Help needed setting timber frame window hole sizes.

[G and J]

Our brains have overheated trying to fathom how much to add on to our window sizes to set the precise hole sizes.

 

We have a mixture of fixed and opening windows. And the frame width (jamb) is 42mm for the fixed lights and 68mm (jamb plus stile) for the opening lights.   We are keen to have sensible anti- cold bridging measures.  Our plans show 25mm PIR plus plasterboard and skim totalling about 40mm.

 

So if we set our holes tight to the frame size then on the fixed lights there’ll be virtually no jamb showing.  
 

So is it ok to add 25mm air gap round the window frame - to be filled with expanding foam tape (thank you to @Nick Laslett for introducing me to that stuff).  If not how much should I add?

 

@craig - J tells me you are the dude to ask this of, just in case you are on here.

 

[Alan Ambrose]

I thought those gaps were more like 5-10mm no? And I think I remember that they’re also specified by the window supplier - I guess they have their build tolerance too. Also smaller for installing into timber frame as presumably that can be made more exact (and adjusted mored easily).

[G and J]

That was my starting point too.  But our SAP report came complete with a document which requires a minimum insulation level on the return. 

Now I could achieve this with 15mm of PIR, then plasterboard then skim, totally circa 32mm.  Marmox multi board comes in 20mm (just under required R value) or 30mm (which would be thicker than the 15mm PIR/plasterboard.

 

These all leave circa 10mm of aluminium frame showing which seems too little to me.  What am I missing?

 

 

[Dunc]

Marmox in 12.5mm + 4mm + skim = 19.5mm? Bit of a faff using the two board thicknesses, but gets you close to the required 15mm PIR?

 

Our window fitter (going into a timber frame) has requested a 10mm tolerance, which I thought was rather large.

[G and J]

I’m ok with to 10mm tolerance, I’d rather that and a strip of expending foam tape all the way round than having to hack bits out of my timber frame.

 

The spec asks for an R value of 0.65 which equates to 22mm of Marmox multi board - unless there is a better board I should be looking at. 
 

If I pad out the frame just where the window is I don’t know if that would undermine the strength of the brackets they use to fit the window.  Otherwise it would simply be a case of specifying the apertures slightly oversize and putting 20mm of wood all round.  

[saveasteading]

25 minutes ago, Dunc said:

requested a 10mm tolerance

All round or across the opening?

 

The latter is only 5mm gap all round.

[Iceverge]

What wall build up have you gone for in the end? Does it include a block fasade?

 

Mount the windows flush with the sheathing and put strip of dense mineral wool or  the outside lapping over the frame and the timber frame studs. It would perform better and be far easier to install. Much like an EWI install. 

 

The as drawn detail will have lots of heat loss as I have shown.

 

 

Then Screw J beads to the window, slot normal plasterboard in and have perfectly cheap and tidy reveals. 

 

 

 

 

[crispy_wafer]

@Iceverge I need to get hold of some of these J beads for my window reveals, do you have a picture of what they look like?  I mean if I go into the merchants and ask for j bead, I want to end up with the right stuff!

[Iceverge]

[Nick Laslett]

@G and J, I still have a small quantity of Illbruck compriband tape in the 3 sizes. You a welcome to have them if you want to play about with them. The tapes are an expensive way to finish the window joint, but I used expanding tape, FM330 foam and Siga Fentrim for my windows. I found a blog post with some testing data, where standard compriband on it’s own was not airtight enough, they have improved the product since then. 

 

 

Due to the way the ICF check reveal work, I had 10mm vertical gaps, then a few openings where I’d been too excited with the EPS rasp, I had some 15mm gaps. 
 

 

Edited February 18 by Nick Laslett

[G and J]

1 hour ago, Iceverge said:

What wall build up have you gone for in the end? Does it include a block fasade?

 

Mount the windows flush with the sheathing and put strip of dense mineral wool or  the outside lapping over the frame and the timber frame studs. It would perform better and be far easier to install. Much like an EWI install. 

 

The as drawn detail will have lots of heat loss as I have shown.

 

 

Then Screw J beads to the window, slot normal plasterboard in and have perfectly cheap and tidy reveals. 

