200mm cavity window installation - my plan!

soldsold

Hi All,

As the number of posters looking at wide cavities is increasing, and the window installation for wide cavities is a hard detail to get right, I thought I would document my plan and see if any of you have concerns about any details, or suggestions for simpler methods. Hopefully it will be of interest to those considering wide cavities.

My priority is structural soundness, followed by airtightness, then cold bridging, finally cosmetic cracks.

Note the detail is for insulated (thermally broken) window frames, which is allowing me to fit the windows tight against the outer blockwork, which suits me as 7 of my windows have a sandstone finish on the outside so I think leaving a 25mm gap between the window and the stone finish would not look right and be hard to finish.

Ive considered using a WBP plywood box, or a water resistant MDF box, but the cost of the carpentry involved has turned me off this idea.

So my plan (see drawing attached) is:

A. Cut a wedge of PU or EPS board to fit above the window into the space created by the horizontal DPC that runs at an angle from inner blockwork down to the outer blockwork. I think this needs to be in place before fitting the window?

B. Attach timber slips to sides of windows, and galvanised straps to front (?) face of window that will be hidden by the blockwork

C. Attach foam tape to front face of window (my best guess, ive never seen foam tape)

D. Put a bead of airtight mastic on the concrete window sill upstand (eg Isovar airtightness mastic or similar)

E. Fit window in place, sitting the window on the concrete sill upstand

F. Strap the window to the outer blockwork with the galvanised straps

G. Cut 60mm thick PU insulation boards to size to fit tightly between the back of the concrete sill and the inner blockwork, making sure to keep the sill DPC lapped up between the sill and the PU piece. Use airtight foam between window and PU sheets (eg illbruck non hardening foam that is gentle and will not push the PU pieces out of position)

H. Cut a strip of airtightness membrane long enough to fit continuously all around the window cavity and tape this to both the window and the interior blockwork using appropriate tapes/ beads of airtightness mastic - eg Pro Clima/ SIGA/ MOY/ Klober tapes

I. Screw the top and side insulated plasterboards/ cement boards in place to close the cavity - does it matter which is fitted first? Use bolts and contact cement for quinnlites (or just mushroom fixings/ adhesive?)

J. Fit the interior timber window ledge to the inner blockwork using adhesive or screws

K. Fit plaster beading, then skim coat then paint the window reveal
The above is the simplest method I can see, and is based on talking to window fitters, and looking at details of other ways self-builders have closed the window reveals.

Any comments and criticism welcome.

Some notes and questions below.

Thanks!

S

Notes:

1. The membrane will be installed and taped to the window, as I will be using full-fill cavity pumping with fire resistant glass wool I dont see the need for a cavity closer?

2.and 6. Do I need an insulated slab here (eg 22mm total thickness - 9mm+12mm)? I dont think I do...Cement board might be simpler, can I use this cement board as an airtightness layer and tape the window/ inner block to this board?

3. I think I will need something to hold the plasterboard/ cement board in place, this seems the easiest way for angled window reveals, maybe a routered rebate in the window frame would allow ordinary plasterboard to fit without the timber slip?

4. I can cut a chunk out of the inner blocks (quinnlites) if needed, to get access to screw in the straps for the windows. Any comments on types of straps/ how many to use? Strapping to the outer leaf seems the simplest method to use and structurally safer to hold heavy triple glazed windows in place.

5.Expanding foam tape seems the best bet, are there better options?

7. Some of my windows have natural sandstone on the outside, turned in 100mm at the edges to form a sandstone external window reveal. Will moving the straps away from the sandstone till they find solid concrete be ok?

8. How to fix the plasterboard to the window here? Another timber slip on top of the window frame?

9. I have been assured a 20mm thick window ledge will not move if glued or screwed into place.

10. Should I use a different detail for the windows with a plastered external finish to those with a sandstone finish? ie set the window back into the cavity 25mm or so to fill with foam afterwards?

Any other comments?

thanks!

sydthebeat

have you this detail signed off by a structural engineer??

i would see possible structural integrity issues with the distance the outer lef blockwork covers the cavity and thin wedge of quinnlite.

I just did a quick calculation and theres about 270mm from the beginning of the splay on the quinnlite to the end of the blockwork.

This could be very dodgy on large openings

soldsold

Hi Syd, Thanks for taking the time to look over the drawing. My engineer did structural calculations at the start when deciding on the blockwork to use, and the quinnlite blocks on the inner leaf were left on the flat, with one wall tie at every course around the windows, and an RSJ as a lintol for the wider openings. The only large window that has this angled reveal also has 150mm of sandstone outside the outer leaf which I would guess would add some structural stability. So the detail is: 9 inch wide quinnlite as the inner leaf, 4 inch dense block as outer leaf, 6 inch sandstone outside this. The quinnlites are also only used just around the windows - as they were easy to cut with a chopsaw at 30 degrees to form the angled reveal, everywhere else is dense block on the flat as the inner leaf A hollowcore floor sits on top of the inner leaf which I have been told will give "triangulation" strength for the roof and stabilise the whole of the inner wall - but this was a passing comment so I might have taken it up wrong. I do see your point though. Anyway, the answer is yes, my engineer has made sure to calculate before any structural decisions were made, including sizes of RSJ's to support hollowcore floor etc. But whether he took the angled reveal into account Im not sure.

soldsold

Bump!

this is the cross section of the window Im planning to install, hope to get a couple of + or - comments on my planned method to install the windows...

