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Fundamentals: Skylights, roof windows and clerestory skylights

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1 to 26 of 26

  1.  
    • 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.
  7.  
    On a north facing single storey extension say 10m x 4m what would be the best roof and glazing setup?

    We were thinking of no windows in the roof itself but maybe clerestory windows on the east and west gable and above he rear glazed doors.

    Any thoughts on this or will it become a dark area with no Velux or roof lantern?
    • CommentAuthorMike1
    • CommentTimeMay 29th 2026
     
    A disadvantage of single storey extensions is that the old part of room(s) that have been extended don't get as much light; skylights are a good way of mitigating that and triple glazed ones are available, for a price.

    It may also be possible to incorporate north-facing clerestory window in the new roof close to the old rooms, depending on the roof design.

    Due to solar gain I'd be cautious about any significant west-facing glazing that doesn't have a brise-soleil, unless it's going to be shaded by trees / a pergola / other buildings.
    •  
      CommentAuthorfostertom
    • CommentTime7 days ago edited
     
    Horizontal overhead brise soleil won't help with west facing glazing - from the west the sun angle is low so gets in under any overhang. Only useful for SW to SE elevations. Vertical micro-screens work for west/east.
    •  
      CommentAuthordjh
    • CommentTime7 days ago edited
     
    Posted By: VictorianecoOn a north facing single storey extension say 10m x 4m what would be the best roof and glazing setup?

    We were thinking of no windows in the roof itself but maybe clerestory windows on the east and west gable and above he rear glazed doors.
    A lot depends on what you will be using the extension for, and how the space is separated from the rest of the building, and whether you fit shutters and/or blinds and ....

    PHPP or similar will be able to model the heat loss and gain from various designs and give you more insight. FWIW, our sun room has a quadruple-glazed rooflight with polycarbonate glazing that has worked well for eleven years now. We bought it from Lamilux. Another thought: LED lamps mean that natural light is not necessarily as important as it once was. Careful siting of daylight LEDs can supplement real daylight if required.
    • CommentAuthorMike1
    • CommentTime7 days ago edited
     
    Posted By: fostertomHorizontal overhead brise soleil won't help with west facing glazing
    +1. It needs to be a façade (vertical) brise soleil:
      BS.png
  12.  
    Is there a set % of window area to floor area or wall area that we should aim for in a north facing extension?

    And if we had a clerestory window along the roof itself, is there a calculation or dirndls to work out where or how much?

    Thanks
    •  
      CommentAuthordjh
    • CommentTime7 days ago
     
    Posted By: VictorianecoIs there a set % of window area to floor area or wall area that we should aim for in a north facing extension?
    The short answer is no. But any north-facing glazing carries a big penalty in PHPP. You're better adding east or west facing glazing instead. I think you'll need to provide a lot more information about the use and size and position and external environment of the space to make sensible suggestions.
    •  
      CommentAuthorfostertom
    • CommentTime7 days ago edited
     
    Posted By: Mike1It needs to be a façade (vertical) brise soleil:
    True, but apart from that example which obviously exists, vertical is not really feasible because it relies on restricting field-of-view extremely, so the lo-angle sun is preventing from penetrating except for a brief, say half-hour period. Late afternoon summer sun is very damaging to overheating - you've succeeded in keeping direct sun out all day, so the interior fabric is still quite cool, then heat blasts in, giving you a hot night.
    •  
      CommentAuthordjh
    • CommentTime7 days ago
     
    I considered the possibility of insulated external shutters when we were building. External shutters are quite common in France and other countries as are external roller blinds.
  16.  
    Hi djh

    It's essentially a 4m outward by 10m rear extension.

    We were thinking of making this area a 3.5m high internal ceiling and then a low profile sheet roof. We were thinking of 2no x 4m wide sliding doors on the north elevation to bring the garden 'inside:

    But as we were thinking we need to get light into the main area of the house we either go for Velux or lantern. Or was only then I read up about clerestory windows I thought this would be a good avenue as we are detached but the view from the sides aren't anything to write home about, so thought high level windows on east, west and north could add light and depth.

