PIR boards - how rigid?

[ReadySalted69]

Hi there

I'm upgrading my loft for light storage and improved insulation by removing some old T&G chip-boards that were very poorly fitted and way too heavy, plus some old degraded insulation. The loft will only be used for moderate storage and access to a boiler, nothing heavyweight.

I plan to lay 100mm rockwool between the rafters (which are about 70mm deep, yes I know I'm squashing it a bit) and then simply lay Celotex GA4000 boards straight on top (at 100mm thick).

I have already tested this method with some Knauf Polyfoam but then I noticed Celotex come in wider sizes. Can anyone comment if the Celotex boards will be just as rigid as the Knauf ones. I've found the results good with the Knauf and they do not compress on the rafters - not that I will be jumping on them. Will the Celotex ones be just as rigid to stand on? They are both PIR boards. For a neat finish I will add some cheap (lightweight) click flooring on top. Any views on the Celotex option welcome before I spend £250

 

[jadzy]

my flat roof was converted from a cold roof to a warm roof. it now has 125mm PIR on it (can't remember the make, but not celotex). the PIR had a 9mm plywood layer fixed to it. Can walk around up there without any hint of give under foot. Does have 3 layers of felt on top of the ply, plus the PIR went on top of the previous 18mm deck boards. seem to remember most of the manufactures produced a insulation board with a ply backing. seems a way to go if you'll be moving around on it, save accidentally damaging it.

 

[big-all]

my only comment would be
normally when you cross brace with timber its all tied in together so the load is spread over several joists
with foam any load will be transferred over a smaller area before deflection sets in

 

[ReadySalted69]

Thanks for the replies

I've found someone on an AV forum with the same idea as myself, so I'll keep it simple and try the Celotex simply laid on top of loft joists without any plywood.

The joists are only 400mm apart and its worked fine with the Knauf polyfoam I had without any plywood layers in between. I figure the weight is spread on the PIR pretty good and and I can't see any compression damage having walked around a few times now working on some loft plumbing.

I just wanted to know specifically if Celotex GA4000 is equally rigid as standard Knauf Polyfoam Floorboard. I see people on this forum bang on about Wickes so maybe I'm on the wrong forum

 

[big-all]

my concerns are not the compression is the fact you are relying on the foam for structural strength as the joists underneath flex as they are not designed for any load other than holding the ceiling up

the foam wont gradually fail it will crack from underneath and fail under tension and not compression

 

[ReadySalted69]

Thanks Big-All

I had not considered that perspective.

I see Knauf Standard floorboard is designed to withstand 40KPa and 66KPa occassional loads (!).

Would you suggest I lay 9mm plyboard across the loft joists for the PIR boards to sit on and distrbute any load spreads more evenly?

I've not seen Polyisocyanurate board crack, it is described as structurally stable in the long term, but I would like to hear what your experience of it failing under tension is. Sounds like I could be putting too much faith in it holding up long-term?

 

[big-all]

the insulation is fine on point loading[compression] but not so good tension

on a normal floor you could stand on a joist and little deflection will happen
a ceiling is timber is less than half the size so will deflect unless you effectively spread the load

i simply dont know the answer using rigid foam in an application its not designed for
why not contact the manufacturer

 

[ReadySalted69]

On furhter research I see Knauf Polyfoam Floorboard was previously branded as Space board. They provided an instruction sheet showing you can indeed just lay these rigid boards down straight over loft joists, so long as the joists are not spaced more than 400mm wide (mine are 15-16")

I think the Celotext would not be as rigid looking at the tech sheets. Hence I will continue with this method using Knauf Polyfoam Floorboards with and will let you all know if it turns into a disaster without the addition of a plywood filling in this loft sandwich.

Watch out where those huskies go . . .

 

[big-all]

technically its 400mm[16"] centers so 14-14 1/2" between timbers dependent on thickness

but good luck

and remember there quotes are for timbers that wont deflect under load but yours will so may fail only time will tell

 

[JohnD]

the Knauf board you mention, I think is the one I have seen. You can lay it on top of joists but they say lay flooring such as chipboard on top if you want to walk on it.

 

[ReadySalted69]

Thanks

Yes, I've laid some boards all ready and amazing how rigid they are. I will not be jumping up and down on them and yes I will be adding some cheap click flooring on top as they do compress slightly from knees and elbows when crawling in the loft to install plumbing on top.

They are XPS (not PIR). I think this is a very simple solution, ultra lightweight compared to g&t chipboards or plywood, amazing insulation properties. I'll probably use 2 layers at 90 degrees but the 65mm ones I've laid already feel fine under foot. I simulated them in the garden and was amazed how they perform. I understand Big-all's point in this thread but they are also able to flex under tension and it would have to be huge deflections for them to fracture - although I would not employ this method at 600mm centres. I may even take a photo and post it here when complete.

 

[ReadySalted69]

The product link is here by the way if anyone is thinking of trying them in a loft siutation:

http://www.knaufinsulation.co.uk/media/294789/Brochure-Polyfoam-Solutions-Guide.pdf