Pitched roof thickness and insulation

[Moonshine]

I am trying to work out general space requirements for a new build, and one area that i have some questions about is the thickness of the pitched roof.

 

The two areas that look to determine how thick the roof needs to be is the structural requirements of the rafters and also the insulation required, of these the insulation is probably the thickest and the rafter thickness can be accommodated in it (100mm - 150mm)

 

Having looked at a U-value calculator for 0.11 for roofs an indicative build up is as below 

 

https://www.uvalue-calculator.co.uk/calculator/pitched roof/insulation between and over rafters/unventilated/no sarking board/150mm/400mm/180/

 

This would probably be;

- 13mm plasterboard

- 180mm PIR insulation inc rafters

- 38mm counter-batten

- 25mm tile batten
- circa 11mm clay roof tile

- add 10mm for getting rafter and insulation thickness to line up

 

So i am looking at a total of 277mm for total roof thickness, does that sound about right? Also why is the 38mm counter-batten in the suggested build up, and is it really required? Also how do you secure the battens though the insulation on top of the rafters? really long screws or something else?

 

Thanks

[Dudda]

You need plasterboard then battens to form service cavity. It will need to be large battens (75-100mm) if you've MVHR or just about 35mm if only electrics and recessed ceiling lights. Then vapour control layer and the 180mm pir insulation inc rafters, counter-battens, tile battens, tiles, etc.

 

The 38mm counter-batten is required to prevent water pooling behind the tile batten and allow ventilation so yes it's required. Very long screws to fix through the insulation.

[Moonshine]

7 minutes ago, Dudda said:

You need plasterboard then battens to form service cavity. It will need to be large battens (75-100mm) if you've MVHR or just about 35mm if only electrics and recessed ceiling lights. Then vapour control layer and the 180mm pir insulation inc rafters, counter-battens, tile battens, tiles, etc.

 

The 38mm counter-batten is required to prevent water pooling behind the tile batten and allow ventilation so yes it's required. Very long screws to fix through the insulation.

 

Thanks, ah the pooling water aspect makes logical sense, i am surprised that it isn't a fairly standard 25mm.

 

I am hoping that i don't need a service cavity on roof pitches as electrics will be channelled on internal and external walls, with eave walls, and in the ceiling cavity / small loft area at the pitch.

Edited September 21, 2018 by mike m

[Ed Davies]

Generally you want about 50mm of ventilation under the outer rainscreen (tiles in your case). For my roof that'll be made up of 38x12 counterbattens with 38x50 battens across. Both widest face up/down, of course, so total thickness = 12 + 38 = 50mm. Not sure why it's not 25+25 other than, I speculate, that the battens take the weight of the rainscreen crosswise and are only nailed at the counterbatten spacing so need to have as much wood as reasonably possible wrapped round the nails.

[Dudda]

2 hours ago, mike m said:

 

Thanks, ah the pooling water aspect makes logical sense, i am surprised that it isn't a fairly standard 25mm.

 

I am hoping that i don't need a service cavity on roof pitches as electrics will be channelled on internal and external walls, with eave walls, and in the ceiling cavity / small loft area at the pitch.

You still need an airtight layer which the vapour control barrier performs. Having that tight between the plasterboard and underside of the PIR will mean it will have thousands of holes in it from fixing the plasterboard making it useless.

 

I wouldn't use 12x38mm timber counter battens. Timber is cheap in the grand scheme of things and the labor involved won't change. 12mm is very thin and will split from the fixings that go through it. Remember it has fixings that will be circa 150mm long if not more so probably 5 or 6mm diameter that go through the insulation into the rafters. Then additional fixings to secure the tiling battens. If the tiling battens are only fixed to the counter battens, which is usually the case, and not fixed back through the insulation into the rafters then you'll have the tiles supported on thin 12mm timber. First storm and the roof will come off.

[A_L]

4 hours ago, mike m said:

 

Having looked at a U-value calculator for 0.11 .....

 

I think they are a bit optimistic, even with a 10% timber fraction (38mm@400mm centres) I get 0.13

 

[Mr Punter]

It is a pain to fix counter battens through thick sheets of insulation to a roof as you are aiming for a rafter less than 2 inches wide and about 5 inches down.  The fixings will be 160mm and need to be corrosion resistant.

 

May be better to go for most of the insulation between rafters and, say, 30mm underneath, taped with ali.  Easier and safer to fit.

[Temp]

We have around 180mm PIR insulation between 200mm deep rafters. I don't think it's enough. If building again I would look at either a SIP roof or a warm roof (insulation above the rafters or above and between).

[Jeremy Harris]

We have 302mm deep I beam rafters, with an added bit of stud timber nail plated spaced 10mm off the lower I beam edge, giving a total rafter depth of about 401mm,  The outer face of the rafters is clad with OSB, to provide sarking and containment for the insulation, the inside has a layer of Isover VCL membrane, held in place by 50mm x 50mm battens across the underside of the rafters, which also provides a service void.  The space between the rafters is filled with ~400mm of cellulose, which in reality is a bit thicker than this on average, as the pressure under which it's blown in has slightly quilted the membrane on the underside.

 

The advantage of this method is that the ~400mm layer of cellulose provides a reasonable level of insulation, but more importantly, it has a reasonably long decrement delay.  An unexpected advantage is that it also provides a very good degree of acoustic insulation, so the house is very quiet inside, and we can't hear rain on the roof, for example.