Window fitting into Timber frame with external cladding - Compression filler, DPC, membrane?

[oranjeboom]

Have the window installers coming tomorrow to fit some of the 3G UPVC windows and I'd just like to ensure that they are doing it the 'correct way'. We've agreed that they only install the windows with me doing the finishing off later with cills, taping etc. So I just need to be sure that they fit the 88mm frame at the mid point of the 175mm SIP wall. Externally, I will have wall fitted with cladding.

 

Questions

 

So my only questions today are around tomorrow's fitting of the window, all the other weather tightness aspects will come later. But the placement of the window will be important to get right of course as that will impact the follow-on work. So currently in my head, I have:

 

1.  Breather membrane (that's currently sealing up the window opening) to be cut and folded inwards over window reveal.
2. DPC on top of breather membrane extending outwards under cill and over cladding (NB note the 30mm air gap between cladding and SIP). How do I stick it down - or just use the weight of the window frame?
3. Fit frame directly on top of the SIP (on DPC layer) or on compressible layer?  I have concerns about compression/shrinkage of the panels - From what  have read there won't be much compression of the SIP panels but I'd like to cater for any movement and any pressure on the window frames by not filling the gap around the window and SIPs with anything solid. I think I have a 20mm gap  around the window so there is hopefully sufficient allowance for any 'movement'. During installation I'm looking to leave that 20mm gap (i.e. not get the installers to just sit the window frame directly on top of the panel) by putting some temporary packers/shims in there to ensure non of the frame is touching the panel. Once the frame is all screwed in, I was going to fill the gap between the frame and SIPs with foam (Soudal non-expanding type/equivalent). Or should I go for a compressible filler? But what should I be using? I've looked at Compriband stuff, but that is just too pricey for my liking, so was hoping to just use Soudal type stuff which I could do if the window is sitting on removable shims

 

 

I've attached a window detail that I found on the www (from Nordan) that I have amended. Ignore the window frame detail itself - mine is UPVC.

 

If anyone could comment on the above 3 items that would be great. The guys arrive tomorow and I just need to ensure that the correct materials are in place before they start doing it all 'their way'.

 

TIA!

 

OB

[Crofter]

How level and square are your openings? If you sit the window frame directly onto the bottom of the opening, you won't be able to make adjustments easily. The wedge shaped shims are not just there to temporarily fill the 20mm gap but also to allow very fine adjustments to be made.

 

I used low expansion foam all around, I don't think it would be practical to try filling the gap with a tape.

I am, however, considering applying some form of tape on the outside to form the weather seal. This will be fairly easy for me as the outside face of the windows lines up with the outside face of the timber frame.

 

Oh and you've got me a bit worried as I didn't put any DPC underneath the window- although I'm not really sure why it's needed?

[jack]

5 minutes ago, Crofter said:

Oh and you've got me a bit worried as I didn't put any DPC underneath the window- although I'm not really sure why it's needed?

 

It might be belt and braces, but I was told it was needed to protect the frame from condensation dripping off the underside of the cill.  Unfortunately, ours doesn't have any either (installer error), and when I went to install some afterwards found that they'd permanently glued the cills in place!

[Crofter]

In my installation, the cill overhangs the cavity and cladding, but not the frame. So hopefully no problem.

[jack]

Ah, lucky you!  Mine's more or less exactly as shown above, but without the DPM! 

[oranjeboom]

22 minutes ago, Crofter said:

How level and square are your openings? If you sit the window frame directly onto the bottom of the opening, you won't be able to make adjustments easily. The wedge shaped shims are not just there to temporarily fill the 20mm gap but also to allow very fine adjustments to be made.

 

Very level and square. So I think I may just add some over-long battens in there to raise the frames 20-25mm and then remove them once the foam has all set.

25 minutes ago, Crofter said:

Oh and you've got me a bit worried as I didn't put any DPC underneath the window- although I'm not really sure why it's needed?

As Jack alluded to, it's for condensation, though probably a bit of overkill.

 

 

[IanR]

Do you have an airtight layer beyond the inherent "airtightness" of the SIPs panel? What level of air tightness are you trying to achieve?

To me the DPC looks like a potential air leak path, and with the breather membrane there as well, not entirely necessary (other than on windows/doors that start at ground level).

The Sections show straps. Direct fixing through the frame would be better for airtightness. Straps are difficult to seal around. Direct fixing would also help hold the DPC in place if you choose to keep it.

 

I'd say shim the window up to create a gap for a Low expanding foam to fill. 20mm is a big gap. I assume that is 10mm top and bottom (and sides), rather than 20mm top and bottom. The LE foam allows the SIPs to settle without pinching the frames.

