockwool in between. Celotex is good but also expensive. Then chippy on top.

Thanks, I couldn't think of a reason why a gap would be needed but I'm perfectly capable of overlooking the bleeding obvious. True that Celotex is expensive but it's quite a small house so it's not mad expensive overall and the void is quite small (there are sloping eaves in the first floor rooms) so space saving is worth it. Also, I might come back to this and put another layer in (when I've got more money) so saving height matters a bit.

 

[RobKett]

However, the loft void above the ceiling should be correctly ventilated and it is a requirement to have a continuous vapour control layer on the warm side of the insulation.

Hope that helps! Any more questions feel free to contact the Celotex Technical Department - 01473 820850 or [email protected]

Thanks for this - do you mean that I need to put down a continuous VCL underneath the celotex? Since I am only filling in between the joists (rather than cross-battening and putting another layer on top of them too) - the top of the joists will actually be in the cold, it will only be the sides of the joists (and obviously the ceiling too) which will be warm. This makes it sound to me like I ought to just have a strip of VCL under the celotex and up the sides of the joists, and leave the top of the joists clear. Does that sound right?