How To Install A Window In A Cavity Wall

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Trying to decide where, relative to the cavity, to position the windows and doors then how best to prepare them at that place. New masonry build with 150mm crenel fully filled with PIR board (CavityTherm), and want to minimise thermal bridging and achieve good air tightness of course! Hoping that the experienced folks here can help.

I take read with a cavity wall that it is all-time to position the windows off the outer leaf and over the insulation, and to wrap the insulation around the frame if possible. The accredited detail (M11-WD-04) advises to move the frame back off the outer leaf to overhang the cavity by at least 30mm, Denby Dale has windows in the middle of the insulation, and others here have discussed like solutions. My beginning thoughts were that middle of the cavity/insulation would be best merely I found research from Leeds Met that concludes "lowest psi values are achieved when the window head is positioned in line with the external wall crenel". And I noted that, unlike Denby Dale, some people here have stock-still the window to the inner face of the outer leaf. So I accept been trying to model it in Therm and move the frame effectually to notice if there is a sugariness spot. I don't understand how to summate Psi factors, or how to interpret the flux patterns, and so merely been looking at overall u value. The event is that resting 15mm on the outer leaf is improve than not touching the outer leaf at all. A quirk of my model perchance, or my lack of understanding? Can this be correct? Thermal Flux images below simply in case someone can explicate them to me.

Probably going with uPVC windows (on toll for performance and low maintenance grounds), not going to have much frame to wrap in insulation, so not modelled that. Hoping for a slate sill, and withal to work how to close the cavity, window ledge etc. It would be user-friendly if the frame did muffle the crenel externally, although I would withal let loose with foam and DPC to reduce air flow and damp in the cavity. If resting on the outer foliage then do I need a plywood box to back up the frame, or would metallic straps suffice. Mayhap a box would make air tightness easier to achieve.

Vague on positioning, fifty-fifty more vague on fixing, any advice welcome.
- JambTherm01.jpg
I like then on the back of the outside pare hiding some of the frame.
You lot should use something with slightly amend insulation than the cavity insulation to close the crenel to become best results
Protecting the window by setting it back into a reveal has many other benefits, they are less exposed and are more than protected from pelting etc.
Posted By: tonyYou should use something with slightly better insulation than the cavity insulation to close the crenel to go best results

Using PIR in the crenel, so not sure what would exist a meliorate closer, er aerogel I gauge.... :)

Found the original Leeds Met paper that investigated the variation of psi with position of window in crenel. It also shows that the results were quite flat when the exterior of the frame is nearly in line with the inside of the outer leaf. Since I can not easily wrap any insulation or cake piece of work around the shallow frame, I am content to permit the frame just remainder, say 15mm, on the outer leaf.

Fixing with metal straps dorsum to inner leaf seems OK, and filling gaps with PU cream and silicone, but not and so clear most achieving airtightness with straps disrupting the record. Any work arounds?
Employ a plywood subframe to bridge the cavity space and aid with air tightness. 'We' looked at this in some other thread and the plywood has a minimal effect on the PSI value.

Information technology looks similar you tin use Therm - although I must nit-pick and say that your modelling is non 100% accurate - you should be omitting the window frame in the calculation of the PSI value. So do a model with a plywood subframe. Have a wait at BR497 - it is free to download and gives some very handy advice on modelling conventions.

The plywood box will allow fixing and sealing of the window to be done a fiddling more than robustly than merely pumping foam and silicone into large gaps.
Thanks Timber, will have a go at calculation a timber box. My modelling is definitely simplistic if not woefully inaccurate, not really clear what I am doing simply game to attempt. Found a window position in EWI discussed in another thread, only I could take missed something.

Having problem finding BR497 for free, any links?

Posted By: TimberThe plywood box will let fixing and sealing of the window to be done a little more robustly than just pumping foam and silicone into large gaps.

Do you just add packers and screw the window into the ply box, or apply straps onto the box?
Definitely employ a ply box. Brand the external leaf opening smaller. Prepare ply box to window such that the ply extends out a small corporeality and the ply buts against the outer leafage. Prepare box to inner leaf. Employ an EDPM expanding tape to seal gap betwixt window and block/return (dewdrop the render and fit tape after).

