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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthorFloyd
    • CommentTimeFeb 12th 2018
     
    I'm insulating an exterior wall with a false wall of studs and 90mm rigid insulation tightly fitted between. In some places there is about a 50mm air space between the insulation and the 450mm stone wall. Is convection going to be a problem?
  1.  
    Convection - or thermal by-pass - Probably yes (but not certainly)! Is there plaster left on the wall against which the stud frame is fixed? Is there a cellar or sub-floor void? Rubble-fill walls (which yours *may* be) can be very leaky (if Beatrix potter's mice and rats had staircases in them, think how easily a draught - aided by the stack effect) can move from sub-floor to roof void! Is there room for another 25mm of insulation over the studs, to cloak the thermal bridge?
    • CommentAuthortony
    • CommentTimeFeb 12th 2018
     
    The answer to the OP is yes, when it is windy the insulation wont be doing anything.
    •  
      CommentAuthordjh
    • CommentTimeFeb 12th 2018
     
    Fill the gap behind with something compressible, like mineral wool batts.
  2.  
    Wot tony said +1
    If the gap is not too big (I suppose even if it is relatively big) you could fill the space between the insulation and the wall at the top of each insulation slab with the ubiquitous squirtey foam. Don't forget that any gaps between the studs and the rigid insulation slabs will also be detrimental to the performance.
  3.  
    Parge coat over the stone wall, seal the base of the wall/floor joint, and either do as per djh's suggestion (Is the wall completely free of damp?) or find something impervious and break up the height of the void into v small 'compartments, to minimise convection, as per 'dot and dab' (Not!) best practice.
    • CommentAuthorFloyd
    • CommentTimeFeb 12th 2018
     
    Gosh, thanks all! Nick, there is a void underfloor but I've sealed between the bottom stud and the wall with squirty foam so that should be isolated. And there is still plaster on the walls, no doubt mice behind :) but there is no wind coming through. I think mineral wool as per djh for the larger gaps and squirty foam to break up the narrower gaps seems most sensible. Would 1200x600 compartments (following the studs) count as 'v small'?
  4.  
    I cannot offer a seriously scientific basis for this, but I always try to break up the spaces at 400mm vertical centres.

    No wind coming through... Was it windy outside? Try it with a smoke source before you 'box it all up'. You'll only get one try at this!

    I repeat my Q (unconnected with convection, but strogly connected to thermal bridging) as to whether you have space for another 25mm Pu to cover the studs.
    • CommentAuthorFloyd
    • CommentTimeFeb 12th 2018
     
    There is space for a further 25mm insulation board. Calcs suggest it would improve the u-value from 0.26 to 0.21. It seems I'd get a similar improvement by squeezing 40-50mm batts behind the studs. This would cost less and should solve any convection problems. Would batts adequately address thermal bridging or is it better to have rigid boards on the warm side of the studs?

    The smoke source makes good sense. Is there a good DIY option here - I may get some odd looks waving incense sticks over the walls?
  5.  
    Floyd, you said: ''Calcs suggest it would improve the u-value from 0.26 to 0.21. It seems I'd get a similar improvement by squeezing 40-50mm batts behind the studs''.Earlier you'd said: '' In some places there is about a 50mm air space between the insulation and the 450mm stone wall''. I had taken this to mean ''...and in some places there isn't''.

    If there is a good 40-50mm all over, I'd say go for it, but note that if you are going to have something which might 'suck' against a wall whose 'dryness' we are unsure of, I'd suggest putting a breathable membrane against the wall.

    I am unclear how far on you are. On the one hand you refer to a gap between ''the insulation and the ...wall'', which suggests the Pu is already in, and on the other you are suggesting mineral wool behind the Pu, which suggests the Pu *isn't* in.

    I assume from what you said above that the studs (?what size?) are at 600 ctrs. The U value of 0.26 sounds like it's unadjusted for the timber fraction - is that right?

    In terms of the smoke source, ignore the odd looks - incense sticks are fine! Normal caveats re not setting fire to you work apply....

