Rock wool AND Celotex?

[Aerate]

Time to insulate the loft on our extension. I’m thinking 150mm Rock wool between the joists, boarding over the joists with 25mm celotex, 18mm OSB above the celotex and then filling up the remaining space with the general family detritus. Am I going to hit the correct u value? Am I going to have some unexpected side effect of combining the two types of insulation? CMTMB

[skeeterm5]

Don’t the plans specify it?

[Aerate]

skeeterm5 said:

Don’t the plans specify it?

If they did - I probably wouldn’t be asking?

Long story, but I have free reign.

[Gtom]

PIR roughly (very) is half the thickness compared to mineral wool for the the same u value.

Your plan puts you at around 200mm of wool, you need somewhere between 300-350mm.

Can’t you put PIR between as well as over the top?

Also, ideally you need to put some 11mm osb or such like over the joists and then more PIR when boarding for storage, to stop the PIR compressing on the point load of the joists.

[Aluminati]

270 mm of loft roll will give you .16

Not sure what your proposed will give.

We did an estate a few years ago, and the 2 properties that had condensation issues were the ones that had filled their lofts with crap and affecting airflow.

[skeeterm5]

Aerate said:

skeeterm5 said:

Don’t the plans specify it?

If they did - I probably wouldn’t be asking?

Long story, but I have free rein.

Yes, sorry I leaned on send too early, I was surprised that the building regs plans didn’t specify it.

And in the gap somebody else has posted the answer I was meaning to post….

[Elysium]

I would use rockwool where insulation is exposed. Purely because it will not contribute to a fire.

[OutInTheShed]

The celotex is vapour resistant, so you could get condensation problems if the vapour barrier of the ceiling is not perfect. And it won't be perfect.

The celotex will probably crush over the joists when you walk on the boarding?

25mm of celotex is not much thermal resistance. It's equivalent to roughly 50mm of rockwool?

You'd be better off raising the boards on blocks/legs/whatever and having another 150mm of rockwool and correct ventilation.