Go to Forum Home Materials and products Insulation between rafters, was in Multifoil Insulation

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    • #30516
      Anonymous

        David – is there an advantage to using rockwool between rafters rather than kingspan/celotex? And when you say polyethylene between the lowest layer of insulation and the plasterboard, do you mean simple DPM? Does filling gaps with expanding foam and taping all joints, then a final plaster skim still leave significant airtightness problems?

      • #32990
        Anonymous

          That's the method I normally use – but I'm always interested in alternative techniques, and particularly interested in the detailing of polyethylene sheeting above the plasterboard – I've never heard of that and would like to know how it works. How is it fixed without causing an airtightness breach?

        • #32991

          Typical thermal conductivities since PU foam began to be made with ZODP gases are:
          1. Mineral fibre generally 0.036-0.038 W/mK
          2. PU foam around 0.025 W/mK

          U-values for a 100 mm slab:
          Mineral fibre 0.37 W/m2K; PU foam 0.25.

          The gain in U-value will be <0.12 when thermal bridging effects are factored in.
          The gain in U-value will be far less than 0.12 if an insulating material is also used over or under the rafters.

          The extra cost may be of the order of £400-500 in a modest house, making it an expensive measure in £ per kWh saved. One argument for energy efficiency has long been that properly-planned and -costed measures are a lot cheaper than energy supply. But this measure isn't.

          David.

        • #32992
          Anonymous

            That completely makes sense – it's bloody laborious cutting kingspan into each joist. But for most retrofit loft conversions, the rafters are only 100 thick, and the upper 50 of that has to be left to allow airflow – is 50mm rockwool enough?

            And what's the detail of the polyethylene sheet? DPM fixed with clout nails and the overlaps taped? If so, I find it hard to see how that's more airtight than filling joints in the underlayer of insulation with foam and then taping them – and surely the extra fixings act as mini cold bridges?

          • #32993
            Anonymous

              I'm still unclear on the detail of how to fix it. Clout nails? If so don't they compromise its airtightness?

              When you say 'turn the sheet down all around the edges', you mean leave excess poking down, which can then be sliced off just before the walls are skimmed?

            • #32994
              Nick Grant
              Participant

                Dan

                I used stainless steel staples (steel should be fine but stainless can be used for other things) and made sure any joins were over rafters or noggins. The staples just hold the sheet in place so plasterboard can be fixed over. Advantage is one hand is free for holding membrane out of your face. Plastic ackaging strapping (the stuff you use to break into cars!) can be used to reinforce the membrane where staples go through. I'd rely on mechanical seals rather than tapes and glues, ideally both.

                IMHO the weak point is not the staple holes but any joints to wall etc. Purlins are an 'impossible' airtightness detail for retrofit. For new roofs you can drape strips of plastic or Sizalcraft over them and then join to the rest of the membrane later, same for visible trusses etc. Perhaps rafters can be cut free, strip inserted then re-nailed?

                As Neil says avoid any services penetrations but if you must have say a vent pipe or solar water pipes going thru' then I'd suggest a panel of, e.g. OSB or ply behind the membrane. Can then seal the pipe to the ply and trap the membrane onto this.

                Forget trying to seal electrical back-boxes and cables – surface fix or batten out a services void between membrane and plasterboard.

                Another challenge is screwing up the plasterboard without peppering the membrane with holes!

                Nick

              • #32995
                Nick Grant
                Participant
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