I have two Velux skylights (flat, approx 140 X 60 and double glazed and 1.2 m apart from each other ) in a cathedral ceiling angled approx 15 deg and facing north. They are great for admitting light and fine in winter but in warmer weather cause serious overheating problems in our dodgy passive solar 80's house. Our roof is already badly designed (something I've come to realise reading other posts here) as the depth is insufficient and the (old) insulation is packed in with sissilation touching the tiles above (dark terracotta).
The skylights came with reflective internal blinds which are fine for reducing glare but of course do basically nothing to reduce the radiant heat. I experimented with draping shadecloth over them which seemed to do very little if anything. I guessed part of the problem was that the shadecloth was in direct contact with the glass, so my latest effort is two layers of shadecloth separated by a sheet of laserlight roofing that I had lying around, so the two layers of shadecloth are not touching. I have also tried to shade the skylight surrounds themselves which are metal.
I have borrowed an infrared non-contact thermometer and have measured the surface of the internal blind as high as 40 degrees on a 32 degrees day, this being after my shading efforts. For comparison, the unshaded roof area with tiles is around 26 degrees if I measure the pine lining inside. So the skylights are like having two radiators in the roof.
To compound problems I've noticed that the skylight surround (pine) inside the house also gets very hot. When I checked on the outside I realised that while all the flashing is done well, there is basically no insulation other than some small pieces of foam around the skylight, and even these don't seem to be all the way around. I believe the problem is predominantly radiant heat but I think there is hot air getting in as well to an extent, and no doubt the terracotta tiles are baking as well.
I am now thinking the best solution might be to forgo the idea of allowing some filtered light in (in any case I have been leaving the internal blinds shut as well for what little they will do) and just going full on with something full block.
So I guess my questions would be:
1. Would it be best to try and protect the skylights from radiant heat with something highly reflective, or use something like timber to surround and cover them?
2. What is the best material to fill in around and under the flashing to seal up the skylight better from hot air coming in - e.g polyurethane foam pieces? I don't want to go the expanding foam path as that may overexpand into the skylights themselves.
I also wonder how much of my problems are due to the realities of putting glass in the roof and standar, vs how much is poor design/installation. We do have a glass roof in the same room and once shaded that was measuring in at 29 deg late afternoon on a 32 degrees day. So I want to be realistic but hoping I could get the temperature of the skylights down to something like that.
In retrospect I think it was definitely a mistake to install 2 skylights 1m apart (my idea I'm afraid, before I realised how the priority in this place is keeping it cool) so another option I wouldn't rule out is removing one of them altogether. But that might have to be something that waits until (or if) we ever re-roof with colour bond.
Any suggestions appreciated!
<edit> Heat loss in winter through the skylights isn't really an issue, as the passive solar features of the house (multi level slab with slate floor, double brick) work well for admitting and storing heat.