[jack]

42 minutes ago, IanR said:

Straps are difficult to seal around.

 

Never a truer word said.  Wish we'd had some advice about this before our window installation.  Caused a lot of consternation with the people doing the plasterboarding, and we ended up with bits of airtightness tape peeping around the edges of the plasterboard in a couple of places.

[oranjeboom]

2 hours ago, IanR said:

Do you have an airtight layer beyond the inherent "airtightness" of the SIPs panel? What level of air tightness are you trying to achieve?

As good as I can get, but not aiming for anything. This is an extension to an old bungalow which will probably leak like a sieve if tested. Having said that, I will be aiming to get it right where possible, especially with doors and windows. I was going to have a VPC layer on the interior wall - i.e. over the membrane and then up and over the DPC and stuck down to the window frame using airtightness tape. I'll add that to the pic a bit later.

2 hours ago, IanR said:

To me the DPC looks like a potential air leak path, and with the breather membrane there as well, not entirely necessary (other than on windows/doors that start at ground level).

With above approach, I should hopefully get a decent seal with the VPC lapping over the DPC and then sticking down the VPC on innermost window frame edge (next to the No.10 in pic)

 

2 hours ago, IanR said:

The Sections show straps. Direct fixing through the frame would be better for airtightness. Straps are difficult to seal around. Direct fixing would also help hold the DPC in place if you choose to keep it.

 

Yes, not using straps on these windows - I tried to delete them from the pic. So the frame will just be screwed into place (though they'll probably want to use nail guns which I wont allow in case I ever need to adjus/remove the frames).

2 hours ago, IanR said:

I'd say shim the window up to create a gap for a Low expanding foam to fill. 20mm is a big gap. I assume that is 10mm top and bottom (and sides), rather than 20mm top and bottom. The LE foam allows the SIPs to settle without pinching the frames.

 

I'll have to check my tolerances, but yeah should be therabouts. SIPs have been in place for just over a year now, so hoping the biggest contraction/settling/movement has taken place.

Edited September 13, 2016 by oranjeboom
VPC detail added, bracket removed

[Oz07]

nail guns to fix plastic windows. Check for spurs on their boots n all

[IanR]

17 minutes ago, oranjeboom said:

 I was going to have a VPC layer on the interior wall - i.e. over the membrane and then up and over the DPC and stuck down to the window frame using airtightness tape. I'll add that to the pic a bit later.

With above approach, I should hopefully get a decent seal with the VPC lapping over the DPC and then sticking down the VPC on innermost window frame edge (next to the No.10 in pic)

As long as the VCL is installed as an airtight layer (taped joints and edges) then that should work, otherwise it will still leak air, it just won't be apparent around your window frame as the leak path will got through into your service void.

[TerryE]

9 hours ago, oranjeboom said:

Assertion 1. So I just need to be sure that they fit the 88mm frame at the mid point of the 175mm SIP wall. Externally, I will have wall fitted with cladding.

 

Q1.  Breather membrane (that's currently sealing up the window opening) to be cut and folded inwards over window reveall

 

Q2.  DPC on top of breather membrane extending outwards under cill and over cladding (NB note the 30mm air gap between cladding and SIP). How do I stick it down - or just use the weight of the window frame

 

Q3. Fit frame directly on top of the SIP (on DPC layer) or on compressible layer?  I have concerns about compression/shrinkage of the panels - From what  have read there won't be much compression of the SIP panels but I'd like to cater for any movement and any pressure on the window frames by not filling the gap around the window and SIPs with anything solid. I think I have a 20mm gap  around the window so there is hopefully sufficient allowance for any 'movement'. During installation I'm looking to leave that 20mm gap (i.e. not get the installers to just sit the window frame directly on top of the panel) by putting some temporary packers/shims in there to ensure non of the frame is touching the panel. Once the frame is all screwed in, I was going to fill the gap between the frame and SIPs with foam (Soudal non-expanding type/equivalent). Or should I go for a compressible filler? But what should I be using? I've looked at Compriband stuff, but that is just too pricey are in place before they start doing it all 'their way'

 

A1:  Why are you having your windows centred in the frame?  I can't understand the reasoning here.  Surely you should start with outside profile, cladding and all and work backwards from there.  Don't Norden do a custom external cill for this range in which case this will dictate the cill overhang and how far back into the frame the windows sit.

 

Q1.  As previously discussed the VPC is different to the external breather.  The key issue externally is weather protection and how this will be implemented over the breather.

