Cellulose insulation at velux windows - problem

[Dunc]

The cellulose insulation was pumped into pur larsen truss frame last week. The installer noted that at each velux window there is a small-ish gap between the head and foot trimmers and the airtightness board about 25mm. Not possible to pump cellulose into such a shallow void.

There is insulation behind the trimmers, of course.

 

The TF company have dismissed the gap as "too small to have any impact"...I'm disinclined to accept that when chasing marginal gains in Passive type builds this is a reasonable thing to ignore.

Am I being unreasonable?

If not, suggestions for a solution?

 

[Eric]

Could you remove the airtight board and fit 25mm celotex?

Roof colour looks nice 👍🏻

Edited Tuesday at 19:59 by Eric

[Nickfromwales]

Drill a dozen 6mm holes and pump it full of "PIR in a can", eg Illbruck foam  

 

<20 mins per window, job done  

[Dunc]

Two good suggestions there, thank you! I take it you agree it's worth sorting, rather than ignoring.

 

I guess the squirty foam might be easier. Getting the osb off, having already been thoroughly taped up would be difficult. The tape tends to rip the green layer off.

 

Is all expanding foam equal? Not sure whether the expense of the illbruck airtightness foam is necessary, being on the outside of the airtight layer?

[Nickfromwales]

1 hour ago, Dunc said:

Not sure whether the expense of the illbruck airtightness foam is necessary, being on the outside of the airtight layer?

Erm....expense? A few £10's at most. It's a totally different foam, which will be void of honeycombing etc, so buy cheaper foam if you wish, but "seriously!?".  

[Russell griffiths]

Illbruck fm 330 is actually as cheap as the nasty rubbish from screwfix if you buy a box of 12 cans. 
do some google searching and buy a box of it, you will have wished you found it years ago. 

[Roger440]

13 hours ago, Russell griffiths said:

Illbruck fm 330 is actually as cheap as the nasty rubbish from screwfix if you buy a box of 12 cans. 
do some google searching and buy a box of it, you will have wished you found it years ago. 

 

Absolutely this. Not more expensive. But you DO need to be organised as its not stocked by anywhere you can actually go to in person, so needs to be mail ordered.

[Iceverge]

 

On 24/03/2026 at 22:14, Dunc said:

Is all expanding foam equal?

 

It's like comparing this

 

 

To this