I had all sorts of worrys about the weight of the window simply they are rock solid. 18mm ply *only* be really careful of the ply. If information technology is the high spec stuff, check it before you use it. I got some that was rubbish and wished I had not used information technology (Expensive only less than marine ply). I would probably bite the bullet and get for Marine ply next time.

Fill gaps around with Flexi Foam so it will move.
Slight hijack, but what about supporting doors/ sliders in the crenel. Denby dale used a fiberglass box department, but no mention how this was fixed/supported.
No worries Woodgnome, not a hijack at all, I would like advice on fixing doors and sliders also.
Doors I did exactly the same.
Posted By: woodgnomeSlight hijack, but what about supporting doors/ sliders in the cavity. Denby dale used a fiberglass box section, but no mention how this was fixed/supported.

They commodities fixed the fibreglass box to the slab, although its not clear how this was accomplished equally the box was filled with insulation. I assume the insulation was polyurethane expanding foam added afterwards the bolts were fixed.

Especially for level entry thresholds, I retrieve its more straightforward & more than robust to build something in when y'all bandage the slab. If y'all use structural insulation which tin take a wood screw (e.1000. CompacFoam) and then it can be fixed to the outer leaf & slab using wall starter ties (e.g. Ancon Staifix Starter Tie or Crenel Starter Tie). A slice of fibreglass plate or C aqueduct can be glued on top to provide DPM/air bulwark continuity.

David
My finished floor level and thus entry thresholds are going to exist a block grade higher up my slab (insulation above slab, UFH in screed etc.), this makes my doors much similar my windows. I guess I could bolt some structural insulation betwixt inner and outer leaf, just how differently do I need to treat doors? There is the touch of feet to consider, merely is the actress weight of the door itself an outcome?

Borpin - how broad are your openings over how broad a cavity?
Windows are up to 1200 so openings that plus 45mm (18mm x2 plus a plumbing fixtures gap). Build up is, 120mm PIR outside frame, 50mm crenel, block. block is returned against insulation to close cavity.

bank check out photos at www.borpin.co.united kingdom of great britain and northern ireland (way out of engagement though).
On doors, I moved the blockwork out to a normal 50mm cavity so the doors had something to residuum on. Please with the particular - bit of a thermal span, simply non also bad.
Borpin - cheers for the pictures, actually helps to see how others exercise things. Would never have idea of sticking the ply box to the window then installing the lot. Interesting. No straps at that place, but did Denby Dale use straps and ply box?

David - had a expect at Cavalok. Even the non-structural version of BigBlok can take 150kg/g when bridging between inner and outer masonry. That could support a window and interesting for thresholds too peradventure, non the door weight, simply peradventure the human foot autumn.

Suggested a ply box for windows approach to my architect, and his response was "why a box, wouldn't straps do?" Going back to my OP (and being more confident with Therm), whilst the platonic position for the window is aligned with at the back of the the outer leaf, but over lapping the other leaf is pretty much the same thermally, gives some support to the frame and closes the cavity externally. So I don't seem to accept a structural reason for a ply box. Using Therm a ply box is non especially better or worse than metallic straps thermally - steel more conductive than woods, but more wood there. Timber already told me that didn't he!

So the "why a box?" comes downward to air tightness, to which my architect said "why not use record and wet plaster like we are elsewhere". With the rigid board fully filling the cavity, and good masonry, nosotros are non going to have large gaps to fill with cream. Going to think some more then try to produce a drawing.
Posted By: GreenfishSo the "why a box?" comes down to air tightness, to which my architect said "why non utilise tape and wet plaster like we are elsewhere".

The problem is, this is never actually air tight. Look at what they do on the continent to makes these joins air tight. A chip of plaster does not cut information technology IMHO.
Agreed "just a bit of plaster" is not enough for air tightness, and then again neither is just a ply box. Skilful employ of tape seems essential in both cases. Best record for the chore? Or something else?

Look at what they exercise on the continent to makes these joins air tight.

Any links? What exercise they practice?
I used this SWS system http://youtu.be/74HnLT9S3io the comment past the UK office was "we have not washed much with this in the UK as you lot put your windows in from the incorrect side".

A ply box sealed to the inner (light steel) frame with Flexi foam between ply and steel gave me an initial air exam of 0.8 air-conditioning/h.
Borpin, great link! Overnice air exam besides
Now I am wondering what other tape and foam systems/products there might be, but not sure how to search for them. At to the lowest degree I know there is something to search for.