Metal web joists in the thermal envelope

[Norbert]

This seems like a bad idea due to the thermal bridging that a lot of  metal would introduce. However I note that MBC use metal web rafters in their Passive roof constructions and claim U value of 0.10, so there would appear not to be an issue.

My proposed build also uses a suspend timber floor as part of the thermal envelope, so I guess the same construction would be good here too.

 

However it just feels wrong and my instinct tells me that I beams would be better. Any views anyone?

[JohnMo]

Posi rafter has basically the same thermal losses as the same size wooden beam. The metal webs are about 1mm think, so the cross section is pretty small. 

 

I used them in my roof, but then underneath counter battened 100mm and full filled with insulation. To limit any thermal bridges further. Then a vapour barrier and a further 50mm service void.

 

Your first floor, floor build inside the thermal envelope, makes zero difference to heat loss, could be all metal if you wanted.

[Norbert]

Thanks. That seems like a consistent story then.

 

I will have 2 floors constructed using metal webs, the lower being insulated and the thermal envelope between basement and first floor, so much the same as the roof. The middle floor is entirely within the thermal envelope, so indeed that makes no difference. n.b. everything 2 meters above current ground level to get above highest possible flood levels.