Thermal bridge

[bolst3r]

Planning a garden office but condensation in the roof cavity worries me. I'm going for a warm deck and propose making the walls and roof up as illustrated in the image below however I have no idea how to stop cold air being blown in underneath the soffit and entering the roof cavity thus causing condensation. Any pointer or guidance would be much appreciated.

Thanks

Untitled

• bolst3r
• 19 Jun 2013
• 1

 

[pred]

My only recommendation would be a fibre glass as apposed to the rubber roof. They build boats using fibre glass, I could be wrong but I have never seen one made from lead, felt or rubber.

 

[geraldthehamster]

I think in your "End on view" diagram, you need to fill the gaps labelled "Cold air", possibly by fixing Celotex in place with expanding foam. Otherwise you have cold air getting to the back side of your plasterboard, beneath your insulation layer above, which negates the value of the insulation.

Although I've done similar projects I am not a builder, so if there is a better way to do this, perhaps someone can suggest it.

Cheers
Richard

 

[jeds]

Agree with above. Close off the ends with celotex and seal any gaps with sealant or foam.

 

[^woody^]

The wall insulation should meet the roof insulation. And fit an actual soffit board to the soffit

 

[freddiemercurystwin]

It needs a soffit fixed to the U/S of the joists, pack the void with rockwool.

Your battens behind the cladding ought to be vertical btw, then any rain that gets behind can fall down the cavity and not sit on the battens. Use foil backed plasterboard too.

Ignore pred, more poor advice.

 

[pred]

It needs a soffit fixed to the U/S of the joists, pack the void with rockwool.

Your battens behind the cladding ought to be vertical btw, then any rain that gets behind can fall down the cavity and not sit on the battens. Use foil backed plasterboard too.

Ignore pred, more poor advice.

Have you ever had fibre glass fitted on any of your jobs?

 

[bolst3r]

thanks for all the feedback

@pred

keep taking the medication

@geraldthehamster @jeds

Yes that makes sense, just to cut some sort of end caps out of celotex to stop the cold air getting in

@ Woody

Wall insulation can't meet the roof insulation as it's separated by a layer of WPB. Or are you just on about taking the wall insulation right up to the roof then filling any gaps with expandable foam? Guess this would be a nightmare with anything other than celotex/kingspan. Was going to use 100mm rockwool due to cost and not being fussed by slightly thicker walls

@freddymercurystwin
Soffit with rockwool stuffed in behind then Celetox in the end of the roof joists just to make sure... Showed my battons that way as was easiest way of showing gap for air and couldn't be fussed with an angled illustration– sorry. Will do them vertical when I build but thanks for spotting it

Would you use foil backed plasterboard instead of Thermaline basic (TE) 22mm? I was worried about cold spots on the wall timbers and figured the insulated plasterboard would stop heat loss. Is this overkill?

 

[freddiemercurystwin]

It needs a soffit fixed to the U/S of the joists, pack the void with rockwool.

Your battens behind the cladding ought to be vertical btw, then any rain that gets behind can fall down the cavity and not sit on the battens. Use foil backed plasterboard too.

Ignore pred, more poor advice.

Have you ever had fibre glass fitted on any of your jobs?

Yes and lead and felt and edpm and PVC and asphalt amongst others. If done properly they will all give you at least 20 odd years. Though lead will outlast all of the others by about 75 years. But what's your point?

 

[freddiemercurystwin]

Would you use foil backed plasterboard instead of Thermaline basic (TE) 22mm? I was worried about cold spots on the wall timbers and figured the insulated plasterboard would stop heat loss. Is this overkill?

That's my bad (and eyesight) I speed read your tiny writing as gyproc basic 12mm, if insulated boards then yes they include a VB in-between the insul and the PB so no need for separate VB. Overkill? Maybe depends what you want to end up with. If you want it really warm use celotex in-between the studs instead of rockwool. If you're that worried about cold bridging you would probably find it cheaper to use separate components rather than insulated PB.

 

[pred]

http://www.flatroofs.co.uk/flat-roof-systems-compared/

 

[freddiemercurystwin]

But what's your point?

 

[pred]

I agree with the above statement.

 

[pred]

In the wrong location ie, on the roof in plain sight, lead will last no more than five minutes!

 

[geraldthehamster]

I agree with the above statement.

I think I preferred the other chap with his cut price Romanians.

Cheers
Richard