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    • #31769

      Searching for a 'passive' roof window yet yesterday I came across Roto roof windows from Germany.

      Their R8 roof window claims a Uw of 0.86 for the timber version and 0.84 for the uPVC (Ug is 0.56). The installation detail looks significantly better than anything I have seen previously in terms of Psi install value too – see attached detail. The window is double sealed and has an air tightness classification of 3 to EN12207 (3 to 9 m3/h-m2 at 100Pa – double passive house test pressure). Although this is not ideal, we have achieved good test results with class 3 windows in the past where there were just a couple of this type and the majority perform better.

      Has anyone out there used a Roto R8 yet? Or made an accurate assessment of a typical Psi install value?

      Is there anything with better performace out there?

      I have made enquiries with the manufacturer for test information and will share any further info gained.

    • #38808
      David Olivier

      Hello Peter

      I suspect that if you check with the mfcr. you will find that the U-value has the product in a vertical position!! This is just one of the bizarre assumptions that European companies get away with.

      Rooflights seem to be so poorly-insulated that for my house I made my own. It's modelled on the two-part construction used in the 1st Canadian Advanced House in Brampton, 1989. In Scandinavia it would be described as 3+1 glazing.

      I think the NFRC in the USA has published some true in situ U-values for sloping glazing in their climate, under design conditions, as has the ASHRAE Handbook. Needless to say they are worse than European quoted figures.

      I suppose it might still work out, inputting the true U-value of this product, and benefiting from the gains of natural light, but there could presumably be a downdraught if the U-value including the installation psi-value is too high.

      So far I haven't detected a downdraught from mine. I'm writing a book on the house and will probably give the details in there.



    • #38809

      Hi David

      Thats interesting, I hadn't realised the U-value would be different when sloping. I have now read up to 1/3rd less performance when sloping.

      I have a had a little look at what happens in PHPP when you change the angle of inclination in the windows page and it increases the solar gain but does not automatically increase the heat loss. So it would appear that we either need to have data from tests carried out on a slope or to make some adjustment (reduction) in u-value for sloping roof windows? I suspect you are right though and I wil struggle to get sloping test information.

      I have also found a Fakro window that claims to be the best currently available the FFT Thermo with U8 quadruple glazing, claimed Uw 0.58, Ug 0.3 and class 4 airtightness with quadruple seals. It also appears to have a better frame and flashing installation detail than the Roto in terms of Psi install – see attached.

      This one looks like it should work to me. I will ask for test data but any advice on how to model / calculate / make allowance for the reduction in U-value would be really useful.

      I don't have a specific project in mind at the moment but it really should be possible to get roof lights into passive houses…

    • #38810
      Nick Grant


      PHPP can't change the Ug with roof angle But it does flag up a warning to check the Ug if the glazing is not vertical.

      For standard triple glazing that would be 0.6 U vertical I use about 0.9 in sloped position. As David says you need to consider whether any downdraft of cold radiant will cause comfort problems but modest sized skylights should be fine I think.


    • #38811

      Passivhaus certification assumes 45 degree inclination

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