 

Q2.  If your window is properly sealed the DPC below should be optional.  In our case, our frame sits forward of our frame outer surface and closes the 50mm gap between the frame and the stone skin.  I have a vertical DPC from the underside of the widow protecting the upstand discussed next.

 

Q3.  You normally have a minimum 5mm and typically 10 mm on the sides and top to allow differential expansion, etc.  This is foamed and weather protected with a decent exterior silicon.  Windows can be bloody heavy so they are typically designed to be supported underneath and therefore sit directly on some hard support.  IMO, you are asking for trouble if you don't' do this.   I sat my Internorm widows hard on a tanalised 30mm wooden spacer, though most window providers will offer a custom uPVC spacer.  (This was to give me enough room to put in battens to carry my internal cills.)  Don't bother with Compriband here; wrong application.  If you were lining your exterior reveals or overlapping your cladding and needed a weathertight, flexible seal, then Compriband would be one alternative to use for this. 

Edited September 13, 2016 by TerryE

[oranjeboom]

1 hour ago, TerryE said:

 

A1:  Why are you having your windows centred in the frame?  I can't understand the reasoning here.  Surely you should start with outside profile, cladding and all and work backwards from there.  Don't Norden do a custom external cill for this range in which case this will dictate the cill overhang and how far back into the frame the windows sit.

Best to have the window within the thermal layer - in my my case that will be within the SIP frame rather than the void between the SIP and the cladding.

It's not a Nordern window - I just borrowed that detail from them to illustrate what I plan to do.

1 hour ago, TerryE said:

 

Q1.  As previously discussed the VPC is different to the external breather.  The key issue externally is weather protection and how this will be implemented over the breather.

 

Yes, my VPC will be on the interior, with the external breather membrane on the outside. The latter will be further shielded by the timber cladding. Any penetration via the cill will be caught by the DPC...   But there should not be any issues with the cill of course.

1 hour ago, TerryE said:

Q2.  If your window is properly sealed the DPC below should be optional.  In our case, our frame sits forward of our frame outer surface and closes the 50mm gap between the frame and the stone skin.  I have a vertical DPC from the underside of the widow protecting the upstand discussed next.

Yes, only adding the DPC as extra security. Good idea about the upstand.

1 hour ago, TerryE said:

Q3.  You normally have a minimum 5mm and typically 10 mm on the sides and top to allow differential expansion, etc.  This is foamed and weather protected with a decent exterior silicon.  Windows can be bloody heavy so they are typically designed to be supported underneath and therefore sit directly on some hard support.  IMO, you are asking for trouble if you don't' do this.   I sat my Internorm widows hard on a tanalised 30mm wooden spacer, though most window providers will offer a custom uPVC spacer.  (This was to give me enough room to put in battens to carry my internal cills.)  Don't bother with Compriband here; wrong application.  If you were lining your exterior reveals or overlapping your cladding and needed a weathertight, flexible seal, then Compriband would be one alternative to use for this. 

Yes, I did have some concerns about the weight also but was thinking of having a few spacers under the frame to support the weight: 50mm from corners and then every 400mm. That is better than one bracket manufacturer recommends for their window brackets. The largest window for this extension only weighs 100kg, so with with enough support evenly spaced under the frame, it should be okay. The UPVC frame also contains a steel internal frame - less likelihood of warping.

 

My other option would be to use XPS/cellotex under the frame maybe?

[Monty Gerhardy]

Probably a bit late for the OP but might save some others going with jerry-built details in a similar situation....

http://www.nordan.no/vindu/vis

http://www.nordan.no/vindu/vis/vaeggkonstruktioner

It doesn't have to be slavishly copied but if it's good enough for the west coast of Norway the principle should be good enough for anywhere in the UK.

[TerryE]

@oranjeboom it isn't always best to have the window entirely within the thermal layer (as in my case where we've adopted a pretty standard approach for passive houses with stone skin).  OK, for a clad frame it makes sense, but you miss the point of my Q.  Why pick the middle?   As the Väggkonstruktioner link above shows, you need to get the external profile and treatment correct.  If you arbitrarily pick a centre location, then you might have to make up special to purpose external framing to keep the windows and frame weather tight. I just think that you need to think through the detailing otherwise you might face an "oh shit" moment in a week or so.

Edited September 14, 2016 by TerryE

[jack]

All I will say is make sure that the you test fit the cills before finalising window positions. You can see on the drawing you've done that you had to change the angle of the cill from the Nordan detail (which I know you've just adapted)  and chamfer the frame.  Are you sure that you can get the required fall on the cills with your current window position?

