Fitting windows into timber frame

[Carrerahill]

Hi All,

 

I am currently doing my timber frame drawings and sorting out window opening sizes etc. and it got me thinking about the installation - the plan is to get the timber frame up and install the windows almost immediately to secure and weather-proof the extension long before the block-work goes up around it. 

 

I know it is common practise to install windows into the frame before the masonry goes up so I wondered if anyone has an installation detail or some good photos showing how they sit exactly and the sort of depth into the cavity they sit. I don't want to install them then find I have them in the wrong position.

 

I was on a site recently and I saw how they had installed them, albeit from a bit of a distance and it appeared to me they had done this:

 

Typical timber frame sheathed in OSB and wrapped in a building wrap, then circa. 50x50 rough treated had been attached to the outside of the opening round the timber frame, so basically like a picture frame over the top of the building wrap, this I assume was to acct as the cavity closer, the cavity was 50mm, so my assumption was that the window was then going to be installed flush to the outside edge of the 50x50. Then, when the masonry went up, the back brick face would be hard against the 50x50 (as you would expect a cavity closer) and the masonry would be brought right up to the edge of the window frame. The render would then go on thus "sealing" the window in and a bit of mastic at the end (as appears common on new builds around all doors/windows) to seal the UPVC to the render.

 

Does this sound correct? Would you attach band to the window and then fix it from the inside of the timber frame? In theory it sounds fine to me but I just want to check.

 

The next question is the sill, the supplier can supply me with a stub sill or longer versions. Do I get the sill, then build the masonry up to the underside of the sill, bit of mastic and job done? Or should I be thinking about a proper concrete sill to the top of the masonry?

 

I will be able to ask questions of suppliers and people involved when I get there but I need to get some of these things clear in my head before I start the frame drawings as it could make a difference to exactly where I place a window to ensure I get get a whole block or brick up to the bottom of an opening etc.

 

 

Edited January 30, 2019 by Carrerahill

[Redoctober]

Hi - take a look at my blog - 

From the photos down the bottom you should be able to see exactly what you have described above and this is the way forward. You may also be able to see the window manufacturers stub cill resting above our stone cills. Ours was a TF and as soon as it was ready, the windows went in from the inside.They protruded sufficiently to tie in with the battens which framed the opening, These battens are called fire stops which serve the same purpose as those red "socks" you will see in the photos too.

I hope this helps. 

[DonGillies]

I just done some on a project and I was surprised at how easy it was.

 

We had 50x50 battens that are cut to the fit around the windows, they are then screwed onto the outside edge of window frame (so like a frame around the window)

 

You then push the windows into the recess and the battens leave it sticking out by 50mm. You then screw through the battens to the outside of the timber frame. 

 

If you zoom in on the picture you will see what I mean. 

 

This will I’ll have a block skin on the outside of the kit so the windows will be recessed once the blockwork goes up

 

 

[Russell griffiths]

They look nice @DonGillies who’s windows are they. 

 

All i I would add is make sure you look at how you are going to air seal the frames, the days of sticking a bit of mastic in are long gone. 

 

Also so look at using a nice trim to finish the render against the frame, don’t just render up to the frame and smear a bit of mastic over it, well not unless you want it looking a bit moody. 

[DonGillies]

7 minutes ago, Russell griffiths said:

They look nice @DonGillies who’s windows are they. 

 

All i I would add is make sure you look at how you are going to air seal the frames, the days of sticking a bit of mastic in are long gone. 

 

Also so look at using a nice trim to finish the render against the frame, don’t just render up to the frame and smear a bit of mastic over it, well not unless you want it looking a bit moody. 

 

We have fire stop material around the outside and i think it will be expanding foam around any gaps. what would you recommend for air sealing?

 

The windows are triple glazed Ali Clad from a company called Alan Bros. Been pleased with them and price was good in comparison to others.

[Russell griffiths]

I had the SIGA rep at my place today doing a demo on all the sealing tapes and membranes they sell

you should be providing a weather proof tape to the outside and an air tight tape to the inside. 

Squirty foam just doesn’t cut it anymore and will make your triple glazed windows a complete waste of money if not sealed properly. 

[Jeremy Harris]

If you want good thermal performance, then to minimise thermal bridging around the window frame it's essential that the frame be set into the insulation layer, and not project out into the cold area outside the insulation.  Apart from the increased heat loss through what is already a thermally weak area, the window frame, there is also a condensation risk associated with having a cold window frame.

 

If you do need to space windows out, then adding insulation around the edge of the frame, within the cavity, can mitigate thermal bridging a bit.

