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    • #31461
      Anonymous

        Hi all.

        Going back some time I was posting some questions about rooflights. I have now found a supplier who will adapt their design to allow me to add insulation outside the upstand and hopefully reduce the heat losses.

        I understand that rooflights are always going to be poor performers but this is the best I have come up with so far. Even having said that there is one obvious issue with the design, where there is a single pane of glass resting on the wooden frame. I don't think I'll be able to persuade the manufacturer to do anything about this and I'm wondering whether it is a significant thermal bridge or not.

        Details are shown in the attached diagram and I welcome feedback on whether this will be good enough.

        Thanks,
        Mark.

      • #37870
        Anonymous

          there is one obvious issue with the design, where there is a single pane of glass resting on the wooden frame

          It looks like an obvious application for a strip of aerogel tape. Or did I misinterpret the diagram?

          It seems like it would just need a simple planing operation, or a change to the cutting schedule, so I'm not sure why the manufacturer would be reluctant, if they're willing to make other changes.

          Cheers, Dave

        • #37871
          Anonymous

            there is one obvious issue with the design, where there is a single pane of glass resting on the wooden frame

            It looks like an obvious application for a strip of aerogel tape. Or did I misinterpret the diagram?

            It seems like it would just need a simple planing operation, or a change to the cutting schedule, so I'm not sure why the manufacturer would be reluctant, if they're willing to make other changes.

            Cheers, Dave

            The problem is that I think they have the upstand manufactured for them so it comes in as a standard design, already coated and painted and all they have to do is cut it to length with mitred corners and bond it to the glass.

            Your suggestion of aerogel is an interesting one, but I don't think it works. Something needs to ensure the top pane doesn't lift off in strong winds, and I suspect that the spacer and sealant holding the three panes together isn't strong enough, hence the wood/glass bond directly to the top pane.

            I had thought about pushing the second and third panes out across the whole wood upstand but then I'm purely reliant on the inter-pane sealant to hold the top sheet of glass down.

            I'm now starting to think that I'm better off finding a local joiner who would put the frame together and source the glazing. I think a new diagram will be called for.

          • #37872
            Anonymous

              OK, so tackling this from the opposite direction.

              The attached indicates a design that I think would work and would just about eliminate any thermal bridge and I can't see how to make it any better.

              Any feedback on whether this is likely to work, or fall apart and whether I'm likely to be able to find someone to make it?

            • #37873
              Anonymous

                Hmm, I think I made the problem more complicated than it is.

                The problem is that I think they have the upstand manufactured for them so it comes in as a standard design, already coated and painted and all they have to do is cut it to length with mitred corners and bond it to the glass.

                So instead of planing a bit off the top of the timber, just cut the PIR a little bit taller?

                (and as an aside, the least likely thing I'd do to a piece of timber that I was expecting to bond something to would be to paint it. So I guess I'm still not understanding everything?)

                Something needs to ensure the top pane doesn't lift off in strong winds, and I suspect that the spacer and sealant holding the three panes together isn't strong enough, hence the wood/glass bond directly to the top pane.

                There could equally be a wood-glass bond to the underside of the bottom pane. The pressure only needs to be resisted once. Or at the expense of aesthetics, some kind of hold-down strap or clips over the top pane.

                I take it that this absolutely needs to be transparent, otherwise Kalwall or some other form of translucent insulation panel, Okalux etc, would be an obvious alternative.

              • #37874
                Anonymous

                  Hmm, I think I made the problem more complicated than it is.

                  The problem is that I think they have the upstand manufactured for them so it comes in as a standard design, already coated and painted and all they have to do is cut it to length with mitred corners and bond it to the glass.

                  So instead of planing a bit off the top of the timber, just cut the PIR a little bit taller?

                  (and as an aside, the least likely thing I'd do to a piece of timber that I was expecting to bond something to would be to paint it. So I guess I'm still not understanding everything?)

                  I understand where you are coming from, but in the original diagram the PIR is not fitted by the window manufacturer – they just supply the glass and the timber upstand already bonded together (everything inside the red box). If I were to ask them to fix the aerogel strip in between the timber and the glass then there wouldn't be the bond to the top sheet of glass any more, only the presumed bond at the bottom.

                  I accept that we could then try to bond the top sheet down using the PIR/external timber ourselves, but I'd be worried about the basic unit getting damaged in shipping.

