100mm insulation between 5x2 rafters too much?

[Ian H]

With the 100mm boards seeming more available I might put a thinner board between the rafters and 100mm under them. The room is big so I think I could loose a little space for better insulation

 

[John D v2.0]

The air gap is ventilated at the eaves

didn't see that comment, fair enough. Still agree with gerald - anything more is

 

[John D v2.0]

That's a hell of a room! Either that or that's a 1200mm spirit level and you're a midget...

 

[Doggit]

That would mean more weight being supported, so longer screws, and and more of them, but you could go that route. On the other hand, it would help you lose the steel.

How are you doing the wall behind you.

And I'd say about 5'10" or 5'11"

 

[Ian H]

I'm easy 6' 2"

Behind me I'm going to leave the chimney exposed but insulate and board under and over it. I wouldn't mind doing something special up there to give the wow factor.

 

[Ian H]

I've done it now, I put a trim on the rafters to give a 40mm air gap:

 

[Doggit]

25mm with a breather membrane would have been fine, but that's coming along nicely.

 

[John D v2.0]

40mm air gap:

Maybe it's been gone over, but you don't need an air gap at all there, you just need enough space for the membrane to drape between the rafters so the water doesn't get stopped by the tile battens. On that note your membrane is a bit on the tight side.
If you had counter battened, that wouldn't be needed.
Looks neat though!

 

[Ian H]

It was the builder that talked me into the air gap, he said if the felt was touching the foil it would take an age to dry.

He's guaranteeing the roof so I thought it best not to argue.

Worst thing was that I had to swap from Celotex to Ecotherm 3/4 of the way along OCD overload!!

I did a bit of top bodging too, I'll put a single piece in if I have enough left over.

 

[John D v2.0]

Fair enough! Won't cause any problem.
Did you know with the velux there's a recommended detail where you splay out the reveals to let more light in and make them look thinner.

 

[Ian H]

By splay do you mean come off the window at 45degrees?

 

[Doggit]

30 degrees would be fine on that size, 45 if if it was smaller. And no Ian, it's only a bodge if you forget to fill the gaps with expanding foam; that's just creative use of the small bits.

 

[John D v2.0]

By splay do you mean come off the window at 45degrees?

Yes something like that, not sure the exact specification of the splay but they have some drawings of their suggestion.
Not really essential but they're there if you want.

 

[Ian H]

I'll have a look, thanks for the tip.

The Velux in those pics will be above the stairs so it's only purpose is to let light in. The other is in a room and they have done that different with angled noggins which i think is how you mean:

 

[John D v2.0]

I had a Google this morning but can't find the official detail but it's basically noggins as you say and rafters spaced as wide as possible as you can't splay them. With your roof thickness it isn't so important.
Found this diagram that showed the windows can actually defy physics if you splay the reveals!