Cheers!

Steve

atech

Hi Steve,

My thoughts on this would be you may have left out one priority which would be workmanship and the feasability of 2 or 3 things:

1) The cavity is 200mm wide correct? Your frame is about 90-100mm at its widest point. Trying to put your hand and hammer/drill into the remaining 100mm or so to fix the straps to the inside of the outer leaf would be fairly difficult. This may be easier on the jamb because of the angle though.

2) Although the window is strapped to the blockwork at the head and jambs, most of the weight is resting on the 9" cill. For large windows and especially triple glazed they can be quite heavy and may unbalance your concrete cill.

3) Just a small point, In your blow up of the jamb, your vertical DPC seems to cross the straps and is only about 100mm wide, the dpc would have to stay between the wall and the straps and by fixing the straps the DPC would be punctured.

I have attached a PDF of something i'd been working on myself, although with a 150mm cavity.
(Please do not take this detail as what should be done, its for indicative purposes only)

P.S. I realise the window frame looks abit strange but ignore that (the width of the frame is the only important factor in that)

You will notice certain things would have to be done in stages for this to work, i.e cavity closure at cill, conc pad on inner leaf, steel angle and rigid insulation would have to be done prior to the conc cill being inserted. (This is probably not feasible for you at this stage.)

Also if the cavity increased to 200mm and the frame was only 70mm wide (not 90mm as shown) the steel angle may need to increase to 150*75 to allow the window frame to rest on this and the conc cill.

Best of luck with it anyway and keep us posted.

soldsold

Thanks Atech,

Some good points in your reply, Im not concerned about access to the straps, if needed Ill just drill through the inside blockwork with a longer drill but I think I have room anyway.

Workmanship wont be an issue, I'll be doing the detailing myself

atech wrote: »

2) Although the window is strapped to the blockwork at the head and jambs, most of the weight is resting on the 9" cill. For large windows and especially triple glazed they can be quite heavy and may unbalance your concrete cill.

3) Just a small point, In your blow up of the jamb, your vertical DPC seems to cross the straps and is only about 100mm wide, the dpc would have to stay between the wall and the straps and by fixing the straps the DPC would be punctured.

On No 2 above, this is a concern, Ive thought about strapping the cill externally to the outside of the wall, or inside using angle brackets

On No 3, my stonemason advised using wider dpc so the drawing isnt too accurate. Good point though.

I have also noticed that the straps I have shown that strap the window to the lintol overhead cant be used, as the straps would puncture the horizontal dpc over the window.

So my question here is: Are windows usually strapped to the lintols overhead, or just to the sides?

A note on your drawing, if you put the horizontal dpc in and have it crossing the cavity while the blockwork is going on, it will surely catch a load of mortar? This is why I left this DPC built into the inner blockwork but hanging down until the windows are going in.

Cheers,

Steve

atech

soldsold wrote: »

So my question here is: Are windows usually strapped to the lintols overhead, or just to the sides?

A note on your drawing, if you put the horizontal dpc in and have it crossing the cavity while the blockwork is going on, it will surely catch a load of mortar? This is why I left this DPC built into the inner blockwork but hanging down until the windows are going in.

Good point on the DPC tray, I was trying to keep the water as far away as possible from the window head. Hadn't thought about falling mortar. I imagine it would be a similar problem with pressed metal lintels. By right blocklayers are supposed to use a timber board in the cavity to catch these but in reality it doesn't happen to often and thats a big reason why partial fill insulation isn't suitable. Might be best to leave the DPC as you have shown it so.

Regarding the straps, it might be best to ask your window supplier but I would think once you start going over 1m wide they would need to be strapped on top to stop any movement which could affect the mastic seal outside and airtight membrane inside potentially causing draughts.

dfader

atech wrote: »

Hi Steve,

My thoughts on this would be you may have left out one priority which would be workmanship and the feasability of 2 or 3 things:

1) The cavity is 200mm wide correct? Your frame is about 90-100mm at its widest point. Trying to put your hand and hammer/drill into the remaining 100mm or so to fix the straps to the inside of the outer leaf would be fairly difficult. This may be easier on the jamb because of the angle though.

2) Although the window is strapped to the blockwork at the head and jambs, most of the weight is resting on the 9" cill. For large windows and especially triple glazed they can be quite heavy and may unbalance your concrete cill.

3) Just a small point, In your blow up of the jamb, your vertical DPC seems to cross the straps and is only about 100mm wide, the dpc would have to stay between the wall and the straps and by fixing the straps the DPC would be punctured.

