Insulating from the outside and the inside: short version

[Garald]

I think my post from yesterday on the subject was TL;DR, so here is the short version.

 

A wall that should have been insulated from the outside was insulated from the inside. (Not my decision.) I was already thinking of adding insulation on the outside, and now that I've spotted some mold in a closet (due most likely to thermal bridging) the contractor tells me that he has been thinking along the same lines.

 

The question is what to have in mind when adding insulation on the outside without removing the insulation on the inside. Is the main issue to make sure that one chooses a material that (like the walls and the inside insulation) is permeable to water vapor?

 

Longer version:

This is a northern wall, adjacent to a (sometimes humid) courtyard. The insulation on the inside has two layers: ISOLIN HPV reflecting insulation + 45mm BIOFIB.

What do you recommend, keeping the budget reasonable - 10 to 15cm PIR perhaps? What are the alternatives? What are their pros and cons?

 

 

 

 

[Redbeard]

I don't think I have ever seen PIR used as EWI in the mainstream EWI industry. (On Grand Designs behind cladding, yes, but not in the mainstream). Phenolic (marginally better for insulation value than PIR but more expensive), graphite EPS, Rockwool (is fire a concern?), wood-fibre? What will your finish be? Render?

 

I would model it/get it modelled (preferably in WUFI, but any condensation risk assessment software (some on here use Ubakus) is better than none. You are looking for potential for collection of moisture within the wall thickness without an obvious exit route.

[Garald]

9 minutes ago, Redbeard said:

I don't think I have ever seen PIR used as EWI in the mainstream EWI industry. (On Grand Designs behind cladding, yes, but not in the mainstream). Phenolic (marginally better for insulation value than PIR but more expensive), graphite EPS, Rockwool (is fire a concern?), wood-fibre? What will your finish be? Render?

 

Rockwool is actually good against fire, no?

 

The finish will be whatever we choose it to be, I take - this is a courtyard wall, and whatever I end up with will look better than any of the courtyard walls currently, so the other members of the tiny coop should be happy.

 

The current wall is some mystery masonry from the early 1930s. I can upload a photo if it helps. I don't think the risk of fire is high (though the inside insulation (cotton-linen-hemp) is in principle flammable) but what do I know.

 

9 minutes ago, Redbeard said:

 

I would model it/get it modelled (preferably in WUFI, but any condensation risk assessment software (some on here use Ubakus) is better than none. You are looking for potential for collection of moisture within the wall thickness without an obvious exit route.

 

Sure, let me try Ubakus. 

 

PS. I am currently on a brief trip abroad, but here is a photo of the outside before renovations. The main things that have changed are that the stained glass now has additional windows on top and the asbestos pipe has been removed.

Edited May 22 by Garald

[Redbeard]

1 hour ago, Garald said:

Rockwool is actually good against fire, no?

 

Yes, the point I was making is that if fire is a particular concern you may wish to use a completely non-flammable insulant such as Rockwool.

[Garald]

1 hour ago, Redbeard said:

 

Yes, the point I was making is that if fire is a particular concern you may wish to use a completely non-flammable insulant such as Rockwool.

 

What are the downsides of using rockwool outdoors? Cost? Lower R per cm than  phenolic?

[Redbeard]

Yes, lower R per cm. Some suggest that phenolic may shrink more than other plastics though I have found nothing to corroborate this. Don't know about the cost but I would expect phenolic to be pricey. I have used graphite EPS, wood-fibre and Rockwool (the latter only as a 200mm high fire-break above 1st floor ceiling level).

[Garald]

How do I specify a reflective insulation (such as ITR HPV/ATI PRO MULTIHPV) in Ubakus?