RoofInsulation query using cellotex+loft insulation combined

[saaya2020]

Background: last year, we converted our Bungalow into a 2 storey house.
The Bungalow had a attic so the Roof rafters had 50mm cellotex which I was able to salvage.

When we converted to a house, the loft space was not converted into an attic. Put simply, British Gas laid 120m2 of yellow fibreglass loft insulation blanket over the horizontal floor space in my loft. The insulation is 100mm thick

My upstanding roof rafters are at 40 degrees & their dimensions are 200mm by 40mm (8 by 2's) and they are at 400mm centres. I have breathable roof felt.

My proposed solution: In order to have proper ventilation air flow in the roof space, I know that I will need a 50mm airgap from the felt to the 50mm cellotex. Then I am thinking to put the 100m fibreglass on top of the cellotex. Then I put foil backed plasterboard over the rafter & skim.

@Experts - Is my solution ok or will I run into condensation issues? I am simply trying to re-use the existing cellotex and 120m2 fibreglss insulation.

I'd be grateful for any solution or advise to re-use this insulation rather than skip it!

Summary : 200mm roof rafters. So, 50mm airgap + 50mm cellotex + 100mm loft blanket. Then, foil backed plasterboard + skim.

If there are any other solutions to use this cellotex or insulation blanket, pls let me know as well.

Many Thanks in advance

PS - regs not needed for this project.

 

[coxinelle]

Hi there

as far as I know, it's not a good idea to combine the different types of insulation. In Buidling Control terms it might be even illegal- someone mentioned that to me a while back (I'm an Architect).
The timber rafters will bypass the celotex and get in contact with the rockwool- not sure what happens- but depending on where the due point is- you might well creat little pockets of condensation/i.e. the timbers might get moist over time and the cavity is not ventilated. so moisture might well be trapped. I would rather get the celotex in the rafters and that's it...
for a small space- you won't loose too much energy.

hope this helps

 

[saaya2020]

thanks for your comments. Aprreciated. I'm no expert myself ... but wouldnt the foil backed plasterboard act as a vapour barrier to prevent such condensation from forming?

At the moment, the 50mm cellotex is inset into the rafters by 100mm. (for the blanket). If i just fix the plasterboard over the rafter, there will be a 100mm gap. Should I move the cellotex in line with the rafter?

 

[coxinelle]

Thats Fine with the gap. The vapor barrier stops the cavity to be drowned in condensate, however residual moisture is always there from the outside air. Make sure all is perfectly air tight and celotex really snuck with timbers. Needs to properly wedged in, so in theory you should now not be able to move it back. Use a mallet to wedge it in. A 5mm gap means you loose all performance on that particular rafter bay. Good luck

 

[saaya2020]

thanks for your reply.

 

[^woody^]

as far as I know, it's not a good idea to combine the different types of insulation. In Buidling Control terms it might be even illegal- someone mentioned that to me a while back (I'm an Architect)

If you are an Architect, then how can you describe this as being "illegal". Surely you must have a grasp of the Building Act, Building Regulations and building law?

Its not illegal and never could be

 

[^woody^]

There is nothing wrong with combining different insulation. In fact it can be a good thing to combine say the thermal efficiency of a PIR board (Celotex) with the acoustic insulation of a dense quilt such as Rockwool.

Your proposal has an air gap which will deal with condensation, and you could even insulate the entire space between the rafters with no air gap. Lots of threads on it here.