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retrofit cavity wall assumption inaccuracies

Discussion in 'Building' started by DIYedboy, 1 Oct 2021.

  1. DIYedboy

    DIYedboy

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    I'm all for insulating everything I can (including the 55mm cavity walls in my 1960's house with polystyrene beads).....................BUT...............whilst being effective, it probably aint as effective as what people assume.

    2" of any insulation will only slow down heat loss, would anyone know by what percentage heat loss with filled cavities is reduced ? (with only 2" of insulation it won't matter very much which insulation material is used)
     
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  3. ^woody^

    ^woody^

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    The clue is in r values of materials and the watts per m2/k which is a measure of the thermal transmittance of energy.
     
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  4. mikeey84

    mikeey84

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    Insulation follows the law of diminishing returns, so the 1st 2 inches are the most effective, and every 2 inches after are progressively less effective.

    That's why even thin Insulation is worth it, if thicker isn't possible

    And it does matter, as 2 inches of PIR is about twice as effective as 2 inches of rockwool type.

    To give an example, your unfilled cavity wall probably has a u value of about 1.5, if filled this will drop to about 0.6. Current construction will give you about 0.2, so your 2 inches is giving a much larger improvement vs the original build, than your filled cavity vs a new build
     
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  5. DIYedboy

    DIYedboy

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    Wood................when is a clue not a clue?..............when I don't have a clue! those numbers are great...........if you know your stuff, n I don't.

    Mikeey don't forget that PUR foam only has superior insulation properties because of the gas in the bubbles, when that 'gasses off' it's no better than polystyrene.

    So, if 30% of heat is lost through the walls, after I fill the cavity, about 12% will still escape (1.6 down to 0.6), n in a new build (0.2) that 30% loss will be sliced to 4%................does that sound about right?
     
  6. mikeey84

    mikeey84

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    Not sure on the maths tbh, but looks right? So bigger improvement from the lesser insulation, if that makes sense?

    I've seen that said about PUR, and always people arguing against it. I don't know either way...
     
  7. ^woody^

    ^woody^

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    Not in real life.

    A numerical value to indicate a big improvement might be on paper only - so the "big improvement" on paper does not necessarily mean a big improvement in keeping the heat in.

    ie if adding 25mm of insulation gives the the "big improvement" numerically over any subsequent layers, that does not translate in actually reducing heat loss to perceivable levels, so it may be that 4x as much insulation - another 3 layers ("less improvement" layers with this type of thinking) is required to make adding insulation efficient and worthwhile in the first place.
     
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  8. DIYedboy

    DIYedboy

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    Wise words Wood.................all those clever numbers are wonderful in a theoretical situation, but reality has an annoying habit of sticking 2 fingers up at theories.................However, if we just think of wall U values, surely they are an accurate measure of heat loss through a structure (brick/ block etc), so a brand new wall of 0.2 and an old 9* solid wall of 2.2? means the new wall only loses one tenth of the heat therefore the 30% loss through the wall figure becomes a 3% loss in the modern house (we will ignore the detrimental factors of windows, floor, ceiling, draughts etc ).

    The one thing that gets me is when I pick up a 2" lump of polystyrene, I can feel heat radiating back into my finger...........that don't happen when I pick up rockwool or bare celotex!!
     
  9. Mr Chibs

    Mr Chibs

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    I insulated the exterior solid brick walls, on the inside of my room. When I was leaning on the wall (in the corner) after a few seconds you can feel your body heat reflected back to you from the foil coating.
    50mm has made the world of difference to the room, it's actually comfortable to be in there now!
     
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  11. mikeey84

    mikeey84

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    Surely that depends on what other insulation you have.

    If you have 600mm of rockwool in your loft, and 100mm of solid insulation in your floor, and solid 9" walls, your gonna be losing more than 30% through the walls.
     
  12. John D v2.0

    John D v2.0

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    Just on the point of 30% down to 3%.

    If it's a huge percentage through the wall, let's say 50% of 1000w ie 500w for simplicity, and the u value is 2, if you get it down to 0.2 then you'll lose 50w from the walls.
    You think you'll only be losing 5% from the walls now, but that's not how it works.
    The overall heat loss will reduce from 1000w to 550w, so you'll be losing 50w of 550w so about 9% from the walls, not 5%.
    And even that is just the theory, in practice it could be anything.
     
  13. mrrusty

    mrrusty

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    55mm cavity. We have cavity trays, drip points on wall ties, weep vents, and older properties have cavity airbrick vents. Ask yourself what they are there for, and then decide if filing the cavity is a good idea.
     
  14. johnny2007

    johnny2007

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    I speak for my own experience.
    We had our cavity walls insulated by the fluffy white stuff that looks like feathers a few years ago.
    The heating bill noticeably went down by a lot.
    I had concerns about the cavity ventilation, but I was assured that this stuff lets the walls breath because it's not a solid piece of foam but as said, very similar to feathers.
    Haven't suffered from damp, and the boiler now works part time.
    Only my personal experience, maybe the theory is totally different.
     
  15. mikeey84

    mikeey84

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    We could talk calculations and theory till the cows come home, and never get a number that actually reflects what happens in the real world, as there are too many variables.

    You can get retrofit cavity wall insulation wrong, plenty of people with damp can testify to that, but done properly its been proven to have a benefit on keeping your house warmer and your bills down
     
  16. mrrusty

    mrrusty

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    No-one has ever explained to me where the water designed to be in the cavity goes when the cavity is filled.
     
  17. mikeey84

    mikeey84

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