Has anyone got any suggestions as to what is the best way to install roof lights to reduce thermal bridging? I'm looking at triple glazed units but don't want to waste my money if the frame is not efficient enough.

Nothing in there about thermal bridging that I can see

I set the rafters back an additional 100mm and then fill the void with high quality sheet insulation, there will still be some bridges through the frame though.

You can DIY and make the glass 500mm bigger than the hole and get out of almost all the bridging -- have done this for green roof and made them open too.

<blockquote><cite>Posted By: tony</cite>You can DIY and make the glass 500mm bigger than the hole and get out of almost all the bridging -- have done this for green roof and made them open too</blockquote>

hi do you have any pictures of this? does it open outward? how have you attached an opening mechanism? (etc)

thanks

if we're talking Velux/Fakro, the deeper you can set them, by 'recessed' or low-rise flashing kits (Fakro have 3 height options to Velux's 2), the better. The flanking loss thro the upstanding frames is shocking, prob makes nonsense of the new 3G option. We need a new design, where it's either deepset to or beneath the slate/tile surface, or else substantial insulation comes up the sides and across the edge of the top of the velux/Fakro. It's possible to slide a bit of Aerogel in between the timber upstand face and the flashing upstand.

Hi,
So is it worth paying the extra for the Fakro & Velux 3g version, has anybody handled one? fitted one?
Cheers
Mike up North

um
So that’s - yes, no maybe, depends then. I agree when you look them like that, the sticking-outness is inviting thermal losses, apart from being ugly as well. I costed flush non opening units and it was horrendous. There is only one required now, question is, whether it’s worth upping to a higher spec unit or not.

I can calculate values, its interpretation of them against some or other criteria that’s a problem, even if the criteria are fixed (which it usually isn’t).

So putting it another way (for example).
Need one to be fitted, current joiner will do std one, as has some already (probably from another job) at the going price in 3 days time whilst on site.
Or
Pay about + Ă‚ÂŁ100, order through merchant, 6 weeks delivery time and the joiner will be gone so probably another will have to do it on a quote and the first joiner gets the hump because the second one is messing with his roof (despite the fact he too busy to come back 6 weeks later to fit it).

The pragmatic member of our family will go with the former to get it done but usually leaves me feeling like we have had 2nd best for somes elses convienience, I’d go with the latter as I’m always trying to squeeze the best out of things (but I didn’t think we needed an extra window anyway).

Btw - The Uw is 1.4 and 0.94 respectively (centre pane is 1 and 0.5) according to the sales blurb.

So what would you do – this is a real situation. Can’t widen the fitting space to take more insulation. Joiner said the bog standard unit must be ok because he’s fitting them all the time (??), but that’s just more of the usual “its good enough”, “it’ll do” philosophy. They all think I'm barking anyway by fussing about with details to maintain air tightness/counter act bridges etc, Ive already spent ages making up cardboard templates so as to cut accurately thin ply to fit around rafter/hanging posts connect to the knee-wall skeilings (sp), so as to be able to seal and hold insulation. So the extra hassle for me isnt a problem, and probably wont surprise others. Just trying to get an opinion on these 3g roof windows. I prefer the Fakro at the moment but not for any particular reason.

Cheers

Mike up North

<blockquote><cite>Posted By: fostertom</cite>We need a new design, where it's either deepset to or beneath the slate/tile surface, or else substantial insulation comes up the sides and across the edge of the top of the velux/Fakro.</blockquote>

is your idea to use a recessed light with an additional pane flush with the roof?

Posted By: Viking HouseI read this post with interest and I think I may have what you are looking for, I was in the same predicament last year trying to build a Passive House with rooflights. The Velux and Fakro U-values were not the detrimental factor but the fitting factor in the PHPP is what gave them major losses. They were losing 5+ times more heat than a standard window because they are fitted so far away from the insulation line.

This led me to invent an un-openable rooflight that is being passed by the PassivHaus institute, we've used it in 3 Passivhaus projects already. It has a single external glass to throw off the rain and Krypton filled triple glazing inside at the insulation line with the edges wrapped in insulation. It shows no frame as the plasterboard wraps around the latts that hold up/in the triple glazing. The plasterboard inside lines up with the plywood between the glasses to give nice crisp frameless lines, the external glass is oversized by 100mm all round to throw off the rain, the upstands externally are lined with zinc. The internal void is ventilated to prevent condensation on the outer glass.

I hadn't really appreciated the impact of fitting rooflights before this thread and had started to change my mind about whether this was sensible.

However, this proposal sounds very interesting and could be an elegant solution for my house. Do you have any pictures?

what are building regs for unopenable roof lights? i understand they don't allow fixed windows 1st floor+.

Posted By: fostertomWithin that perimeter, the wood and glass bit can be set as high or as low as you like, relative to the roof plane.

hmm er where does the water go when it falls on glass that is below the roof line?

Posted By: Viking HouseThis led me to invent an un-openable rooflight

i take it you can open the lower window for cleaning?

Posted By: spoonandforkwhere does the water go when it falls on glass that is below the roof line?

It's still higher up than the bit of the roof that's downslope from it, so a lead-out 'ramp' does it

Posted By: fostertomand that has to drain out at bottom (again, can differ depending on pitch)

ah, perhaps the salesman misinformed me (presumably he doesn't sell fixed windows)

re: setting back the joists and setting the rooflight deeper to reduce thermal bridging, is this drawing what you mean? where the lower one is better. The rafters are outlined in blue, the roof is corrugated.

that sounds good but quite involved to design and install if you want the window to open. I think I'll try to install a standard rooflight as best I can, then have secondary removable glazing underneath if necessary (and remove it in summer, as mentioned above). However, it seems to me that the lower diagram in my above illustration would not be significantly better than the upper and wouldn't warrant any extra effort required to do it.

my first drawing is supposed to represent a typical installation so shouldn't leak. I'm just wondering if setting the window deeper into the building (with a smaller upstand) and setting the rafters further away from the window makes any significant difference. This is a picture from fakro which is clearer. Yes I understand using a unit that oversails is better, but that's beyond most people's DIY skills if you want it to open.

ok, it just seems that increasing the width of red bit would have a negligible effect in the above situation as the heat can pass through the rest of the frame (it's a short route from warm to cold just above the red bit)