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Housing - Renovation: Sealing floor void using tanking slurry

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    • CommentAuthori_DaveJ
    • CommentTimeNov 6th 2019
     
    Interested in members views on this.
    I'm planning to fill the void under the ground floor boards with insulation (possibly loose fill cellulose) and add EWI which will be taken down to the footing, beyond the DPM and blocking off the air bricks. The under floor space is dry (based on observation) but I am wondering whether to seal the concrete and bricks under the floor with a sealant. The floor boards will be coming up so I can access the void to apply the sealant.
    Thinking of a crystalline tanking slurry that can be applied with a brush (or sprayed on but I don't have a sprayer).
    There's a couple of brick courses under the dpm and a concrete slab which I assume has been laid on sand without a damp proof membrane.
    I'm in two minds. Is it better to leave the space to breath or damp proof? I guess I'm worried about damp in the insulation as there will be no ventilation of the void.
    Appreciate any comments, thanks...
    •  
      CommentAuthordjh
    • CommentTimeNov 6th 2019
     
    I think you're right to be worried. Damp in the space is likely to come from above as much as from below, I'd have thought. I'd use some rot-proof insulation at the bottom of the fill just in case but I think I'd probably leave the bottom surface unsealed.
    • CommentAuthori_DaveJ
    • CommentTimeNov 6th 2019
     
    Just read my post and it may be misleading in terms of the construction. I don't want to suggest that the walls sit on the slab - they don't. I have attached a quick sketch of the void/wall/footing.
    I don't think it changes anything but just in case...

    Posted By: djhI think you're right to be worried. Damp in the space is likely to come from above as much as from below, I'd have thought. I'd use some rot-proof insulation at the bottom of the fill just in case but I think I'd probably leave the bottom surface unsealed.


    djh, In terms of damp coming from above, are you thinking about spills and/or damp/condensation as a result of temperature gradient and relative humidity at the bottom surface?
      SusFloorSection.png
    •  
      CommentAuthordjh
    • CommentTimeNov 6th 2019
     
    Posted By: i_DaveJdjh, In terms of damp coming from above, are you thinking about spills and/or damp/condensation as a result of temperature gradient and relative humidity at the bottom surface?

    Both
    • CommentAuthorPetlyn
    • CommentTimeNov 7th 2019 edited
     
    If your concern is that cellulose will absorb too much moisture, then an alternative would be expanded glass beads. They are also free-flowing and have added advantages such as the have high compressive strength and therefore do not settle, are non-flammable, inert, with no degredation of insulation value and importantly do not take on more than 15% of moisture.

    Yes, glass beads are more expensive than cellulose initially but in a situation such as a floor void where you wouldn't want to repeat the insulation exercise again in years to come, perhaps a 'fit and forget' approach is worth spending a little more money on now.

    If you lay a geotextile layer at the base of the insulation, this would allow any moisture in the void to find it's way out downwards.
      Glass granules 9.jpg
  6.  
    @petlyn - don't suppose you know where I could get some?? :wink:
    • CommentAuthorPetlyn
    • CommentTimeNov 7th 2019
     
    The expanded glass beads are widely available and used throughout Europe but we have a one-off availability of around 30m3, residual from our low carbon self-build (long story!) and as such, are available at an attractive price.

    If you are interested and would like a sample - let me know and I can post some to you in a small bag.
    • CommentAuthori_DaveJ
    • CommentTimeNov 7th 2019
     
    djh, I saw your comments on another thread
    "Whatever you do, you want some way for liquid water (or wine!) to flow through to the ground rather than collect in or above the insulation."
    That's the thing that worries me. The likelihood of a spill is remote but if it happens...
    I'm leaning towards not sealing but have no idea how I would create any form of drainage in the unlikely event that it happens. I guess I could drill holes through the concrete but there is no fall and for this to be successful it would end up perforated like a tea bag. And I'm not giving up wine!
    Renovating 60s bungalows was never meant to be easy, ay!
    Pragmatically, it looks like rockwool or knaufwool laid under the joists and then another layer between the joists. Both are inorganic so should cope with any moisture (hopefully temporary or not at all). Then comes down to cost/conductivity/practicality.
    @GreenPaddy, I wondered when that was coming. :wink: indeed

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