Insulation of roof and soild wall - Spray foam or board?

12 Oct 2012
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United Kingdom
Hi Guys,

Trying to figure out the best way to insulate the bungalow we are renovating.

Its about 70% solid wall (other 30% is cavity due to a later added extension) and has a loft conversion (2 bedrooms). Roof construction (hip and valley) is concrete tiles/battens(i assume)/felt/15mm sarking board/115mm rafters. It has ridge vents the length of the roof but currently the fascia and soffits are completely sealed!

i've done a search and read some posts about spray on insulation and noted some of the concerns over rot etc.

i've had a couple of quotes from different companies and they have all said their recommended product is "breathable" so does not require normal ventilation gap. Pricing is ranging from £9-23/m²
Only 1 company mentioned the need for ventilation gap but said they use 15mm cardboard spacers to maintain an air gap, i know regs usually call for a 50mm gap, so my question is would a "Breathable foam" with a 15mm air gap to the sarking board provide adequate ventilation? Would the finishing plasterboard then have to be "breathable" too?
i'm worried about condensation especially as the roof will have 2 bedrooms.

My initial plan was to use 75mm PIR board between the rafters (leaving air gap to the sarking boards) tape and foam sealed and covering with 25mm insulated PB. This is straight forward in theory but on measuring the rafters, no 2 spacings are the same! Im worried about maintaining a good seal and lots of waste.

There is a closed cell spray foam offered which claims the same insulation values as kingspan etc. Im now thinking of installing some sort of spacer system to maintain the recommended 50mm gap to the sarking board and then spray over the top of everything, To me this is the same result as using boards without the waste, easier to install and much better air tightness. Would a VCL layer be needed between the foam and rafter or between the insulated PB and the foam or both?

If going the spray foam option im also considering doing the walls at the same time (ceilings and loft floor will be removed to ensure full height of wall is insulated from floor to roof, avoiding cold bridging/condensation), in theory this would seal the whole envelope but leave the roof ventilated.

don't know what sort of issues this might cause down the line? only thing im seeing as a potential issue is the that roof/loft floor joists are no longer ventilated.
I know i would also then need to think about dedicated ventilation systems etc.

This is a minefield! lol

Cheers Neil
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Believe you need to ask them to prove that their foam is breathable ( I don't think so ). For PIR/PU. their closed-cell nature makes them vitually waterproof ( believe that total immersion for 30 days gives a weight increase - i.e. water penetration -of something like 0.8% ) and polystyrene is not much different.

Don't know too much about roofs, but if anyone is saying breathable material right up to the covering is acceptable as an alternative to a 50 mm gap, that that is just balls. The 50 mm gap is there to allow air to blast through, not seep through at microscopic rates.

Since you can't spray over the joists, you will have small gaps where the foam won't bond/shrinks back from joists so you would need to tape or use VCL. You are complicating matters ( and increasing cost ) by using insulated pb: I'd say use insulating boards of whatever type up to flush with joists, then tape/VCL, then pb.

It will not be possible to foam flush with the joists because of expansion, so you would need to trim that flush and then tape/VCL before pb.

If you foam the walls, then the only problem I see is access to any cables which may run up walls and that's not too likely.

What does this foam cost and can you compare amount needed to make 1 m2 x 100 mm with cost of same in PIR/PU/PS ?
Thanks for the reply,

the products i've been looking at are -

theres a few others but i believe they are just different trade names. producing either a closed or open cell foam with approx U values of 0.25 and 0.35 respectively. they are all applied by trained installers, not really a cost effective DIY option for larger areas.

All claim to be BBA approved etc but i can see what you mean about air gap vs a breathable material. I know from outdoor clothing, breathable = still soaking on the inside lol.

Good info on using a separate insulation from the PB, suppose should work out much less expensive :)

Cost wise, i had of quote of £1100 to spray 105m² of roof structure to a depth of 115mm (full rafter fill as ventilation isn't needed :rolleyes: !) working out about £10.48/m² finished in a few hours.

Best price i can get on 75mm Xtratherm board is £7/m² that i will need to batten (£1/m²) out for ventilation then cut the board and fix/seal with expanding foam (£0.20/m²) so £8.20/m² + time for installing, free if DIY.

Should also mention most other quotes are £20-23/m² however one of these included the cardboard spacers for ventilation (15mm). In the grand scheme of things its probably similar sort money (excluding the cheapest quote i had) for a finished job ready for tape/VCL etc.

If im honest, im skeptical, id prefer to use tried and tested methods - board/foam/tape but this appeals due to labour saving and (claimed) airtightness benefits.
Thinking if my cheapest quote checks out, i should be able to install a spacer system to maintain the proper ventilation gap but specify a closed cell foam getting all the other benefits with out the down sides and probably a lower finished cost.

Cables are currently tracked into the brickwork in conduit and dropped down from above. Seems like a good option for the wall. Would it normally be recommended to have an air gap between the wall and a PIR/PU board? do you see any issue by spraying foam on directly, eliminating any air gap?

Under no circumstances use a spray foam insulation, its a con and will leave you with a rotting roof. Do a search of the Roofing forum or follow links in this thread //

Just use 65 Kinspan or similar between your rafters leaving your 50mm air gap, seal any gaps with expanding foam, (its a PITA but that's what you get when renovating so you'd better just suck it up), then use say 50mm under the rafters screwed into the rafters, if your insulation is foil-backed then just tape over the gaps with a foil tape, then fix your plasterboard up, screwing through the insulation and into the rafters. Ensure you open up those eaves vents.
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Thanks for that,

I think ill stick with the board and tape method. eaves will be opened up in due course, when replacing the fascia and soffits. Still a while off yet as chimneys need rebuilt before i can do anything with the roof.

Same sort of thing for the soild walls then? Seen various methods of attaching the board using dot and dab, timber battens or fixing the boards directly to the wall.

Is there a preferred method for internal wall insulation? Read various reports, some recommend an air gap others aim to eliminate any air gap.

cheers Neil
This answer assumes you mean the internal surface of an external wall.

If you are using foam boards there is no point in an air-gap, but you need to have a vapour-barrier.
Yep thats what i meant lol.

Am i right in saying if using a foil backed board and its foil taped and sealed at all joins (floor/walls/celing) that no additonal VCL is needed?

Remember being told if going for internal insulation to make sure it is sealed at the celing and continued above to remove the cold bridge and prevent condensation on the wall above the celing.

If im maintaining the air gap in the roof between sarking and rafters, soffit and ridge etc would i be ok to seal the top of the wall boards to the roof insulation making for better air tightness?
Only issue i see with this the celing/loft floor joists are bridging from warm insulated space through the wall insulation to the cold wall plate, would this be a condensation/rot concern?

Apologies for all the questions and long posts, first time doing any of this so ive been doing a lot of research to make sure its done right to achieve the best results and wont cause any long term issues.

Thanks again


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