Celotex between Rafters

[FRAN1870]

Loft conversion question ....

The designer has spec'd 3x2 rafters need to be doubled up with 75mm Celotex in-between and 75 under that with PB on top.

Trouble is that the rafters are only 400mm apart so if I double them up I will have gaps of 350mm ( which seems fiddly ) and rafters 4inch wide and 3inch deep... Won't this cause thermal bridging ?

My idea was to instead of sistering the rafters with 3x2 I thought maybe use 2x2 ( will of course check with the SE ) fixed to the underside of the rafter with a thicker ( 125mm celotex between ) and a 25mm with PB under that. The cost is about the same give or take £5 a sheet.

What are your thoughts ?

 

[^woody^]

Have the plans been passed?

 

[geraldthehamster]

Are you sure he doesn't mean the rafters to be doubled up from 3x2 to 6x2 (rather than 3x4), in order to fit the depth of insulation required?

Cheers
Richard

 

[FRAN1870]

plans are with BC as we speak ...... but even if they do pass it then its still fiddly.

I don't need an air gap because its got breathable sarking felt but the plans call for 75mm between and 75mm under the rafters .... 6 inch deep rafters would be too deep so it means to double them up. 4 wide x 3 inch deep leaving a 350 wide gap that is 75mm deep for the first layer of celotex

 

[freddiemercurystwin]

You still need a minimum 25mm gap even with a breathable membrane.

 

[FRAN1870]

You still need a minimum 25mm gap even with a breathable membrane.

How come ? Is that advisory or reg ?

 

[^woody^]

Why dont you just get your designer to redesign it, send the redesign to the council and get the redesign checked and approved?

 

[roboughton]

You still need a minimum 25mm gap even with a breathable membrane.

How come ? Is that advisory or reg ?

Thermal bridging for one reason. The roof still needs to breath or you will get rot.

As woody says you need to redo your plans and resubmit to BC. On my extension we put 100mm of Kingspan roof boards between rafters and 30mm on the underside.

 

[FRAN1870]

I'm using a private BC company ... Just wanted to be clear about what's been designed and why. If you remember this designer is a bit hit n miss if I'm honest and this is just another issue I've identified that should've been designed out.

He's not been very helpful now he's been paid either ... Just points me to BC but I don't want to be going back n forth if there's a definative logical solution.

I don't know the designer client BC relationship should work tbh

 

[ey143]

You still need a minimum 25mm gap even with a breathable membrane.

How come ? Is that advisory or reg ?

We had to also leave 25mm air gap under our breathable felt recently too.

 

[^woody^]

Without knowing what the rest of the roof is like, the timber sections, or the available space it's not just a case of suggesting this or that.

There are several options, but whether the specified one is better or worse that other methods is unknown.

 

[FRAN1870]

Photo for reference ... 3x2 rafters at 400mm ctrs

 

[^woody^]

Why can't you put 75mm between no air gap, and 60mm or so below?

I can't see why the rafters need to be doubled up.

 

[FRAN1870]

Yes I'd be happy to do that but I was Just going on what the drawing said from the SE's specs

 

[^woody^]

From your previous posts, it seems that you can't rely on your designer or SE.