Advice on filling gap around new window frame.

JohnB47

I'm soon to fit new uPVC window in a brick built shed. The manufaturer recommended  taking (minimum) width and height measurements then subtracting 25mm to each. 

I'll use the normal external sealant but there's likely to be a bigish gap here and there.

I seem to remember reading about an expanding foam that was more contrallable than in the old days, which can keep going like an alien invasion.

Any advice?

subjecttocontract

I assume you mean there will be some gaps on the inside, not the outside of the frame ? You can buy plastic extrusion with various profiles. Just pick one that suits, buy a length of it and fit it with either silicon sealer or super glue. 

grumpy_codger

JohnB47 said:

....

I seem to remember reading about an expanding foam that was more contrallable than in the old days, which can keep going like an alien invasion.
...

I think it's this sort of expanding foam tape (you have to freeze it first otherwise it expands too fast):

https://www.youtube.com/watch?v=V6lZXjRR-1k

Then you cover it outside with silicone.

Nearlyold

 "Low expansion foam" is the stuff you've  read about - much easier to control. I've found the Soudal version works well.

bjorn_toby_wilde

Nearlyold said:

 "Low expansion foam" is the stuff you've  read about - much easier to control. I've found the Soudal version works well.

+1 for this, although you may be better with foam sealant backer rod. No idea why it’s called that because it’s not a rod!  Google it and you’ll see what I mean. Just squeeze it into the gap then seal over the top.

I’m not a big fan of thick sealant beads myself. It looks neater IMO to use upvc quadrant to cover the gap.

JohnB47

subjecttocontract said:

I assume you mean there will be some gaps on the inside, not the outside of the frame ? You can buy plastic extrusion with various profiles. Just pick one that suits, buy a length of it and fit it with either silicon sealer or super glue. 

Nope, I mean on the outside of the frames - all around between the frames and the brickwork.

JohnB47

Thanks all. I'll have a look at the foam sealant backer rod but I may go for the "Low expansion foam".

stuart45

Expanding foam is used because it grips the frame better and gives it more stability. 

WIAWSNB

Hi John.
Where in the walls do the windows 'sit'? I presume they straddle the cavity to some degree? Were cavity closers fitted - I noticed these were used on our extension? Or, is it a solid wall?
If using foam, is there an unwanted risk of bridging the cavity? I have to say, where to install windows is a bit of a mystery to me - I've only ever done so in single skin walls. 
However, foam does seems an attractive method to me, if done carefully. It not only adds extra support all around the perimeter, but will be an effective insulation layer the full depth of the window frame thickness.
If using just a pressed-in foam strip, or - worse - just a bead of silicone, it'll surely mean the gap immediately surrounding frame will barely be insulated? A bead of silicone, and a thin strip of PVC trim - and that's it.
That's how it seems to me in any case, but I don't know.
What profile beading do your windows have - IE, the beads surrounding the glass units? Mine were 'ovolo', so I bought a length of quadrant PVC beading, which was a good match, with which to cover the side gaps when I recently tidied them up - stuck in place with StixAll. A lot smarter than just a frame-sealant bead.
When I cut away the old silicone, it exposed gaps right into the cavity - hardly well insulated!
So, if fitting windows myself, I'd be tempted to foam-fill any gaps to insulate and draught proof them, trim this flush with the outside, and then cover with the most suitable style of PVC trim.

bjorn_toby_wilde

JohnB47 said:

Thanks all. I'll have a look at the foam sealant backer rod but I may go for the "Low expansion foam".

Sounds like a good choice.  If you’ve not used it before I’d mask the frame and the wall next to the window. Low expansion doesn’t mean no expansion and it sticks like the proverbial 😬

Also, you may already be aware, acetoxy cure silicone isn’t suitable for UPVC.  I’ve seen posts on here where people refer to using any old silicone for windows but you should use a neutral cure. It will say on the tube whether it’s suitable for UPVC.

JohnB47

Thanks for that extra advice. The building is actually brick built, single skin, so no cavity to worry about. Beading won't be needed either - just a simple sealed finish on the outside edge of the windows. On the inside? Well, we'll see if I bother covering up where the brackets are fastened to the walls.

Yes, I'm planning to mask the windows extensively. I've not heard that about the silicon. I'll make sure to check.