Expanding Foam for Glazing.

[paro]

Hi All - I have someone installing the glazing for me at the moment (I was on the fence about doing it myself and thought it better to get some kind of warranty). Is a lot of expanding foam around the whole frame the norm nowadays? There are some pretty heafty gaps of 20-30mm in places which seems a lot. I appreciate that it helps with cold bridging but it seems to be the only thing other than the timber cladding I'll be installing that will be holding back the weather. I had kind of anticipated some kind of trim that was sealed to the block with mastic or just something else. I've used foam for other external bodges around the house until I get to it later but after a while they start to look weathered and like it wouldn't hold up long term. Maybe just a newb worry over nothing. 

 

Could anyone point me to a reference guide on what is right / wrong for installing doors and windows?

 

Thanks, Paul

[Mr Punter]

You are right.  Foam is not suitable for long term exposure.  It needs to be covered with some trim, like plastic, timber, metal etc or rendered.  Also, most foam is not closed cell and will let water through.

[SBMS]

7 minutes ago, paro said:

Hi All - I have someone installing the glazing for me at the moment (I was on the fence about doing it myself and thought it better to get some kind of warranty). Is a lot of expanding foam around the whole frame the norm nowadays? There are some pretty heafty gaps of 20-30mm in places which seems a lot. I appreciate that it helps with cold bridging but it seems to be the only thing other than the timber cladding I'll be installing that will be holding back the weather. I had kind of anticipated some kind of trim that was sealed to the block with mastic or just something else. I've used foam for other external bodges around the house until I get to it later but after a while they start to look weathered and like it wouldn't hold up long term. Maybe just a newb worry over nothing. 

 

Could anyone point me to a reference guide on what is right / wrong for installing doors and windows?

 

Thanks, Paul

30mm is too much. You could put trim on and foam behind it. 

[paro]

Thanks. I'll definitely do the trim thing then.

[Nickfromwales]

4 hours ago, paro said:

Thanks. I'll definitely do the trim thing then.

You mean you'll ask them to do it as 'makeup' ? They should finish this so your external finishes go up to their makeup, if being left exposed for any length of time.

[paro]

Well I want to take the view that the cladding that I'll be adding is purely an asthetic facade and in no way is intended to replace the weatherproof finish that I am expecting them to leave in place. So basically if I never added the cladding the frames would remain weathertight indefintely against the blockwork. I've been reading through the FENSA guide and found te below.

 

..."A suitable backing material such as closed cell foam or PU foam should be used in the gap between
the window and wall to ensure the external sealant is set to the correct depth and will therefore adhere
to the frame and aperture (but not the backing foam) when applied. Suitable external sealant should be
applied to cover and form a water resistant joint between the frame and wall"...

 

I hadn't given this much thought, tbh, I'd assumed that by paying for a professional install it would be done right. I should have learnt this lesson by now. 

 

[craig]

On 07/08/2025 at 08:13, paro said:

I've been reading through the FENSA guide and found te below.

 

Fensa is irrelevant for a new build, you should refer to the GGF guidelines and also look at what was and what wasn't included within the installation. Unfortunately, not every installer and or supplier understands that PU foam installation only is not a suitable method. There should be a waterproof breathable membrane and or a compriband finish, then PU foam up to the that. This is asking for problems further down the line.

Edited 18 hours ago by craig