Velux, and insulation round light tunnel

7 Sep 2018
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United Kingdom
We have had a velux put in, nd carpenters coming to make tunnel. However we have to put the insulation around it. This has gone into a lean to pitched roof. Building control want to see the insutlaiton and have mentioned 140mm,

We can’t get our head around this. Do we put 140mm of rockwool up on the sides and tack onto the wood, or use masking tape to fix, then plasterboard over the top, slotting into the grooves? Or do we use that Celotex stuff, confused,

Help appreciated
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The lightwell will need to be insulated to current insulation
standards ie. Celotex/kingspan
Thanks, I think we know that. BC are saying 140. From what we can tell people put it around side below the plasterboard, and Stuff itin. Wondered if any more tips on it,
If they are classing the sides as a wall then 140 of wool may be enough and would be a lot cheaper then celotex. The main thing is keep it continuous and avoid gaps where heat or air can travel through to the cold side.
High up in the ceiling would be the worst place for thermal bridges as that's where damp air will rise up to and condense. You wouldn't want to end up with condensation and theremould up.
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Well you don't use masking tape to fix insulation ! You need a vapour barrier on the warm side of the insulation, between plasterboard and insulation.
Have you looked on Velux website to see if they have any standard details for lightwells below roof windows ?
Thanks. Yes saw some said about a vapour barrier, but views seem to be mixed. We have read the velux manual but very vague will try calling their helpline
Not sure why there are mixed views on a vapour barrier, you need a vapour barrier to minimise the risk of condensation in the roof space ,or within the insulation as interstitial condensation. You could use an insulated plasterboard ,incorporating a vapor control membrane, on the inside of the timber framework forming the lightwell ,in addition to insulation between the timber framing. You could ask Rockwool or Celotex on recommendations for insulation, but how is the rest of the roof insulated ? Is it ventilated, what details have been submitted to Building Control ?
Hi, to be honest they aren’t that interested. This is an old lean to roof. 4 metre by around . It had 100 rockwool in the rafters so they asked for 200 to be put over top, and they asked for 140 up around the sides of the lightwell. We want to do it right though. I have seen pics of the membrane, behind the plasterboard. BC have suggested rockwool, so we thought we would just use some of the 200 we have left and try to cover as much of the sides as possible. But just dint know how to attach the rockwool to the wood struts of the loghtwell to make sure it doesn’t fall down after we close it up. No access after, it’s not a loft hatch.

Re ventilation Bc said allow 50mm near the eaves, it’s a single story lean to, so fairly easy to see. Velux is right on the edge. So can’t get much insulation in that side. I just don’t want condensation or the insulation falling out after we seal up,the plasterboard.
There’s a pic,


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See what you mean. Pity they had not installed the Velux a bit higher up the roof I think. The insulation is between and over the ceiling joists which are the horizontal timbers, but they have cut the ceiling joists without trimming the opening with joists !
Perhaps one of the more practical builders on here could advise how to support the quilt insulation on the sloping /vertical sides of the lightwell ; but how do you get access to install the insulation.
(Sorry you are getting more questions than answers).

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