False Pitch Roof - Insulation Approaches

[GoodDIYjob]

Earlier posts mentioned a lantern, not a flat roof light. Lanterns would tend to have cills, but flat lights would not and instead have some sort of proprietary shroud. So what this means is that the kerb could potentially be thinner (50mm timber ) for a flat light than for a lantern.

But whatever it is, it would be up to the manufacturer to specify the kerb requirements for their product. So that should be determined first, and then the kerb and the light well and structural requirems would be translated on to the drawing.

I'm not against box sections. In fact I made these as lintels for my timber frame instead of using double timbers, for the very same reason - to allow insulation within the box. But what I am saying is think about whether its actually necessary in context of the relatively small amount of exposed area.

Thanks. Yes other posters mentioned lantern but it is a roof light shown in the section and that is what we are using.

I've looked at quite a few and as I say, most have fairly similar kerb profile requirements between 75 and 100mm before plasterboard. See for example vitral attached which is 100mm (before plaster) and suggests ply frame. But I can totally see what you are saying about diminishing returns given the glass losses above it!

Some manufacturers seem offer additional purchase of insulated kerb but most I've seen expect the builder to do it. See attached for Vittral skyvision but these seem quite pricey for a double glaze unit..

Thanks a lot for all your comments.

 

[Notch7]

I would just do a timber upstand. If you want to improve the insulation put some 25mm celetex around the outside.

If you think about it, you have a big sheet of glass on the lantern thats probably got a u value of 1.2 if double glazed.

70mm softwood is about 1.4 from memory, so the heat loss from thevupstand arra will be tiny.

 

[noseall]

70mm softwood is about 1.4 from memory, so the heat loss from thevupstand arra will be tiny.

I was more fearful of a cold spot rather than the heat loss. I guess the neighbouring glass will take the hit there though.

 

[GoodDIYjob]

I would just do a timber upstand. If you want to improve the insulation put some 25mm celetex around the outside.

If you think about it, you have a big sheet of glass on the lantern thats probably got a u value of 1.2 if double glazed.

70mm softwood is about 1.4 from memory, so the heat loss from thevupstand arra will be tiny.

Yes ok. I just found a website that allowed selection of materials and while it seems pretty generic in that all the timber types I selected came up with the same U value for same thickness, it was exactly what you said.

http://www.changeplan.co.uk/u_value_calculator.php

An 18mm plywood upstand, 36mm timber with 40 or 50mm of insulation is going to be better though? I was thinking for condensation risk as well. Seems a real skimp to just put a timber upstand there with all the effort and detail on the roof...

Funny I also found a thread on piston heads devoted to this very subject - timber vs insualted upstands on roof lights. No conclusion but similar statements to here and some confusion over the exact U values....

Thanks also for that Dupont Tyvek document. Good reading, I need to read some of it a second time but the eaves detail is really good. Do you think a sealed roof is going to be noticeably better U value performance wise than unventilated, especially when it is only a small pitch around the edges but airflow and air infliltration could impact more than the false pitch I suppose all the way in the ceiling void below the warm flat roof...

 

[pilsbury]

My flat roof lights had 75mm kerbs. So I just used 2x4 to build up then fixed the kerb on top. I then fixed some 18mm osb to bulk out the outside a bit before fitting some timber fillets to get an angle to GRP over. Seeing as timber isn’t too bad in transmitting heat, I figure that layered 2x4, Osb and timber fillet with GRP over isn’t gonna be too bad at heat loss..... I think......

 

[GoodDIYjob]

My flat roof lights had 75mm kerbs. So I just used 2x4 to build up then fixed the kerb on top. I then fixed some 18mm osb to bulk out the outside a bit before fitting some timber fillets to get an angle to GRP over. Seeing as timber isn’t too bad in transmitting heat, I figure that layered 2x4, Osb and timber fillet with GRP over isn’t gonna be too bad at heat loss..... I think......

Yes, I feel less worked up about it now, I just still feel even if a timber upstand is say 1.4 u value against 1.2 or or 1.3 for the roof light. It is still the worst performing element which probably makes it most susceptible to condensation even if the overall performance hit is not that noticeable. Also, compared to the actual warm roof itself the upstand could be 6x worse. It didn't seem a good starting point.

Thanks for you photos, the roof membrane GRP and detail stuff looks good to my non builder but technically focused eyes.#

Pilsbury, given woody has contributed quite a lot to this thread helpfully and credited you with being the flat roof to pitch roof expert. What do you think I should do on insulation arrangements of pitch rafters and immediately underneath. The image I attached originally has produced some differing opinions. The designer when I initially queried didn't really offer alternatives or much substance to any explanation so I would really like to be armed with the most preferred set up to go back to him as a suggestion..... I can't make my mind up really.

Just deal with false pitch as warm roof, get required insulation there. Have warm roof and ceiling level insulation as per designer. Fill false pitch and ceiling beneath entirely. Void above ceiling level insualtion or not. I am still clinging that is wrong. Mostly I am almost aligned to woody's suggestion to jsut fill the entire false pitch area to ceiling level with any insulation.......

