Low pitch vaulted ceiling single storey construction and insulation

[mr-gobby]

I'm at plate height and considering roof specification. No architect as such, drew plans ourselves but effectively copied similar extension nearby and have calculations for steel ridge and steel across bi-fold opening. I agreed with building inspector to provide details at each stage which so far has been fine. We have building width of 5.25m width 17.5 pitch and is 5.7m long. Two velux will go in, one either side. Glass area calcs are okay. My architect friend (who specified the one nearby) said 150mm rafters at 600 spacing was fine and complies from tables for building regs, I'd decided on 400 centres and the carpenter I'm using suggested that in his opinion 200cm joists would mean no chance of any sag later and also take adequate insulation if I used a breathable membrane for regs.

 

Friend has just installed a flat warm roof and used 120mm celotex which passed inspection. When I mentioned 120mm for in between my joists the inspector said I'd need much more and that it was common to insulate under the rafters as well. Bit confused as to why if a flat roof can pass with 120mm I need so much more. Okay so the timbers are insulated above in that situation but really? I thought timber was thermally quite good anyway. Can it be done within a 200mm rafter?

 

Anyone had similar experiences or can shed any light 

 

Thanks

 

Edited October 12, 2019 by mr-gobby

[Temp]

If you put 200mm insulation between 200mm rafters you really need counter battens to raise the main tile battens off the membrane. This is to allow water to run down the membrane and not pool above the tile battens causing them to rot. 

 

The alternative is to fit 150mm insulation and allow the membrane to sag between rafters forming a gap water can run down. 

 

Not all insulation and membrane manufacturers say they can be in contact. I believe Kingspan say one of their membranes can be installed in contact with insulation. 

 

We have something like 160mm or PIR in some parts of our roof and wish we had more. For some reason insulation at rafter level isn't as effective as at joist level. Perhaps because the rafters cause gaps or bridge the insulation? We get overheating in summer in "room in the roof" rooms even without significant glazing.

Edited October 11, 2019 by Temp

[mr-gobby]

Yes read some specify about 25mm gap into which you sag the breathable membrane, I figure it's a cold bridging of the rafter thing rather than insulation thickness. There are quilts which are insulative and act as a breathable vapour barrier to go on top and batten over thats thin enough not to need counter battening but it's quite expensive, in rolls of 1.5x10m a few brands think kingspan do one, superfoil is it? Makes a warm roof as opposed to insulated PB making a cold roof and loosing some albeit small amount of headroom (vaulted ceiling so just issue at lowest point really. Not got much to play with above though as ridge and flashing is close to a window corner. Thanks for replying Temp.

[Russell griffiths]

Can you not put 50mm celotex on underside of rafters. 

[Mr Punter]

I assume this is an extension.

 

The rafter size depends on the span, the proposed loading and the timber grade and needs to be properly calculated.  You may be able to reference a span table for this. I assume the steel ridge is at right angels to the main house wall and the roof pitches both sides.  It may be better  to have a flat ceiling and insulated a t ceiling joist level.

 

You could even do the roof with lightweight trusses and lose the steel beam.

[mr-gobby]

The rafters can be 150mm based on span tables and architects spec as supplied for same extension few doors down . Carpenter is quite belts and braces and so priced for 200mm to avoid any possibility of sag later, he's done a few jobs I know of and has a very good reputation. It's having a couple large velux rooflight installed either side (size UK04)  It's only got a 17.5 degree pitch, overall 5850 wide so internally 5250. I had a truss company quote, price was great but at the span the tie couldn't be high enough to give us the vaulted effect we want. Yes you can insulate under rafters and is likely option it was just a bug bear that a flat warm roof only needs 120mm but an inbetween rafter roof needs a lot more. The steel ridge is 178x102 so not huge,  I have it on site now and we installed pad stone in house wall today.