Flat roofs, celotex and downlights...

[Octopus]

Got an extension to fit down lights into - can anyone please point me in the direction of the latest regs and guidance regarding this thorny issue

Thanks

 

[stummish]

I've asked a couple of building inspectors about this. If you take the whole roof area and m2 and calc the u value, then minus off the total u value lost by cutting 100mm holes for downlights at (as a rough guide) 1 every 1000mm on x and y axis, the total uv lost is almost nothing.

I normally have a chat to the builder and try make sure there's a minimum 70mm gap between plaster and insulation, then cut out 100mm hole but only 20-30mm deep into the kingspan. Don't remove at this stage. On 2nd fix if I need the space I'll then knock out the cut section. I almost always fit the collingwood halers h2 pro 700's so you need less fittings per m2, they're only 60mm deep so usually they have enough breathing space. Plus integrated led dls dissipate their heat better than canned gu10s. And if all else fails cut straight through the kingspan. Get the building inspector to calculate the lost uv to prove it's negligible loss.

Also the seal fairy's cousin, the kingspan fairy, may appear if you're lucky!

 

[Midwest]

I had a similar installation a while back. Phoned my LBC and was told in no uncertain terms, not to cut holes in the celotex/kingspan.

A while later, had another similar installation, this time emailed LBC and got the response that stummish & Jim received, in this case the customer was having velux installed, and was given them as an example, effecting the U value.

Then the BO turned up to inspect part of the extension, and the subject of DL in the Celotex was mentioned, and we were told we should not. I showed the email advice, and he back tracked somewhat. In the end, my customer decided not to bother.

Clearly with all the wonderful manufacturers pictures you see of DL's in vaulted etc ceilings, its allowed, just getting LBC to commit.

So Murdoch, I would get LBC approval in writing, before proceeding.

 

[stummish]

As @Midwest says, getting it in writing will always win the he said she said battles.

 

[Pat H]

Yes the effect on a few downlighters on the whole ceiling U value is negligible. But you will end up with cold spots round and of the DLs and depending on the design of the DL you may well punch a draught hole straight through from the air gap to the room.
Its a real issue. I've tended to stick with surface mount lights and just avoid it.
But I'd agree its down to what the LBC will sign off.

 

[Midwest]

Think there is an issue with damp forming by warm air going into a cold air designed roof void (not the correct terminology) thru DL, but I was told by my LBC these roof designs are not very common.

 

[johnduffell]

Strictly speaking you have to worry about airtightness and limiting u values as well as the overall u value, which will be fine.
The limiting u value is about 0.8 for ceilings and is there to prevent black mould on cold spots or interstitial condensation, hence why you always get some insulation below or above the joists. But even 25mm of celotex would be fine for that, or a velux window would be OK.

The airtightness is just avoiding the heat bypassing the insulation, otherwise it's a bit like walking around wearing a down jacket but forgetting to zip it up. Or leaving an open velux window. That's the important one for heat loss and loft condensation.

Hope that makes sense! But the building inspector isn't trying to be difficult.