    I did read CIBSE suggest no more than 40% of wall area should be Windows in this context
  17.  
    We have three huge Velux windows in our dormer bungalow (approx 1.2 x 1.2 metres each) and the heat gain in the summer is incredible. The bedrooms reached 32C during the recent heatwave. The heat gain is the main reason I would never buy a dormer bungalow again, at least not ones with Velux windows as opposed to "proper" dormer windows.

    The other disadvantage is the noise when it rains. Some people love that apparently (!) but for me it's a no-no. Thankfully we sleep downstairs - the upstairs bedrooms are for visitors!
    •  
      CommentAuthordjh
    • CommentTime6 days ago
     
    One other thing I might mention if you do go for clerestory windows is flyscreens. Cross ventilation from east to west (or vice versa) is very effective and flyscreens make it very usable. We have tilt and turn windows with external flyscreens, and top hung (external opening) windows with internal flyscreens. The tilt and turn flyscreens just work; the tophung ones require manual intervention every time because of the need to access the window latch.

    The design spec for our sun room was "must get over 30°C as often as possible", and the rooflight is a big part of the reason that spec is met. It also opens electrically (and has a flyscreen) so helps reduce the temperature when required.
  19.  
    In England there is a newish Part O of Building Regulations about overheating (sorry don't know what applies in Wales)

    It specifies a maximum window area to limit solar gains, there is a table depending if it's north facing and whether there is through ventilation from the opposite side of the building, and location, maximum is around 18% of floor area. Unclear if different for roof windows.

    There's also a minimum area that must open to allow shock ventilation, minimum is around 9% of floor area but again a table with lots of factors.


    As alternative you can submit an analysis in software, but probably needs certified consultant etc to submit that.


    https://assets.publishing.service.gov.uk/media/6218c5aad3bf7f4f0b29b624/ADO.pdf



    Also ofc the more window area you have the harder it is to meet winter energy target in building regs SAP EPC
    • CommentAuthorMike1
    • CommentTime1 day ago edited
     
    Posted By: fostertomvertical is not really feasible because it relies on restricting field-of-view extremely
    Don't let the French know! They must have been getting it wrong for the last few hundred years :)
      shutter 2.png
    •  
      CommentAuthorfostertom
    • CommentTime1 day ago edited
     
    Thet's horizointal, not vertical. When the sun's only say 15o elevation, you can't block it with horizontal louvres, unless you almost completely close them. In the Mediterranean, that's mainly OK because it's usual to keep rooms dimly shaded for coolth, in summer.
    • CommentAuthorMike1
    • CommentTime1 day ago
     
    Posted By: fostertomThet's horizointal, not vertical.
    The point is that a brise soleil facing west needs to be parallel with the facade; I should have made that clear in my initial post. The slats, if any, can run horizontally or vertically as long as they are / can be angled to block the sun; sliding / folding screens and shutters are also used - after all, brise-soleil just means 'sun break'. If you were ask someone in France to draw you a brise-soleil, it's likely that they'd pick one of these types, rather than the horizontally projecting variety, as they're so common on traditional buildings and - thanks to Le Corbusier* - on quite a lot of modern ones in France too.

    *see his Gustavo Capanema Palace: https://www.moma.org/collection/works/82483
    •  
      CommentAuthordjh
    • CommentTime1 day ago edited
     
    That looks like it has a set of external horizontal brise soleils over each window, and apparently with the refinement that they can be adjusted, like a Venetian blind. But it's not relevant to the discussion of east and west facades, since it's north facing (in the southern hemisphere).
    • CommentAuthorMike1
    • CommentTime1 day ago
     
    Posted By: djhit's not relevant to the discussion of east and west facades
    You're right, we're getting off track. Le Corbusier was just the first modern architect to start using the concept instead of letting people fry behind glass.
    • CommentAuthorMike1
    • CommentTime1 day ago
     
    Returning to the main subject, when I mentioned this...
    Posted By: Mike1It may also be possible to incorporate north-facing clerestory window in the new roof close to the old rooms, depending on the roof design.
    I had this sort of thing in mind:
      Example.png
    •  
      CommentAuthordjh
    • CommentTime1 day ago
     
    I don't think PHPP would like that. Increases the surface area and has north-facing glazing too!

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