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  1.  
    After going over my fabric heat loss figure of 3474W (-5 outside;inside 20C) I am looking to improve insulation in every last area possible (NB: this is not a new build, but rather extensions and renovation). In one of the SIPs extensions I have 175mm SIP panel (with 153 PU foam sandwiched with 11mm OSB) and wondering whether to add a further 25mm PIR in any redundant service voids internally.

    Currently my u-value is 0.138:

    18mm WRC cladding/ with 25mm void / breather membrane / SIP panel / VCL / 25mm service void (battened) / double plasterboard 24mm

    So my plan was to add 25mm PIR into the service void where I won't have cabling or any future need for cabling.

    This will bring down the u-value to approx 0.124. I don't want to lose any additional interior space by going with thicker PIR and also realise that will move inwards the dew point. I can't add insulation on the outside as the WRC is already up. Critical area is of course around the sole plate and that will have an insulation 'skirt' around the exterior base/slab area as in the attached detail.

    Am I asking for trouble wrt condensation risk if I now start adding the 25mm insulation internally?
      Floor-slab detail (1).jpg
    • CommentAuthortony
    • CommentTimeMay 7th 2018 edited
     
    No risk of condensation
  2.  
    Putting the additional insulation as you suggest will improve your u value by 0.014 which at a delta of 25 deg gives 0.35W or 8.4W over 24 hours (per M2)

    What cost of installation over ROI. At what point does diminishing returns kick in?
  3.  
    @Tony 'no' to what? i.e. no risk or no to installing?

    @Peter - thanks for the Watts. That's 840W then per day saved. 12p (ish) per day saved.

    I have some spare 25mm offcuts that I can squeeze into the voids, which is easy enough to do but getting the detailing done will be the fidgetty side of things (sealing all gaps to try and create maybe a 2nd VCL layer. So probably not worth it in terms of ROI but perhaps from a 2nd vcl perspective I should proceed.
  4.  
    Posted By: kentishgreen@Peter - thanks for the Watts. That's 840W then per day saved. 12p (ish) per day saved.

    Err - not quite. An decrease of 0.014 u value at a delta of 25 deg is 0.014 * 25 = 0.35W , 0.35W over 24 hours is 8.4W so your saving is 8.4W per day per M2.
  5.  
    Ermm... 0.35Watts over 24 hours is 8.4 Watt-hours, which is 0.0084 kWh saved per day per m2, but only on the coldest day of the year.

    Over a year (2000 degree-days depending where you live), reducing U by 0.014 W/C saves you 0.014 x 2000degreedays x 24h/d / 1000 = 0.7kWh which is about 10p of electric
  6.  
    Posted By: Peter_in_Hungary
    Posted By: kentishgreen@Peter - thanks for the Watts. That's 840W then per day saved. 12p (ish) per day saved.

    Err - not quite. An decrease of 0.014 u value at a delta of 25 deg is 0.014 * 25 = 0.35W , 0.35W over 24 hours is 8.4W so your saving is 8.4W per day per M2.


    Should have mentioned my area there (approx 100sqm) so 8.4W = 840W per day (approx 12p) for those few days here in the SE that there's a delta of 25C.
  7.  
    Think we're getting tangled by the units, try to stick to kiloWatt-hours.

    0.014 W/m2C x 25degC = 0.35W/m2

    0.35W/m2 x 100m2 x 24h / 1000 = 0.84 kWh

    saving 12p of electric.

    100m2 of 25mm insulation is about 45 sheets at £20ea so £900 materials, plus labour to cut and fit... may take a good while to pay back....

    Is your void exactly 25mm wide? Planed 1" battens are usually about 22mm .
    • CommentAuthorborpin
    • CommentTimeMay 8th 2018
     
    Whatever the ROI, I put 25mm internal insulation on my SIPPs and I've had no issues with condensation over the last 4 years. I do have a whole house MHRV system though.

    If you only looked at ROI, you'd probably not do half the things folk here do (which is why the commercial big boys don't unless forced to and then they do it badly). It is the theory of marginal gains - not just for you individually; but collectively.
    • CommentAuthortony
    • CommentTimeMay 8th 2018
     
    A MHRV will stop internal condensation but will not stop interstitial condensation

    There is a small increased risk of this to the wallplate area but in the op’s case this has already been mitigated
    • CommentAuthorSigaldry
    • CommentTimeMay 8th 2018
     
    The U-value is fairly irrelevant - what it will do there is mask the heat loss through the sole plate, reducing the thermal bridging
  8.  
    Posted By: SigaldryThe U-value is fairly irrelevant - what it will do there is mask the heat loss through the sole plate, reducing the thermal bridging
    Plus all the other Cold Bridging areas in SIPs panels like structural timbers in each panel (similar to standard Timber Frame), where each panel meets, around windows and window heads, wall roof junctions and as mentioned by Jon the double sole plate.
    • CommentAuthortony
    • CommentTimeMay 10th 2018
     
    Which unlike the other wood sits on cold blockwork ...
    • CommentAuthorSigaldry
    • CommentTimeMay 11th 2018 edited
     
    I was just focused on the bit highlighted yellow in the service void next to the soleplate shown on the section drawing.

    Obviously masking any bridging of an insulation layer, with an internal or external layer of insulation is great for reducing cold bridging as well as the overall wall U-value, but the sole plate is one of the poorest performing junctions and any timber frame detail or SIP detail masking that is good for reducing the wall/floor junction losses.

    (The other typical poor performing junction is lintols, I've seen some good insulated ones recently where a thermal break is used between the inner and outer leafs, but insulating internally and into the reveal would also be good).
    • CommentAuthortony
    • CommentTimeMay 12th 2018
     
    I would put the marmot under the middle of the wall plate, run the internal insulation vertically downwards , fill any voids next to the marmot with eps.

    Even do I think there will be a condensation risk on top of the dpc under the wall plate.

    I like the ides wrt lintel bridging.
    • CommentAuthorPeterStarck
    • CommentTimeMay 12th 2018 edited
     
    Posted By: tonyI would put the marmot under the middle of the wall plate, run the internal insulation vertically downwards , fill any voids next to the marmot with eps.


    That's just plain cruel!
    •  
      CommentAuthordjh
    • CommentTimeMay 12th 2018
     
    I don't understand the design in the drawing. There's a cold slab on top of the piles. OK. Then there's insulation on top of that and finally a presumably warm conductive screed. But alongside that, there's a pile of cold blocks next to the insulation, then some insulating blocks alongside the screed, and then some timber. So 'cold' can flow up the cold blocks, around the corner into the screed and then around the corner into the timber. The 25 mm PIR masks the upper of those corner flows somewhat but it seems like a bolt on fix to a basic poor design. Why not start with a good design?

    The insulation needs to extend underneath the wall somehow. Either make a passive slab type construction on top of the cold slab, or extend the wall downwards instead of using blockwork. Or use insulating blockwork of some type. Or some other way of insulating that junction more substantially.

    BTW, you say you won't have any requirement for cabling at the base of the service cavity, but how are you going to run power to your sockets?
  9.  
    ''That's just plain cruel!''

    Thanks Peter! That's saved me a job! :bigsmile:
  10.  
    OK, so where do we put the EPS? :bigsmile:
      Marmot.jpg
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