Sloping Roof Insulation

[welshcottage]

Hi all

Next questions hopefully someone can help

We are building a single storey extension, with a sloping roof. The architect has specified a ventilated roof i.e. One with a gap above the insulation, but with 6x2 rafters. According to my fag packet calculations we would need 150mm of insulation therefore there is not enough room for the air gap needed.

If I'm wrong that fine how much insulation should I have?

If I'm rights I can foresee two solutionsc build up the beams by 2 inches or use an insulated plasterboard over the top.. is there a preferred option?

 

[r1flyguy]

Whats wrong with 100mm celotex boards? Just use 8x2 wood joists, This will pride you with the sufficent gap you need and is what we have used on ours

 

[welshcottage]

Whats wrong with 100mm celotex boards? Just use 8x2 wood joists, This will pride you with the sufficent gap you need and is what we have used on ours

6x2 already in place based on the spec which means there is a #### up, so what I now need to do is to make sure that the insulation meets min regulations.100mm would be perfect amount to depth for the 6x2 there but is this enough how can I calculate it to show to the 'lovely' building inspector ?

 

[r1flyguy]

Apoloies for being dumb here, 6" is 150mm, so 100mm celotex flush with the bottom of your joists provides a 50mm air gap above.
I appreciate you may have a cut out for lights but why are you need 150mm of insulation?? Has the architect specified this or building control requirement in your area?

 

[r1flyguy]

Heres an extract of my architect spec sheet

c) Roof with pitch of more than l5 degrees - where insulation follows the rafters and ceiling line (warm roof). Then a 25mm air gap (continuous) with gauze fly screen and pre-formed eaves ventilator to maintain a 50mm air passage at the pitch with a 5mm air vent at ridge, slate or tile at 2.5m. centres. Note see drg/details for Mfg. information Celotex (GA4000). Add plasterboard to form a continuous ceiling to the underside of rafters, all to give a min. ‘U’ value of .16 (Bld.Regs. Part L1).

 

[welshcottage]

Heres an extract of my architect spec sheet

c) Roof with pitch of more than l5 degrees - where insulation follows the rafters and ceiling line (warm roof). Then a 25mm air gap (continuous) with gauze fly screen and pre-formed eaves ventilator to maintain a 50mm air passage at the pitch with a 5mm air vent at ridge, slate or tile at 2.5m. centres. Note see drg/details for Mfg. information Celotex (GA4000). Add plasterboard to form a continuous ceiling to the underside of rafters, all to give a min. ‘U’ value of .16 (Bld.Regs. Part L1).

Builder thinks 100mm is fine, building control mentioned 150mm architect ( who has done a poor job) has not specked roof insulation!

Useful quote, I need to ensure I can meet part l as you say. A few calculators on line so will have a crack using one of these. I don't want to skimp, just want to make sure we are doing what's needed and can show this when needed by Bc.

 

[r1flyguy]

Sorry cant be of any more help, hopefully someone will post up who can advise you better, Hope it works put for you

 

[oldbutnotdead]

Worst case (if you end up having to do 150mm) is do as you are doing anyway (100mm between rafters) then add 50mm below rafters. Up to you whether you go with 50mm insulated plasterboard or do them separately- if someone else is doing it the time saving may well outweigh the extra cost of materials. Brucie bonus- no cold bridges from the timbers

 

[welshcottage]

I think to get a full 150 then 100+ 50 on the plasterboard makes sense as easiest.

There are a few calculators online but getting confused s they keep asking for insulated plasterboard depths.

How do I calculate if 100mm is enough to start with, if this makes sense?

 

[Elastomeric]

I've fitted 140mm with TLX Gold underlay. That's in a 220 Rafter. This gives me .16

No need for insulated plasterboard as did not want to lose height. Not sure an extension will need to meet new build regs.

 

[catlad]

Insulated plasterboard over the rafters is the norm for building inspectors round here
with vaulted ceilings.