Skylight/velux window install advice

[bobbyb]

Hi Everyone!

Looking for advice and tips

 

Im currently in the early stages of fixing my son's room's insulation, which I think was done back in the 80s not to the best standard sadly. 

 

But my question is really about the skylight. Now I've never installed a skylight/velux before (installed plenty of windows mind) so this is a new thing for me, so im looking for advice really. Hopefully, if you watch the attached video, it should give you a good idea of what I need to do. 

 

cheers!

[Nickfromwales]

Some pics of the doubled timbers each side of that window would help. May be an option to delete the one immediately next to the opening and add one the other side, to gain a wider opening.

 

For insulating the reveal, you can just install 20mm of Marmox backer board and skim straight to that, so no need for the additional PB  

 

And use Illbruck FM330 foam for all the gap filling of the Ken Dodd's teeth that is your existing PIR lol.

[bobbyb]

Thanks for the help, Nick

 

And yeah - looks like the window is supported by an internal/non-structural timber frame. I had not noticed until you asked about that

 

So with that in mind this just got a little easier as if i make a small measurement mistake, I can simply build or amend the timber frame to make it all work with the Marmox backer board

 

One question on the Marmox backer board - never used it, I assume it's all fine to mix/match this with standard plasterboard?

btw, the Ken Dodd's teeth comment made me laugh! im pretty happy that the current insulation is pretty bad. I was worried that if I pulled a lot of the plasterboard off id discover it was done really well and then Id have to fix a problem caused by me 

[Nickfromwales]

lol. Yea, good news they’ve closed those timber openings in, so you can reverse that.

 

Yes, the Marmox is compatible, just needs your plasterer to scrim tape on the Marmox side of reveal to deal with the fact that the skim is going over some exposed/bare XPS there. If you need a sketch to explain I can sort that later.

 

Pull all the PIR boards forward, so they’re flush with the stud work, and then foam everything with the product I mentioned. It’s way better than the regular dogshit foams, plus it is airtight (closed cell) and won’t bridge damp/moisture. Leave that to cure and then cut it all back flush. 
 

Then do NOT foil tape the joints, only foil tape the joints of the 25mm layer that you say you’re flying over this lot with. Don’t buy insulated plasterboard for this, as you’ll want to foil tape the joints and stagger the joints in the plasterboard too, which you can’t do with ins PB. Remember you’ll need to order in some extra long PB screws   
 

Put some PIR on top of the purlin too vs just screwing PB to it, but I’m guessing you know to do that.

 

In an ideal world you’d foam each outside edge of every piece of PIR that you fit, so there’s no gaps at bottom/top/sides/junctions at walls etc. Cold air infiltration will delete the insulation work so pay proper attention room to this. Foam between the PIR and the floorboards, to stop any thermal tenting. These things will yield better results than most can appreciate.

 

If you get one of these motorised, even though you can reach them, on very hot days you can get a controller that’ll open then an inch of the temp gets too high (as long as it’s not raining ).

[ETC]

Insulated plasterboard across the rafters.