Expanding foam and condensation

[Kelvin]

The builder was back today final sealing the Velux windows. I noticed he’d done a poor job of one of them so I removed the foam and noticed it was wet behind it up against the external flashing. I figured he’d sprayed water to help it cure however I didn’t recall him having anything with him to do that. Just called him and he said he didn’t so where has the water come from? 
 

It’s been hammering down all day today  and been very wet on and off for weeks.  If water was leaking in there would be evidence of it somewhere as eventually it would drip in somewhere. I can’t see any obvious water marks and I can’t see anywhere that it might be running inside the building. I’ve gone around the area with the moisture meter and no indication that anything behind the structure is wet.
 

Does expanding foam produce moisture as it cures? I’ve not seen any evidence of that when I’ve used it but then I always spray some water. Could it be condensation forming on the back of the flashing after the expanding foam was injected to fill the gap? 

[Mr Punter]

I don't really see much expanding foam on Velux.  Normally rigid insulation and plasterboard around the reveals.

[Kelvin]

It’s as a consequence of a Heb Homes balls up that didn’t leave enough space to get rigid insulation in. 

[SteamyTea]

1 hour ago, Kelvin said:

Does expanding foam produce moisture as it cures?

It shouldn't.

While water is often the blowing agent for PU resins, the oxygen gets stripped of the two hydrogen atoms. The free oxygen and hydrogen atoms then combine with carbon atoms that have a free electron.

Then you get a mix of CO, CO₂ and CH₄. Why you are told to keep the area well ventilated.

Thermoplastics use hydrazine, much more fun.

[Kelvin]

Ok so if it isn’t leaking (as far as I can see) and it’s not coming from the foam itself then it must be condensation which seems rather odd in itself. The house is very warm and it felt quite humid today. I might strip the foam from one of the other windows and see if it’s the same. 
 

 

[Dave Jones]

most likely the dew point is occuring in the foam. Exactly what happens to incorrectly constructed warm roofs, rots them out.

[eandg]

27 minutes ago, Kelvin said:

Ok so if it isn’t leaking (as far as I can see) and it’s not coming from the foam itself then it must be condensation which seems rather odd in itself. The house is very warm and it felt quite humid today. I might strip the foam from one of the other windows and see if it’s the same. 
 

 

If it is humid and you've not hooked up your MVHR to ventilate the place then you'd expect any poorly sealed rooflight to be the coldest place in the house and the site of condensation?

[Kelvin]

Yes it seems like interstitial condensation because it wasn’t done very well allowing the warm moist air to permeate the insulation and hit the cold surface of the cladding. 
 

We are a few months away from hooking up the MVHR unit. 
 

It happened very quickly though as they only applied the foam this morning. 

[Susie]

Since it sounds like condensation….

Sometimes we miss the obvious so just asking have you brought anything in to dry out e.g. wood floor, wet boots or coats or have a newly plastered wall.

 

 

[Kelvin]

11 hours ago, Susie said:

Since it sounds like condensation….

Sometimes we miss the obvious so just asking have you brought anything in to dry out e.g. wood floor, wet boots or coats or have a newly plastered wall.

 
 

 

It’s a new house just at the first fix stage. Screed was poured a few weeks ago. There was a lot of moisture in the air but we kept it well ventilated after the initial 24 hours. If rained a fair bit yesterday after I cut the foam out and there was no evidence if water ingress. We are a while away from plaster boarding so will leave it open and see how it goes. I’ll try and pour water over the window and cladding to see if any comes in. I’m now pretty confident it’s not rainwater though as there would be a lot more off it given the heavy rain we’ve had. 
 

If it is condensation, and I’m confident it is, I assume it will be fine when re-foamed and fully sealed airtight?

Edited August 1, 2023 by Kelvin

[Conor]

58 minutes ago, Kelvin said:

 

It’s a new house just at the first fix stage. Screed was poured a few weeks ago. There was a lot of moisture in the air but we kept it well ventilated after the initial 24 hours. If rained a fair bit yesterday after I cut the foam out and there was no evidence if water ingress. We are a while away from plaster boarding so will leave it open and see how it goes. I’ll try and pour water over the window and cladding to see if any comes in. I’m now pretty confident it’s not rainwater though as there would be a lot more off it given the heavy rain we’ve had. 
 

If it is condensation, and I’m confident it is, I assume it will be fine when re-foamed and fully sealed airtight?

It's condensation. It takes months for a house to dry out. Keep windows open 24x7.