 

 

 

 

Downstairs from outside:

render

75mm blockwork 

50mm airgap

breather

9mm osb

140mm stud filled with mineral wool.

vcl

60mm battens with RW3 mineral wool between

11mm osb

plasterboard and skim.

 

Upstaurs from outside:

vertical wood cladding (fire treated)

battens

counterbattens

breather

cement board

9mm osb

140mm stud filled with mineral wool.

vcl

60mm battens with RW3 mineral wool between

11mm osb

plasterboard and skim.

 

 

 

[G and J]

2 hours ago, Nick Laslett said:

@G and J, I still have a small quantity of Illbruck compriband tape in the 3 sizes. You a welcome to have them if you want to play about with them. The tapes are an expensive way to finish the window joint, but I used expanding tape, FM330 foam and Siga Fentrim for my windows. I found a blog post with some testing data, where standard compriband on it’s own was not airtight enough, they have improved the product since then. 

 

 

Due to the way the ICF check reveal work, I had 10mm vertical gaps, then a few openings where I’d been too excited with the EPS rasp, I had some 15mm gaps. 
 

 

I will take you up on that kind offer Nick, thank you.

 

[Nickfromwales]

These may come in handy, for injecting the 330 foam into ‘nooks & crannies’   

 

https://amzn.eu/d/736GNiG

[G and J]

I finally nailed down the head tech chap at the window company.  He chuckled, agreed with my assessment, and then mentioned that the window system we are planning to use has a thermally broken frame extension option for just this purposes.   These add 20mm to the frame width and are applied on both side and at the top, with the cill performing the same function across the bottom.  
 

So with these to be added we simply increase the aperture size to suit, allowing 10mm for fitting.  
 

Job done it would appear.  
 

Sigh.  

Thank you for all the replies folks. . 

 

[craig]

On 17/02/2025 at 22:24, G and J said:

So is it ok to add 25mm air gap round the window frame - to be filled with expanding foam tape

 

Yes.

 

It's how that gap is dealt with, it's a lot of PU foam and you'll probably be cheaper going with a product such as illbruck TP654.

[Nickfromwales]

9 minutes ago, craig said:

 

Yes.

 

It's how that gap is dealt with, it's a lot of PU foam and you'll probably be cheaper going with a product such as illbruck TP654.

Is that based on the assumption that airtight tape from frame to fabric will detail the airtightness?

 

I’d personally still go with the 330 foam as it is airtight, purely as a second fail safe layer in case the tape is compromised. 
 

If it’s <£5 a can cheaper then I’d just roll with the 330 (tbh if it was more than that I’d still use 330, if it’s an airtight house). 

[craig]

No, 25mm is a big gap to fill using PU foam, it expands and fills the void and is airtight as well and the time it takes to foam this versus using TP654.

 

Edited February 19 by craig

[G and J]

Rightly or wrongly I can’t feel good about designing in foaming stuff.   I get that it’s a useful tool sometimes but it doesn’t feel like the best starting point.  We are going for as airtight as we can manage, and my head is all full of membranes and tapes.  
 

Should I view foam more positively?

 

(We haven’t had the costs back for the frame extensions yet, so if it’s that scary I might be reading up about foam!)

[Nick Laslett]

I did not have the confidence to use drawings for ordering windows. I waited til I had the rough opening to measure. But I did make sure I’d started the process with the fabricator and had all other details agreed. They are at the mercy of the glass manufacturer in any case. 
 

I appreciate this delays the build, but the house isn’t weather tight until the roof is tiled. You can always polythene the window openings to let work continue. 

[G and J]

5 hours ago, Nick Laslett said:

I did not have the confidence to use drawings for ordering windows. I waited til I had the rough opening to measure. But I did make sure I’d started the process with the fabricator and had all other details agreed. They are at the mercy of the glass manufacturer in any case. 
 

I appreciate this delays the build, but the house isn’t weather tight until the roof is tiled. You can always polythene the window openings to let work continue. 

Snap.  One of the attractions of the company we are using is that they measure, make and fit, without it being months and months waiting. 
 

But me being me I wanted to ensure we had the best chance of ending up with what we are planning to end up with.  No alarms and no surprises please.  
 

And especially no surprises with egress windows