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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

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Fundamentals: Skylights, roof windows and clerestory skylights

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    • CommentAuthortony
    • CommentTimeJan 11th 2016
     
    Roof windows seem to cool rooms too much.

    U values assume steady state and an indoor and out door temperature

    But skylights can radiate heat to outer space which is at absolute zero.

    This heat loss dominates on any clear night.

    Should heat losses from these be reassessed?
    • CommentAuthorSilky
    • CommentTimeJan 11th 2016
     
    I would always have fitted blinds with Velux stlye windows ( I have 7 in a 1 bed open plan roof flat ) and close the blinds at night to prevent heat loss. I've just checked and apparently the Velux ones prevent '34%' heat loss.. whatever that means, I guess radiant as they are so thin. You can insulate more by fitting roller shutters on the outside, but they are expensive. My advice when designing would be not to go too mad, you don't want too much glass in the roof and I find the previously mentioned 7 windows too much. I'm thinking of losing the biggest one as I never open the blind, it causes too much overheating in the summer
    • CommentAuthorEd Davies
    • CommentTimeJan 11th 2016 edited
     
    Isn't there already an increment to the U-value for roof windows (0.1 W/m²K ???) to take this into account? I think I read somewhere that the values Velux quote already take that into account, but I'm not sure.

    To weigh against the increased heat loss per m², roof windows also let in more light so a smaller area is possible.

    Posted By: tonyBut skylights can radiate heat to outer space which is at absolute zero.
    Well, about 2.7 K, not that that makes much difference from a building perspective.

    More substantially, the effective radiant temperature of even a clear sky is a bit warmer than that though still pretty chilly at something like -30 or -40 °C. Obscure thing often referred to as the greenhouse effect. Anybody hear of it? Not that the UK winter sky is clear that often, particularly in winter when it matters, so usually it's effectively a bit warmer.

    Vertical windows usually see at least a bit of sky, too.
  4.  
    If the skylight outer pane of glass is reflective or tinted how does that affect any loss by radiation if at all.
    • CommentAuthordickster
    • CommentTimeJan 12th 2016
     
    We've gt a 3m rooflight on the ridge with the expected heat loss. It's only 2g with no blinds, but it's a lovely feature. The point of posting is to mention the unexpected circulation of air, up from the WBS, along the underside of the ridge, down from the rooflight and back towards the stove. A very gentle but suprisingly effective air stirrer. Who'd have thought it?
    • CommentAuthorEd Davies
    • CommentTimeJan 12th 2016
     
    Posted By: PeterStarckIf the skylight outer pane of glass is reflective or tinted how does that affect any loss by radiation if at all.
    Depends - the reflectivity or tint might have a lot of effect at short wavelengths (e.g, visible light and near IR) but no effect at longer thermal IR wavelengths.

    I suspect it wouldn't make much difference - if there was a coat that could be put on the outside or in the outer pane that reduced the thermal IR emissivity significantly they wouldn't bother with expensive and (somewhat) delicate coatings on the internal faces.

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