Spray foam on cold bridge steels

[NCXo82ike]

We have most of the steelwork up in our wraparound extension and loft conversion. 1905 mid-terrace, solid wall construction. We have picture frame steels sucking heat into the ground and steels bearing on to (and/or supporting) cold brick walls. I'm looking to minimise the impact of cold bridging.

 

The wraparound is brick and block with mineral wool cavity, pitched vented roof. The loft conversion is timber, vented cold roof (unfortunately necessary).

 

Priority 1 is to avoid condensation, priory 2 to reduce heat loss. No cold steel near bathrooms, but plenty near the kitchen in the wraparound.

 

The builder suggested PIR or mineral wool around the beams, but I worry if we insulate without a vapour barrier against the steels it will worsen interstitial condensation? I wonder if spray foam is a sensible way to do this, cutting the cold bridge with a continuous vapour barrier I can tape against other membranes. 

 

I've sent the horror stories but get the impression the issue is where critical ventilation e.g. cold roof is compromised by the spray foam.

 

Other relevant measures are:

-internally insulating solid walls (planning on vapour-open with thermactive plus wood fibre).

-windows uprated

-We'll be getting as airtight as reasonably possible

-MVHR system fitted which I will control based on internal humidity

 

Grateful for any thoughts- couldn't see similar in the forum surprisingly

Edited 13 hours ago by NCXo82ike

[JohnMo]

Steel, connected to ground will be a cold surface. Wrap in PIR and tape the joints, this gives vapour barrier. 25mm is normally enough but the more the merrier. Or insulation and wrap and tape a vapour barrier. Condensation will only form from constant contact with air

[NCXo82ike]

27 minutes ago, JohnMo said:

Steel, connected to ground will be a cold surface. Wrap in PIR and tape the joints, this gives vapour barrier. 25mm is normally enough but the more the merrier. Or insulation and wrap and tape a vapour barrier. Condensation will only form from constant contact with air

Thanks John, on that basis my question is whether there's a reason not to use spray foam since there's a lot of complex junctions, joists etc

[JohnMo]

I would all to can in PIR and where needed apply spray foam to fill gaps.

[Super_Paulie]

I have a goalpost type setup with the post flush to the block. I attached a sheet of aerogel to that and foamed the edges before plasterboarding, attached with foam rather than traditional adhesive. The upright leads to a comically oversized beam that has been left exposed over 8m. 

 

I can tell you, the beam is cool but never cold and it seemingly has very little effect on the rooms temperature which I have at 23° daytime and 21 evening.

 

Aerogel might seem expensive but could be an option?

 

Edited 3 hours ago by Super_Paulie