I have attached a PDF of something i'd been working on myself, although with a 150mm cavity.
(Please do not take this detail as what should be done, its for indicative purposes only)

P.S. I realise the window frame looks abit strange but ignore that (the width of the frame is the only important factor in that)

You will notice certain things would have to be done in stages for this to work, i.e cavity closure at cill, conc pad on inner leaf, steel angle and rigid insulation would have to be done prior to the conc cill being inserted. (This is probably not feasible for you at this stage.)

Also if the cavity increased to 200mm and the frame was only 70mm wide (not 90mm as shown) the steel angle may need to increase to 150*75 to allow the window frame to rest on this and the conc cill.

Best of luck with it anyway and keep us posted.

atech,

A couple of questions about your detail;

1. Have you supported the cill across the cavity or is it just resting on the outer leaf? would cill support not help prevent rotation of the cill.

2. Is there not a risk of rotation and cracking under the 100*100mm pad onto which the angle bracket is fixed? what revents rotation of this as the load of the window comes onto it?

3. Have you closed the side reveals with blockwork as is the norm. Wouldnt it make closing the cavity and fixing the window a lot easier and structurally better if this was done?

Thanks

atech

dfader wrote: »

atech,

A couple of questions about your detail;

1. Have you supported the cill across the cavity or is it just resting on the outer leaf? would cill support not help prevent rotation of the cill.

2. Is there not a risk of rotation and cracking under the 100*100mm pad onto which the angle bracket is fixed? what revents rotation of this as the load of the window comes onto it?

3. Have you closed the side reveals with blockwork as is the norm. Wouldnt it make closing the cavity and fixing the window a lot easier and structurally better if this was done?

Thanks

Hi dfader,

The detail is a work in progress aimed at eliminating cold bridging and the structural viability of it (and cost) would need to be looked at, I think 1 & 2 above may depend on the actual load of the windows.
I'll answer as best I can anyway.

1) The cill doesn't span the cavity as that would cause cold bridging.
I hadn't shown the cill strapped down to the outer face of the outerleaf blockwork as the DPC tray under the cill would need to be pierced to fix the straps. The load of the window would ideally be split between the cill and the angle bracket, but if the load transfered to the cill was deemed sufficient enough to cause the cill to rotate it may be necessary to strap it down alright.

2) Again it would depend on the load coming onto the bracket but the concrete pad could easily be strapped down to the internal blockwork similar to what is done with wallplates.

3) The internal block could be returned as normal in the jambs to within 50mm of the outerleaf and the void closed with vertical DPC and rigid insulation.


The rotation issues are definitely something that would need to be looked at so if there is any structural engineer on here they might give their thoughts on it and do some calcs for different load windows.

soldsold

To anyone who knows aluclad timber triple glazed window installation....

Does my drawing attached look right?

Im ordering windows direct so have to take my own measurements and provide finished window sizes, the supplier says its so there is no mix up, but Im not getting great assistance mainly due to the language barrier and their lack of knowledge of cavity walls and concrete cills.

They say there is a 25mm rebate they can supply under the window for the sill to sit into, but I cant seem to find out if that has an aluclad finish outside or get a drawing of it.

Anyone come across this before?

Also, the corner window piece they have sent a drawing of is also in the pic attached, am I missing something or is there a lack of dimensions on this for me to be able to work out the corner window size from this drawing?.

I cant afford to mess this up, 40k of incorrect window sizes would be a big mistake...

Thanks!

S

Captain Bluebear

40K is a pretty big risk to take !
The detail you show isn't great. Chances are the bedding mastic will get 'lost' in that rebate and be ineffective. There is no drip cill showing which would provide a secondary line of defense for poorly applied bedding mastic. It would not be a problem to have a packer factory fitted to the bottom of the window to allow you to face fix a drip cill that will extend down over the junction of the window and cill. The drip cill being sourced here in Ireland and preferably finished to th esame colour as your window.
Not sure where you're windows are coming from but you would want to ensure that the supplier is fully aware of our building regs - I have witnessed some shockers. Good luck

soldsold

Captain Bluebear wrote: »

40K is a pretty big risk to take !
The detail you show isn't great. Chances are the bedding mastic will get 'lost' in that rebate and be ineffective. There is no drip cill showing which would provide a secondary line of defense for poorly applied bedding mastic. It would not be a problem to have a packer factory fitted to the bottom of the window to allow you to face fix a drip cill that will extend down over the junction of the window and cill. The drip cill being sourced here in Ireland and preferably finished to th esame colour as your window.
Not sure where you're windows are coming from but you would want to ensure that the supplier is fully aware of our building regs - I have witnessed some shockers. Good luck

Thanks for the reply.

Im attaching photos of the actual window frame in position, I borrowed a "tradeshow cross section sample" and took pics of the window in position on the 3 different sills (concrete, granite, granite with thinner sill lip).

Any comments would be appreciated. Also my first time to see the window in the flesh (didnt think there was an Irish distributer) and Im well impressed.

Cheers,

Steve