Air tightness membrane or spray foam for attic conversion in A3 home?

OfflerCrocGod

I want to get the attic converted in an A3 home and would like to maintain as high a BER rating as possible.

Cutting the hole for the staircase will breach the airtight membrane installed on the attic floor and I'm guessing will reduce the BER rating. The conversion companies I've contacted use different insulation technologies either rigid board, earthwool or spray foam. It looks to me like I'd have to ask for the installation of an airtight membrane for the rigid board and earthwool types but the spray foam insulation type looks to be more air tight (also it looks like they spray it all the way to the bottom of the eaves/wall plate).

Does that make sense? Would it mean I'd be better off going for spray foam insulation for the conversion or would rigid board with an air tight membrane be the best solution? Or is it impossible to airtight an attic conversion enough that I could maintain an A rating?

Wartburg

Whenever it is possible, bring your insulation and air tightness layer down to the wall plate level. It´s never a good idea to leave cold crawl spaces behind the knee walls. The thermal envelope should be leveled with the building envelope.
I´m still surprised about the quantity of spray foam insulation here in Ireland. Have a look at other European countries, were you do not find this method in the construction at all. There must be a certain reason for not being popular (or probably not allowed) in the most central European countries.
There was an interesting article not to long ago in the Passive House plus magazine about potential health issues. https://passivehouseplus.ie/blogs/new-research-raises-spray-foam-health-questions
I have always concerns about fire rating, vapour control (spray foam without additional vapour control layer) and long term performance (chemical deterioration) in conjunction with spray foam insulation.

Calahonda52

if now a three storey, have you signify on meeting fire regs?

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see post 7 here
https://www.boards.ie/vbulletin/showthread.php?t=2058010253

Such a job is tough to get right, the execution of the proper workmanship and detailing is key.

Maintaining the rating is a crude target, easily enough achieved.
Getting the A/T insulation and wind barrier layers properly done is another ball of wax, especially in current climate with shortage of skilled labour


Good luck

OfflerCrocGod

OK, we'll get the insulation and air tightness layer down to the wall plate level.

I've asked about that with all the attic conversion companies I've talked to and apart from the spray foam ones they seem against it. Some saying that it's pointless as there is no heat source in there so the temperature would be the same as outside or that it would lead to condensation in the eaves and that they've had to take it out before.

It won't meet the regulations for a habitable space, it's going to be an office and I'm under no illusion about treating it as an extra bedroom when selling the house etc. It will follow most of the regulations in the Loft Conversion Leaflet from the Dep of Housing. Linked fire alarm, window placement/sizes, it's only one room with less than 50m2 of space, engineer cert of compliance. I'm not getting the plasterboard replaced on the first or ground floor as that makes the cost spiral out of control and I can't justify it for an office/non habitable space (TBH I don't know if it even needs replacing, it might already be up to a 30min fire resisting standard).

The more I read the more I'm put off from using spray foam insulation. I think I may ask for rigid board with an airtight membrane as that seems the best combination/approach. I thought that maintaining the A3 rating would have taken some effort. I mean if I get a conversion with no airtight membrane (not that I want to) would it not be much harder to keep the rating?

I'm worried about the quality of the work around the insulation but I'll inspect the work every day and hope to catch any issues. I'm guessing very few of these companies put in airtight membranes so I'm hoping they might get someone in to install it if I insist on it.