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Spray Foam Insulation vs Drylining (PIR Plasterboard)

  • 21-10-2021 11:06am
    Registered Users Posts: 1,981 ✭✭✭ aFlabbyPanda

    I'd posted before about trying to insulate our 80s dormer bungalow. I've got quotes for 3000 to 3500 to spray foam the area behind the knee wall (cold attic) or we can dryline almost the entire house (external facing walls downstairs and all walls in the dormer area) for 2500. Spray foam install will add ducting to retain ventilation. Of course each supplier is telling me I should do their solution first but I was wondering if anyone has any advice or if someone has done similar before?

    My main concern about doing the spray foam first, is that this will still leave the pitched ceiling in the dormer area without insulation as there is nothing behind the plasterboard right now.

    I've heard that dormers are notorious for draughts etc but would anyone have any advice either way? EWI is beyond our budget right now. External walls have already been pumped and attic has 2 layers of rock wool insulation.

    I've attached a rough image of what we have at the moment.

    PS I posted this in DIY initially but they said here might be more appropriate.


  • Registered Users Posts: 2,584 ✭✭✭ MicktheMan

    You've heard correct. Dormers are notorious for heat loss by air leakage, aka draughts. Not addressing this at the outset will likely render any "insulation" improvement a wasted effort.

  • Registered Users Posts: 1,981 ✭✭✭ aFlabbyPanda

    So what should I look at first? I had been advised that the spray foam would solve the draught issues by sealing the attic space that boundaries the warm walls in the house.

  • Registered Users Posts: 2,584 ✭✭✭ MicktheMan

    "So what should I look at first?"

    The first thing to do is quantify how your heat is being lost and address the largest culprits first.

    Dormers = air leakage

    Insulation does not address air leakage, even spray foam. Yes the guy selling the sprayfoam may well "advise" that it will address air leakage , but it won't. How do I know? I've tested very many houses (including many dormers) with sprayfoam applied to the roof and have yet to find one where the sprayfoam had solved the air leakage.

  • Registered Users Posts: 1,981 ✭✭✭ aFlabbyPanda

    Is there a specific type of trade I should engage with since I'm not really getting a straight answer from anyone. I understand that heat is being lost due to a combination of draughts (we have new attic doors being installed) and lack of insulation (nothing under the dormer floor or pitched ceiling) so apart from pulling the ceiling down or the floor up I'm not really getting a solution. Each vendor has their own solution which they are pitching with vested interests.

    So for the houses you tested what was the solution? Even just some examples.

  • Registered Users Posts: 2,584 ✭✭✭ MicktheMan

    Your first port of call ought to be a heat loss survey. Make sure it is an independent survey not tied to the sale of any other services or products. This survey should leave you in no doubt about your next steps and who/what trade to engage with.

    I've found the most effective solution (of sorts) is the gaining in understanding by the homeowner of the issues at play and an appreciation of their relative importance for their house and what they want to achieve given their budget. Being able to feel what is happening and the solutions explained in lay-mans terms is key and in many cases completely changes the owners perspective on how to go about addressing.

    This may appear strange to read but in the vast majority (<95%) of dormers tested over the years, adding / improving insulation would typically not be in the top 3 items on the improvements list in terms of both bang for the buck and comfort improvements!

    Post edited by MicktheMan on

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  • Registered Users Posts: 2,149 ✭✭✭ Yellow_Fern

    I am no expert but I dont like spray foam as I like houses to be like lego, dismantlable. Spray isnt at all. Now in the US they use it a lot and it can be done well but not for me. I'd rather use PIR and some insulated over it. Also I agree airtightness is more important than insulation.

  • Registered Users Posts: 377 ✭✭ billy_beckham

    Any examples of solutions as the OP asked for? Or just a recommendation to get a survey done. If you've tested many houses you must have some idea of the most common issues/fixes??