                  The upstand is described as: “Solid resin impregnated timber and black plastikote finish”. The latter is effectively a paint but whether this is applied to the top of the upstand isn't obvious. The top of the timber appears black through the glass, but this could just be the sealant used to bond the two together.

                  Something needs to ensure the top pane doesn't lift off in strong winds, and I suspect that the spacer and sealant holding the three panes together isn't strong enough, hence the wood/glass bond directly to the top pane.

                  There could equally be a wood-glass bond to the underside of the bottom pane. The pressure only needs to be resisted once. Or at the expense of aesthetics, some kind of hold-down strap or clips over the top pane.

                  If we were to rely on the wood-glass bond on the bottom pane then we're implicitly relying on the spacers and the sealant around the edge of the frame to hold all the panes together. I wouldn't have thought that the sealant would be designed or strong enough for this.

                  I understand about possible straps or clips, but thought I had found an attractive and workable solution. Adding these introduces a further complexity.

                  I take it that this absolutely needs to be transparent, otherwise Kalwall or some other form of translucent insulation panel, Okalux etc, would be an obvious alternative.

                  Preferably transparent, but not mandatory as long as light transmission was high. However, I don't see how Okalux or Kalwall would help. Their thermal performance is no better, and probably worse, than the 3G panel I'm proposing, and I still need some insulated way of fitting it to the roof so that I don't get leaks. I guess it's my turn to be missing something.

                • #37875
                  Tom Foster
                  Participant

                    The first .pdf design looks no worse than any Uw1.1 Scandinavian-style 3G window – which ain't bad by rooflight standards. To improve on that, why not line the room face of the 70mm struct timber with insulation faced internally with pbd?

                  • #37876
                    Anonymous

                      If we were to rely on the wood-glass bond on the bottom pane then we're implicitly relying on the spacers and the sealant around the edge of the frame to hold all the panes together. I wouldn't have thought that the sealant would be designed or strong enough for this.

                      I'd think it would be, but I don't know. Certainly best to check!

                      Preferably transparent, but not mandatory as long as light transmission was high. However, I don't see how Okalux or Kalwall would help. Their thermal performance is no better, and probably worse, than the 3G panel I'm proposing, and I still need some insulated way of fitting it to the roof so that I don't get leaks. I guess it's my turn to be missing something.

                      I was thinking of Kalwall Nanogel which has a stated U value of 0.3 W/m2K. And I was thinking of some Okalux product that doesn't exist – I misremembered it's U value, sorry.

                    • #37877
                      Anonymous

                        Preferably transparent, but not mandatory as long as light transmission was high. However, I don't see how Okalux or Kalwall would help. Their thermal performance is no better, and probably worse, than the 3G panel I'm proposing, and I still need some insulated way of fitting it to the roof so that I don't get leaks. I guess it's my turn to be missing something.

                        I was thinking of Kalwall Nanogel which has a stated U value of 0.3 W/m2K. And I was thinking of some Okalux product that doesn't exist – I misremembered it's U value, sorry.

                        I just looked again at Kalwall nanogel. Yes it has a U-value of approximately half that of a 3G panel, but has a light transmission of between 12 and 20%, which is much worse. For the equivalent light transmission I'd need much bigger panels and the heat losses would also probably be worse as a result. I recall looking at this a year or so ago, and dismissing it then for the same reasons.

                        It still wouldn't get around the need to have a well insulated upstand with minimal thermal bridges.

                      • #37878
                        Anonymous

                          The first .pdf design looks no worse than any Uw1.1 Scandinavian-style 3G window – which ain't bad by rooflight standards. To improve on that, why not line the room face of the 70mm struct timber with insulation faced internally with pbd?

                          This would be an option, but it means the insulation isn't “joined-up” and doesn't feel a very elegant solution. Still leaves the thermal bridge through the timber, but it is admittedly a long path.

                        • #37879
                          Anonymous

                            For the equivalent light transmission I'd need much bigger panels and the heat losses would also probably be worse as a result. I recall looking at this a year or so ago, and dismissing it then for the same reasons

                            Ditto, but sadly I didn't remember before posting. Oh well. Are lumens per watt the appropriate figure of merit, the same as for lamps?

                            Tom's idea of a bit of insulation inside the timber does seem to make sense, though.

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