What about AVCL arrangements under the false pitch and interface to warm deck.. Still so undecided and confused from what can't be that uncommon can it??

Cheers

 

[^woody^]

makes it most susceptible to condensation even if the overall performance hit is not that noticeable.

Hence the thin layer of celotex.

But a lower u-value does not equate to a higher condensation risk. The surface temperature of timber gives it a higher dew point regardless. I suspect the greater risk will still be the roof light glass or frame.

 

[pilsbury]

Thanks for you photos, the roof membrane GRP and detail stuff looks good to my non builder but technically focused eyes.#

Pilsbury, given woody has contributed quite a lot to this thread helpfully and credited you with being the flat roof to pitch roof expert. What do you think I should do on insulation arrangements of pitch rafters and immediately underneath.

Cheers

Couple of little points I should let you in on.... the internet is a dangerous place.....Don't believe that all people on here are experts. Your first point about it looking good to your non builder eyes... I too unfortunately have "non builder eyes".... and hands, and brain etc etc. I am a complete novice. As of last May when I started this insane project, my building experience was with sand at the beach.

Secondly, spending more time on this forum, you will understand Woody, although immensely knowledgeable, is the king of sarcasm. His quote of me as the flat roof to pitched roof meister, relates to the many painful posts he replied to when I was working out the same madness you now find yourself in.

But, you seem to have your head screwed on and are contemplating all the options. You just have to sift out the best ideas and make a decision on how best to put that into practice on your particular situation. Good luck.

 

[GoodDIYjob]

Aha got you thanks for your reply. Back to thinking about this whilst lying in bed!!

 

[pilsbury]

Just for giggles and to make yourself feel better about your designers efforts, here are my efforts I posted just before I started - remarkably similar roof design..... but I suppose any flat roof hidden behind a pitch is....

https://www.diynot.com/diy/threads/building-regs-drawings-my-attempt.478756/page-2

 

[Notch7]

If you think about it, timber windows are jusr timber between the cold outside and inside the house. They dont suffer condensation, at least not on the timber!

My main business is building timber orangeries, we do a 90mm glulam timber upstand with 25mm celetex externally.

 

[GoodDIYjob]

Just for giggles and to make yourself feel better about your designers efforts, here are my efforts I posted just before I started - remarkably similar roof design..... but I suppose any flat roof hidden behind a pitch is....

https://www.diynot.com/diy/threads/building-regs-drawings-my-attempt.478756/page-2

Well at least you appear to get on and have a good communication line through to your builder and designer!

Yes, that false pitch timber detail you appear to have started with looks much the same.

So even though you may not be an expert, but have improved you sand castles somewhat, what have you done with the warm deck and insulation in that false pitch, warm deck overhang space?.

Has the warm deck been brought out to intersect the false pitch rafter, as your initial plans and my plan also indicates. Seems a lot of detailing to slot the deck, insulation, the valour layer around the false pitch rafter struts and then refix and rejoin all that round the back of the false pitch struts and support the deck out to rafters.

Are you planning on putting insulation between false pitch rafters?

 

[pilsbury]

The plans were just that. Due to inexperience, what seemed logical on sketch up did not translate easily in practice.

My celotex for the warm deck finishes where the vertical back of the false pitch meets the flat roof. This leaves the thin, narrow “loft” uninsulated. I simply then stuffed loft insulation (270mm) in this gap which conveniently tucked between the ceiling joists and firring strip under the warm deck giving a 6” overlap of the cold/warm deck.

I have no plans on putting insulation between the rafters. I’ve got breather membrane under the tiles I want exposed. I’m fairly confident this method will be fine. Before I put the lift insulation down, wind whistles through my build. Loft insulation down and no wind and much quieter so must be doing something.

 

[GoodDIYjob]

Mmmm. Yes, thinking back to start of thread you already told me that. I forgot that was you!

It does look a lot more logical and better separated having the two distinct roof spaces and not dealing with all the extra work to get the warm deck extended, for what, in my design case maybe an extra 150mm cover and the big AND it still leaves either rafter insulation of loft insulation anyway so doesn't even solve the problem.

I'm starting to be convinced with ventilated loft and 270mm loft roll for outer perimeter. It is simple and decomplicates the deck structure which, again with my non builder eyes looks very complicated detailing wise for a problem it doesn't even solve. There is still a secondary insulation approach required for the gap remaining left over the false pitch....

 

[pilsbury]

Are you building this yourself or is a builder doing this?

If the former, you have a lot of time to mull this over as you construct. I had little idea what I would do until I started to cut the roof timbers. I then started to see it take shape and had the idea of loft insulation. In addition, when BC came to inspect, he made a throwaway comment “what you doing in the pitch? Loft insulation?” I of course replied with confidence..... but for me, this sealed it. He seems quite a stickler for the rules so I assumed it would be ok.

If the latter, it’s good you are getting your head round it, but take advice and discuss with your builder. He’s probably seen similar before. No harm in being clued up yourself.