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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

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    • CommentAuthoralm
    • CommentTimeJan 17th 2022

    I have looked on past discussions, but aerogel hasn't been mentioned for a few years so I was wondering if anyone had more recent experience.

    I was pretty set on using aerogel around some bay windows - we have brick piers so need a fairly thin product. However, I just had some advice from a specialist retrofit installer who said he see's a lot of damp issues with aerogel and its not as breathable as you would think - particularly where the boards join. He recommended we use 20mm wood fibre for the piers, and 60-80mm for the rest of the wall. My issues with this is some pretty awkward detailing, and the windows looking a bit strange. And damp issues where the two insulation depths meet.

    I've seen past discussions where aerogel has been used and nothing came up on damp. So just wondering if anyone who used it a while ago has seen this, or any other advice around the window. We have a lot of original features (It's an Edwardian semi) and it's important to us to keep as many of them as possible, whilst improving the thermal performance of the house.

    For the record the house has 225mm solid walls, external wall insulation is not an option.

    • CommentAuthortony
    • CommentTimeJan 17th 2022
    Aerogel is well expensive, any breathable insulation will let moisture through to the masonry and it will condense on the plaster, I wouldn’t use wood-fibre if moisture was a worry.

    I can’t see how any of this would meet building regulations and it seems it should.

    Bay windows are difficult, would you consider having vapour barriers?
    This was a government study where several flats in a building were each insulated with different insulation materials, including aerogel, wood fibre and a few others and monitored for performance and dampness. Results were good for both

    We have used Aerogel window reveal lining boards e.g.

    to line the reveals on the ventilation slits in our barn conversion (now fitted with glazed units to provide very small windows). There wasn't much space so this product was a good solution to keep these and make a feature of them. if you were too obsessed then you wouldn't have bothered keeping these but we think they add interest to a conversion scheme where there wasn't a lot else of interest that could be kept and made a feature of.

    It's early days yet because all the plastering and decorating isn't finished yet, but they do tend to be condensation magnets when there's a lot of moisture around so they are obviously the coldest place. However when its drier they seem okay and once the MVHR is up and running and the place is finished and dried out I'm hoping that they will be just fine.
    It is expensive but I think it may be better than nothing in your situation. we used the one with Magnesium Oxide board, it's a bit bendy. You can get it backed onto plasterboard but that would be thicker overall. We haven't decided yet whether to paint the MgO or leave it as is. I think we will end up painting it but it will be awkward because some of the gaps are only about 100mm wide but 450mm deep because of the thickness of the stone walls (300 ish high)
      ventilation slit.jpg
    The stuff we used blunts your tools when cutting it by the way. I bought a pack of 25 jigsaw blades to cut it up and only got about 3 or 4 cuts before the teeth were blunt (as in actually worn away as opposed to just dulled). The dust is superfine and desiccant, I cut it outdoors after the first cut!
    • CommentTimeJan 18th 2022
    Posted By: Dominic CooneyI think we will end up painting it but it will be awkward because some of the gaps are only about 100mm wide but 450mm deep
    Looks good. :bigsmile:

    Painting shouldn't be too difficult as long as you mask the window thoroughly. Then you can slop a thinnish paint around as much as you want. :devil: A clay paint would work well I think, as should limewash.
    • CommentAuthoralm
    • CommentTimeJan 18th 2022
    Thanks all for your comments,

    Dominic, the windows look great. I think I would have made the same choice to keep them! Interesting that you say its a condensation magnet, have you got pretty high u-values elsewhere? I hope they do work fine once the rest of the work is complete - you'll have to let me know. Thanks for the tips on cutting them as well, I actually thought they looked like they would be a lot easier to install than the wood fibre boards, but it seems perhaps not!

    Will, thanks, I did read that actually (I think you might have posted it on a different discussion!) And there was no mention of damp, which is hopefully positive. Thanks for the link.

    tony, moisture isn't really an issue at the moment. We get a lot of condensation but we have single glazed windows! In terms of regs, it'll depend what we can get away with using internal wall insulation. We obviously dont want to go too thick to risk interstitial condensation. I have contacted a few suppliers of the wood fibre to see what they recommend, but general advice seems to be 60-80mm. As well as that its a balance between retaining as many features as possible and getting the best thermal results.
    Posted By: almcondensation magnet

    Mainly the frame to the glazed unit because it’s just a small aluminium u-channel 15mm deep to keep it as small as possible, set into the stone, so is actually a terrible cold bridge. It would probably have been better with no frame.
    The rest of the walls are 100mm PUR and the large windows have 25mm on the cheeks, some have 50mm on the sills , and the frames on the large windows are wood. So the 10mm of aerogel and 3mm MgO board is the least amount of insulation in the place.
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