Warm loft - insulation between and under rafters or just under?

Discussion in 'Eco Talk' started by hourglass, Jan 21, 2021.

  1. hourglass

    hourglass New Member


    I got a quote from a insulation specialist for warm loft insulation. They quoted for 40mm PIR friction fitted between joists (leaving a 50mm air gap to the roofing membrane) with 100m PIR attached under the rafters.

    It was quite a good value quote, split out very carefully for labour and materials, and I accepted.

    They have now called me and told me that they'd prefer (for obvious reasons) just to install 120mm instead of 100mm under the rafters and forgo the 40mm board between the rafters. They didn't offer a discount.

    That would lose me 20mm of insulation (not hugely important in my eyes), 20mm of headroom (not ideal but it's just 20mm) and make for a much quicker installation and less need for precision in installing.

    They say the benefit for me is that I would get a larger air gap, since no insulation would go between the rafters.

    So I wanted to check with you experts - what do you think of what they're offering? Is there any functional difference or deviation from best practice? Or are they just proposing a more efficient method (in which case why wouldn't they quote for that to begin with?)

    Last edited: Jan 21, 2021
    Delacierva likes this.
  2. stevie22

    stevie22 Screwfix Select

    They are trying to BS you: the air gap is irrelevant.

    They will save a lot of money but the job will be fine. I would ask them for a reduction!
  3. glob@l

    glob@l Active Member

    If you're happy with the changes consider getting a further 2 quotes to compare against the revised works.
  4. Hans_25

    Hans_25 Screwfix Select

    They're trying to make their lives a lot lot easier and make a big profit from you while delivering a sub standard job compared to what they quoted for. Cutting the PIR insulation to fit between the rafters is a time consuming job, I know, I had about 160 pieces to cut for my loft!

    How important is headroom in the loft? Using 120mm on the underside will lose you another 20mm of headspace and leave you 20mm short of insulation compared to what they quoted.

    As above, get a couple more quotes, perhaps ask them to quote for (a) the original idea [40mm + 100mm] (b) just the 120mm (Or 140mm if the headroom will take it)

    Or you could do it yourself, its not hard just very time consuming.
  5. stevie22

    stevie22 Screwfix Select

    You're not actually losing 20mm as the wood reduces the overall efficiency of the material between the rafters.
  6. JasonCSmith

    JasonCSmith Member

    Yep, and there will be fewer gaps too if the whole of the thickness of insulation is underdrawn.
  7. rogerk101

    rogerk101 Screwfix Select

    I would put in 60mm between the rafters and 100mm below the rafters. 35mm is more than enough for an air gap.
    They're a bunch of cowboys and have made the silly mistake of trying to take the pi88 even before starting the job. The good cowboys at least wait until they're half way through the project and then screw you over.
    I would certainly view this as a blessing in disguise, move on and find someone with a few more business ethics.
  8. Alan Wright

    Alan Wright New Member

    The ventilation gap is fine without needing more. This will cut not only their material cost, but also massively cut their time. As mentioned above the rafters are a point to look at, they do not really cause a hole in the insulation between them as the timber will have some thermal resistance, but it does cause a bit of a cold bridge where some can come through hence sheeting over as well as between. I think I'd be looking to get a few other quotes before getting back to them, see how the next people act and talk as they do the survey and you'll be in a better position to make a good judgement of their character.
  9. Delacierva

    Delacierva New Member

    I am not a professional on construction techniques but it is true any insulation system you use will mean a big improvement in comfort at home.
    We have a house in Portugal in the countryside and we insulated the ceiling with some kind of rockwool and the result has been astonishing, about 20 mm layer if my memory is working fine,
    It was installed inside.

    Waking up in the morning is different now, about 4° Celsius more or even 5° more
    Just this so far
  10. koolpc

    koolpc Screwfix Select

    I wouldnt have them anywhere near the house after that.

    Get a few quotes from elsewhere.
  11. Shaquel

    Shaquel New Member

    We can help you, we are TrustMark approved and BBA regulated. If you want to know how much you could save on your energy bills you can use our free EPC checker that will give you an instant result.

    Also here is an image that displays how heat escapes properties. Hope this helps.

    Last edited by a moderator: Jul 23, 2021 at 9:03 AM

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