 

We had an absolute world of pain over this point.

[oranjeboom]

13 hours ago, TerryE said:

@oranjeboom it isn't always best to have the window entirely within the thermal layer (as in my case where we've adopted a pretty standard approach for passive houses with stone skin).  OK, for a clad frame it makes sense, but you miss the point of my Q.  Why pick the middle?   As the Väggkonstruktioner link above shows, you need to get the external profile and treatment correct.  If you arbitrarily pick a centre location, then you might have to make up special to purpose external framing to keep the windows and frame weather tight. I just think that you need to think through the detailing otherwise you might face an "oh shit" moment in a week or so.

Middle of the frame seems the most optimal position in my case - slap bang in the middle of the SIP. I also didn't want too much sill either internally or externally. They are inward opening so I'd like the internal cills cluttter free (not that we'll open them up much with the MHVR). And I'm thinking that as I am having cedar cladding rather than some specialised system that needs costly 'specialised fixings' the cladding is pretty versatile to cut to size around the window frame. If I come across an 'oh shit' moment I'll flag it up here so that others don't follow in my errors. Watch this space!

 

13 hours ago, jack said:

All I will say is make sure that the you test fit the cills before finalising window positions. You can see on the drawing you've done that you had to change the angle of the cill from the Nordan detail (which I know you've just adapted)  and chamfer the frame.  Are you sure that you can get the required fall on the cills with your current window position?

 

We had an absolute world of pain over this point.

Cills not even ordered yet so I was not able to play around with them on the window. But they'll be bespoke alu ones so I can determine size. slope etc once all windows are in. There is sufficient fall with my actual window profile which also has a 'cill extension profile' underneath the main frame (in red below). The alu cill will attach to this and I will have sufficient slope. The first windows are in at least - no major dramas. Onto figuring out how to fit the others that will be fitted on brackets outside of the cavity brick walls (then finished off with EWI later).

[TerryE]

50 on the inside is a too tight, IMO.  You want at least 20mm clearance between the top of the inner cill and the bottom of the opening light.  I would recommend 25mm cills as IMO anything else looks mean and you need ~20mm for the levelling battons and foam.  So I make that a minimum of 65 mm.  

 

If your window faces are 70-60 set back from the face of the cladding then you've got no room for the external cill's upstand and a sensible draining gradient.

 

This type of inner opening window has an in-build drain which either drains vertically in front of the spacer that you've marked in red (assuming that the outer cill sits on this or out of the front of the window.  If you've got your windows centred then you will need windows with front drains.

 

But we've gone round the buoy too many times on this.  You are way past the point where you could address these issues by design so you are going to have to address them by retrospective fixes, if possible.   

Edited September 14, 2016 by TerryE

[Monty Gerhardy]

14 hours ago, TerryE said:

@oranjeboom  I just think that you need to think through the detailing otherwise you might face an "oh shit" moment in a week or so.

Indeed. Given the amount of money people are spending on windows it’s surprising that the installation detailing appears to be an afterthought in many instances. IMHO the hierarchy of requirements for a window installation should be

1. robustness of installation - window stays in place

2. robust detailing - it keeps the water out

3. thermal robustness - thermal optimisation

This is particularly so in areas of severe and very severe exposure to wind-driven rain ie large parts of the British Isles - http://www.nhbc.co.uk/Builders/ProductsandServices/Standardsplus2016/#135

What I would describe as THERM fetishists spend an inordinate amount of effort ensuring that the window installation is thermally optimised to the detriment of the weathering and physical robustness. All well and good for a Passive House in central Europe where the winters are not only a lot colder but they have less 'horizontal' rain. Pretty isotherms on a computer screen may not look so good in situ on the western seaboard in the middle of January.

Furthermore, the simpler the detailing the easier it will be to execute on site.

As you’ve pointed out in an earlier post the ‘compressible fill’ underneath the window is daft - as it is in the original Nordan detail. I would only envisage it being effectively used underneath a window to allow for differential movement between a timber frame and a masonry outer-leaf, and even then it would only be between the masonry and the window cill - http://www.gov.scot/Resource/0047/00479931.pdf  - detail 3.08 (E3) on page 9

[Silage]

I know this is an old topic but i'm guessing that unless the windows are fitted flush with the outer frame (or overhung) then water ingress protection of the outer window reveal is needed, hence the DPC in @oranjeboom 's original drawing. This maybe more of an issue with a timber clad option due the reduced depth compared with brick or stone. What did you go with @jack?

 

 

Edited March 15, 2018 by Silage