    Finally, re the batts, you said: ''This would cost less and should solve any convection problems.'' I agree some stuffed mineral wool is better than nothing at all, but it is not an air-tightness 'tool', so do your utmost to ensure that all air movement is sorted *before* you 'stuff'.
    • CommentAuthorDarylP
    • CommentTimeFeb 13th 2018
     
    ....smoke pellets are a good source of smoke, used for flue/draw testing, as well as smoke matches.
    Plumbers' merchants, SF, TS etc etc:wink:
  6.  
    DarylP, I agree but, even with the matches, often much more smoke than you need.
    • CommentAuthorDarylP
    • CommentTimeFeb 13th 2018
     
    ...open a window....:bigsmile:
    • CommentAuthorFloyd
    • CommentTimeFeb 13th 2018
     
    Nick, thanks for your time, it's much appreciated.

    I've 38x89 CLS studs at 600 centres and noggins at 1200 centres. The studwork is in place but not the Pu. The gap behind the studs varies from 25-50mm, probably averaging 40mm.

    The U-values were calculated at http://www.changeplan.co.uk/u_value_calculator.php and supposedly took the thermal bridging into account.

    I could conceivably put a membrane on the wall though it's going to be a pain to fix - I guess PVA and a few pins/staples would do the job. I was intending to put a membrane behind the plasterboard anyway - would I need both?

    It's just occurred to me that I could push the Pu through the studs and in contact with the wall. Then fill the voids in front of the Pu with mineral wool (or not, as I suspect convection will be less of an issue on the warm side), breather membrane and plasterboard. That would be a lot easier to do and would mean the mineral wool isn't against the cold, potentially damp, outer wall.
    •  
      CommentAuthordjh
    • CommentTimeFeb 13th 2018
     
    Posted By: FloydIt's just occurred to me that I could push the Pu through the studs and in contact with the wall. Then fill the voids in front of the Pu with mineral wool (or not, as I suspect convection will be less of an issue on the warm side), breather membrane and plasterboard. That would be a lot easier to do and would mean the mineral wool isn't against the cold, potentially damp, outer wall.

    It would also mean you have nothing suppressing convection behind the studs. i.e. totally defeat the purpose of the mineral wool batts.
  7.  
    I would be inclined to push the Pu through the studs to the wall and fill top and bottom of each slab with squirty foam. If the studs and noggins are in place then a good squirt of foam onto the wall and then press the PU into place. The aim would be to stop and air flow up behind the Pu going in at the bottom and out at the top. I don't think that any internal convection currents behind an individual slab of Pu circulating between the bottom line of foam and the top line of foam will make and difference (if that happens at all) Make sure that there are no gaps behind the stud work that would allow air to pass up behind the timbers. And then you can add wool at the space at the front if you wish - or leave it for the electrician to use!
  8.  
    Folks Our place has granite walls, with a 3-4" air gap and then originally lath and plaster on studs, now kingspan between studs.

    The way I understand that stone walls work is totally different to modern impermeable materials.

    Moisture comes mostly from outside, as driving rain. Most drips off, but some soaks into the stone and mortar which holds it like a sponge. Any excess water that makes it through to the inside face of the stone, is drained down the gap between stone and lath, this would only be during very wet weather. This air gap is a drainage plane, equivalent to the cavity in a modern brick/block wall. Any condensation onto the inside face of the stone is absorbed or drained down the gap in the same way.

    A lot of cold dry air from outside circulates in the gap between stone and lath. This draught is important to dry out the stone on dry days. The moisture also evaporates to the outside through the lime mortar layer.

    Heat leakage is not required from the inside to keep it dry, as traditionally most of the rooms were unheated or had only radiant heating from open fires. The same construction without the lath was used for unheated outbuildings and barns. The low room temperature meant that the air didn't carry much moisture. The walls have stood 150 years so far like this.

    To reduce energy loss, it is important to stop cold air from the gap behind the lath from getting into the house, while preserving the air circulation and the drainage plane. EG filling all the gaps underneath the skirtings, between floorboards, around electrics and pipes, fireplaces etc. But no insulation stuffed behind the stud, a previous owner tried that and it got sodden as it blocked the drainage plane.

    I see no problem with replacing the lath with say 100mm of PIR and plasterboard between studs, which is the standard construction for barn conversions round here. We did that in two rooms without problems.
    • CommentAuthorTimSmall
    • CommentTimeFeb 15th 2018
     
    I was thinking along the lines of what WillInAberdeen said... Try to make sure there's no way for air to convect around the PIR board (i.e. from the front of the board to the back of it), by sealing it into the stud work with squirty foam, but I'd be inclined to have some ventilation to the face of the stone walls (unless there's a rain screen on the outside of them).
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