Extra battens for PB vs Thermal Bridging

Andy7 · 29 Mar 2013

Hi,
I'm weighing up adding some extra studs in my timber construction garden office to help stabilise the plasterboard verses the thermal bridging that will cost me.

At present I have (from outside)

20mm british cedar cladding
25mm treated battens to hold cladding
vapour membrane
18mm OSB
{70mm studs+ 65mm foil backed celotex}

I spaced my framework studs as wide as I could so that I could take full advantage of the insulation and in places the separation is up to 1200mm centres.

My celotex is lovely and flush with the framework, so I guess I won't get much bowing of the PB if any as it's in contact throughout.

I happen to have half a bucket of contact adhesive from installing the EPDM roof membrane (that was a JOY to put on BTW!)

I wonder if a liberal slap / dab of that both sides of the celotex (on the foil not the PIR, I guess the glue will melt the foam?) will help to stabilise things.

Any thoughts?

Project is coming along a TREAT, photos to the forum shortly when I get a chance to breathe.

pinenot · 29 Mar 2013

Extra studs v cold bridging doesn't quite equate, I'm sorry to say. Cold bridging is where cold from an exterior surface, finds a passage of thermal transfer (= bridging) to an interior surface, this can cause all sorts of problems like condensation, cracking just to name two. Extra studs equates to strength v flimsiness. You describe your wall build as having 1200mm between some studs, the norm is 600 if using 1/2" plasterboard and 400 for 3/8" plasterboard, at the spacing you describe yours walls will have little ability to withstand pressure or movement if for instance someone were to fall against it, or more worrying the buildings ability to withstand a storm or gale conditions. I hope I'm not distressing you, that's just the way I see it...pinenot

r896neo · 29 Mar 2013

1200mm between studs is madness. If your dead set on continuous insulation then use sips panels. You simply can not do this kind of construction as you specified.

I assume by your post you are midway through though so sips are out.

600mm centres minimum. To combat thermal bridging you should fill between the studs and then over the face of them like detailed here

http://www.uvalueinsulations.co.uk/uploads/images/K7.jpg

Its often easier to fill the studs then go over the face of them with insulated plasterboard which kills 2 birds with one stone.

In a building like you are constructing air-tightness is as important as insulation. Badly details membranes etc will be the thing to make it cold rather than bridging over studs.

r896neo · 29 Mar 2013

1200mm between studs is madness. If your dead set on continuous insulation then use sips panels. You simply can not do this kind of construction as you specified.

I assume by your post you are midway through though so sips are out.

600mm centres minimum. To combat thermal bridging you should fill between the studs and then over the face of them like detailed here

http://www.uvalueinsulations.co.uk/uploads/images/K7.jpg

Its often easier to fill the studs then go over the face of them with insulated plasterboard which kills 2 birds with one stone.

In a building like you are constructing air-tightness is as important as insulation. Badly details membranes etc will be the thing to make it cold rather than bridging over studs.

Also pinenot makes an important point about wind. Timber structures are very prone to racking and twisting so thought is needed regarding stiffness.

Andy7 · 30 Mar 2013

Sound advice, and that settles it for me. Extra studs going in on Monday to 600mm centres.
I'm putting vapour barrier under the PB anyhow, so should be pretty airtight.

Thx

Andy7 · 30 Mar 2013

Andy7 said:
Sound advice, and that settles it for me. Extra studs going in on Monday to 600mm centres.
I'm putting vapour barrier under the PB anyhow, so should be pretty airtight.

Thx

Oh, I ought to add that there's only a couple of places where the studs are 1200mm and the timber part is an extension of a sturdy block work construction so it's already pretty stable. But point taken nonetheless.

pinenot · 30 Mar 2013

There's no such thing as a stupid question in my book, only the occasional stupid answer

joe-90 · 30 Mar 2013

Thermal bridging from timber? Timber is an extremely good insulator, if you were to fill the whole void with timber it would be a very warm room.

freddiemercurystwin · 30 Mar 2013

pinenot said:
There's no such thing as a stupid question in my book, only the occasional stupid answer

And just to prove the point!

joe-90 said:
if you were to fill the whole void with timber it would be a very warm room.

joe-90 · 30 Mar 2013

Are you saying it would be a cold house? Is timber a cold material? Try it Freddie - touch your head.

freddiemercurystwin · 30 Mar 2013

Compared to concrete it is warm, however you should ask yourself why you can get cold spots showing through the plasterboard under a flat roof when there is only insulation between the joists and none under.

jeds · 30 Mar 2013

Timber has a thermal conductivity of about 0.13W/m.k where PUR foam is 0.022 - i.e. about 600% better.

joe-90 · 30 Mar 2013

Nobody was suggesting that he replace the stuff with timber - just that the difference is negligible in the studding of a wall. Ie it won't lead to thermal bridging. Think log cabin.

freddiemercurystwin · 30 Mar 2013

What, cold and damp?

joe-90 · 30 Mar 2013

WTF's he going on about then?

diynot

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Extra battens for PB vs Thermal Bridging

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