Loft insulation - worth it?

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I have an old,  cold house and the loft has a couple of rolls of what looks to be cheap insulation. 

I'm looking to put more down but I'm wondering if there's a point whereby adding extra doesn't actually help?

I don't mind spending a good few hundred on a lot of rolls if each one will help,  but would obviously rather not if there's very little difference between say 2 or 3 layers. 

Any thoughts/ info would be greatly appreciated!

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  • molerat
    molerat Posts: 32,007 Forumite
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    edited 12 March 2021 at 11:15AM
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    270mm is considered the optimal depth, beyond that the benefits / savings become less.
  • Tallerdave
    Tallerdave Posts: 321 Forumite
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    The last time I insulated a loft I used loose rockwool to replace thin stuff off a roll because joist spacing was just so random. It seemed very dense compared to rolls. I used a garden rake to spread it around, filling just to joist height (~20cm??). The difference was very noticeable. 
  • brewerdave
    brewerdave Posts: 8,518 Forumite
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    Living in a "relatively" new detached house (25 years old),  we definitely noticed the difference when we had the insulation topped up. Much warmer upstairs with a knock on effect downstairs. Got it done when all the utility cos. were pushing for free however, so didn't have to justify financially.
  • DoctorStrange
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    molerat said:
    270mm is considered the optimal depth, beyond that the benefits / savings become less.
    Thank you. 

    Do you know the rate of reduction beyond 270?

    I guess what I'm really trying to work out is if the layer that takes it from 0 to 170 is for arguments sake 100% effective, the layer from 170 to 270 is 50% effective, is the layer from 270 to 370 nearer 25% or nearer 2.5%?

    Appreciate there's perhaps no exact answer to this, and should probably just spend what I'm happy to spend but I've been stung before buying stuff that I don't actually need to  I like to try to find the "sweet spots" these days 😊
  • matelodave
    matelodave Posts: 8,616 Forumite
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    edited 12 March 2021 at 12:35PM
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    We have approx 350mm in our loft (200mm between the joists +150mm overlay) and it makes a tremendous difference. not only to our heating cost but to comfort as well (we have a bungalow with 140m2 loft so can lose a lot of heat straight out of the roof as we don't get any benefit from heat rising from the groundfloor to the bedrooms

    You will always get the law of diminishing returns, so you could do a cost benefit analysis using the cost of the insulation, the heat loss without, the loss with 100mm then+100mm then+100, take into account the cost of the heat that's being lost, for every step and then cost of the three thicknesses of insulation. Whats more difficult to quantify is the comfort factor that the house says warmer as it doesn't cool down as quickly and warms up faster

    This is just for illustration (use proper figures based on the insulation value of the proposed insulation)

     if you assume that first 100mm will save the most (lets say halve the heat loss), the next 100mm will save 50% of the half that's left, so say 25% of the original heat loss and then the third 100mm will save half of that so only 12.5% and another 100mm will only save 6.25%, so based on that premise, 100mm would be a significant improvement, 200mm will be a lot better and 300 is probably about optimum but it's probably not worth the cost of 400mm.


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  • The_Green_Hornet
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    A cost / benefit analysis can be found on the Energy Saving Trust web site.
  • FreeBear
    FreeBear Posts: 14,818 Forumite
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    Thanks for the thread - I had been toying with the idea of shoving a bit more insulation up in the loft. This thread prompted me to measure how much is up there already. Currently have 200mm. The choice is to get 6 rolls of 100mm or 7 rolls of 170mm. For the sake of £17, I'll go for 170 and board out the loft while I'm up there.
    Yes, a total of 370mm is way more than current recommendations, and I'm not going to save that much on heating. But 370mm will future proof the loft when recommended levels of insulation are increased.
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  • matelodave
    matelodave Posts: 8,616 Forumite
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    edited 12 March 2021 at 6:48PM
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    A cost / benefit analysis can be found on the Energy Saving Trust web site.
    Looking at the EST website it guesses that installing 270mm of insulation where there was none in an average semi would cost about £300 and save around £150 (the inference is that if houses lose about 25% of their heat through the roof then the  heating cost is around £600 a year -  so it should pay back in around 2 years which is well worth doing.

    However, if you've already got around 120mm then adding 150mm would still cost around £290 but the saving is only £14 so the payback is around £20 years.

    This sort of implies that if if you don't have any insulation, or its really thin or poor then it's probably not going to cost you a great deal more to put in a decent thickness of 270mm in one go but not such a good idea to supplement it with another 150mm if you've already 120-150mm. 

    a quick shufti on the web shows that 100m2 of 200mm insulation would cost around £300 and the same area of 150mm could cost about £230 (so I dunno where the EST get their prices from). to that you might have to add installation costs if you aren't doing it yourself.
    Never under estimate the power of stupid people in large numbers
  • OC_66
    OC_66 Posts: 9 Forumite
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    Loft insulation is worth it, it can save you up to £580 a year on bills and there are free UK Gov grants to help cover the installation costs. 
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