 

 

[Carrerahill]

1 hour ago, DonGillies said:

I just done some on a project and I was surprised at how easy it was.

 

We had 50x50 battens that are cut to the fit around the windows, they are then screwed onto the outside edge of window frame (so like a frame around the window)

 

You then push the windows into the recess and the battens leave it sticking out by 50mm. You then screw through the battens to the outside of the timber frame. 

 

If you zoom in on the picture you will see what I mean. 

 

This will I’ll have a block skin on the outside of the kit so the windows will be recessed once the blockwork goes up

 

 

 

 

Thanks. This is exactly what I saw on a site and how I was thinking of doing it - although thinking to the future they are going to be a pest to get back out for replacement... I was thinking I could screw from inside the frame out into the battens so in the future the window can be removed from the inside.

 

 

 

Edited January 30, 2019 by Carrerahill

[Carrerahill]

16 minutes ago, JSHarris said:

If you want good thermal performance, then to minimise thermal bridging around the window frame it's essential that the frame be set into the insulation layer, and not project out into the cold area outside the insulation.  Apart from the increased heat loss through what is already a thermally weak area, the window frame, there is also a condensation risk associated with having a cold window frame.

 

If you do need to space windows out, then adding insulation around the edge of the frame, within the cavity, can mitigate thermal bridging a bit.

 

 

OK I am listening... let's see if I have interpreted what you are saying correctly. I would install the window into the timber frame... That then keeps it in the "warmer" side of things, then I have to firestop/cavity close between the frame and the block, but what do I do about rendering back to the window as I now have a big external reveal... or have I totally misinterpreted this?

 

The BC drawings I have do appear to show the window unit sitting on a wooden batten within the cavity... but a call to BC can soon agree any small details like this as I have already found.

[Jeremy Harris]

1 minute ago, Carrerahill said:

OK I am listening... let's see if I have interpreted what you are saying correctly. I would install the window into the timber frame... That then keeps it in the "warmer" side of things, then I have to firestop/cavity close between the frame and the block, but what do I do about rendering back to the window as I now have a big external reveal... or have I totally misinterpreted this?

 

The BC drawings I have do appear to show the window unit sitting on a wooden batten within the cavity... but a call to BC can soon agree any small details like this as I have already found.

 

It's a thermal detail that isn't often picked up properly, AFAICS, but it is important.  Window and door frames are the most thermally conductive parts of a window, so to minimise heat loss through them, and perhaps more importantly reduce the condensation risk, the minimum amount of frame needs to be exposed on the cold side.  If there is insulation all around the edges of the frame, so that only the outer face that's visible is exposed to the colder environment outside, then that improves things a lot.  If the frame projects out with no insulation around it, then the area of frame exposed to the cold side is greater (the outer face plus the area of the edge that projects outside the house insulation).

 

When our windows were fitted I made sure they were set into the insulation layer, so the frames don't project out at all, as I was OK with having relatively deep reveals in order to get much better thermal performance from the doors and windows. 

 

If you choose to accept a deeper external reveal, then you need to close the cavity around the aperture with a firestop plus a batten to support render board, for a rendered finish.  For a brick finish then the brickwork can be returned in to provide both the firestop and close the cavity externally

 

An alternative, if you want the windows and doors to project out, with shallower reveals outside, is to fit them rather like an insulated Velux, with peripheral insulation so that the heat loss from the projecting edges of the frame is reduced.  To do this probably means using fixing straps to secure the windows to the frame, then adding a suitable thickness of decent insulation all around the frame, within the cavity. 

 

 

[DonGillies]

1 hour ago, Carrerahill said:

 

 

Thanks. This is exactly what I saw on a site and how I was thinking of doing it - although thinking to the future they are going to be a pest to get back out for replacement... I was thinking I could screw from inside the frame out into the battens so in the future the window can be removed from the inside.

 

 

 

 

I also worried about removal in the future, although the Project Manager on the job said they should last at least 20 years so can worry about it then. 

[Carrerahill]

13 hours ago, DonGillies said:

 

I also worried about removal in the future, although the Project Manager on the job said they should last at least 20 years so can worry about it then. 

Ouch! 

[Carrerahill]

I got my quote in for my windows and I spoke to the supplier, who is also an installer and he confirmed the 50x50 strap method - however, he can supply the windows with pre-fitted insulated firestops which I may just go for. I will need to confirm the detail with him but assuming it will fit then that will do.

 

I think I am going to get the window with a stub sill and then build a concrete sill up under the window when it comes to block-work - I am going to sit down with CAD and carefully work ou my sizes so I am full courses to the underside!