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      CommentAuthordjh
    • CommentTimeJul 23rd 2017
     
    Some people install windows from inside, some from outside. But windows are big and heavy, so I think it pays to think about the mechanics in advance. Probably more than we did, although we got away with it. We installed from inside, mainly because manoeuvering them through the scaffolding outside seemed like it would be a problem.

    You don't want a substantial timber 'box' because the whole point is to minimise the thermal bridge. Sticking things makes them MORE difficult to remove because you have to break the entire glue line somehow.

    If the windows ever need replacement then you have to choose either to replace from inside or outside, again. If from outside then you'd have to remove the insulation over the frame and having made such a mess, removing a bit of insulation to access the outside of the box at the screw positions doesn't seem like a big issue, especially if you marked the screw positions when you installed the window. Replacing from the inside means removing the window board and all inside trim so you could access the screws that fix the box to the house fairly easily. But removing the window frame in little pieces seems like the most probable outcome and then figuring out how best to install the new one as a fresh problem. Straps might well seem an attractive option at that point.

    I appear not to have any details showing how we actually installed the windows. Lots of draft details but as I said we didn't think it through in advance well enough!

    Our installation was somewhat unique, since the windows are embedded in straw bale walls. We have timber uprights alongside the windows so the box sides are screwed to the window and then to the timber using packers as required, with some of the dreaded foam as filler where required. The sides of the box extend from the inside of the timber past the window almost to the outside of the bales to provide support for the woodfibre insulation that covers the outside of the frames. Underneath the window there's a piece of OSB almost bale width that sits on the bales. Attaching that, with membrane above it, to the window and then lifting the whole lot into place was the trickiest part of the operation. Above most windows there's a void to the box beam above, thats filled by a lightweight timber-framed structure with an OSB floor all built after the window is installed. (In some windows where the head abuts the box beam we used standard window straps to restrain the head of the window.)
    • CommentAuthorgoodevans
    • CommentTimeJul 24th 2017
     
    Thanks DJH - I think my plan A is to fix the cement board box to the frame (with membrane between) - lift the whole lot from the inside - with someone outside to stop it toppling out , pack/wedge/level between the wall and the cement board fix flashing above the top of the cement board just in case. render/plaster to the membrane inside and out - overlapping insulation on the frame externally if possible and using reinforcing mesh internally over the gap between the cement board and the block.
  1.  
    Our frames were fixed without the glazing installed and the triple glazed units were put in afterwards. In my last house, with timber double glazing, I put the frames in without the sashes and screwed those in afterwards.
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      CommentAuthordjh
    • CommentTimeJul 24th 2017
     
    Fitting without the glazing is fine if the windows are not supplied pre-glazed. In our case, the suppliers also suggested that it was a poor idea to remove movable sashes before fixing too, although we did do that in one case. I never really understood their logic, but I did want to be able to rely on their continued support in resolving any issues.

    So horses for courses, I suppose.
    • CommentAuthorborpin
    • CommentTimeAug 2nd 2017 edited
     
    Posted By: goodevansI think my plan A is to fix the cement board box to the frame
    You asked if I would use the cement board as the box - NO. I don't think it is strong enough. I did exactly what you are suggesting; box to frame - sealed with Silirub (iirc). insert from inside, pack and level; seal between box and house frame. At this point you could use a tape round the box if necessary. The supalux is fitted on the outside of the box flush to the frame to close the exposed insulation.
    • CommentAuthorgoodevans
    • CommentTimeAug 3rd 2017
     
    OSB appears to be only a little stronger than cement board in bending strength - and about the same stiffness. The limiting factor for this application is deflection so it's the stiffness that is important.

    However as cement board appears to behave better if it does get wet I think it would be a better substrate for a wet plaster finish.

    Or am I wrong?
    • CommentAuthortony
    • CommentTimeAug 3rd 2017
     
    Plaster should not be applied to osb, it could be with building paper, laths and expanded metal, but most peps would use plasterboard. The wet as in wet plaster is very transient, then dry for its whole life.

    Something is wrong if window linings get wet.
    •  
      CommentAuthordjh
    • CommentTimeAug 3rd 2017
     
    Posted By: tonySomething is wrong if window linings get wet.

    Indeed so, but water penetration especially through the bottom corners of windows is a fairly common fault, so belt and braces isn't silly, IMHO.
    • CommentAuthortony
    • CommentTimeAug 3rd 2017
     
    Is that belt and braces design to stop the water getting in or belt and braces in case it does get in
    •  
      CommentAuthordjh
    • CommentTimeAug 3rd 2017
     
    Belt and braces does not admit of OR.
    • CommentAuthorMikC
    • CommentTimeAug 4th 2017
     
    Has anyone ever thought of using phenolic ply to box out their windows? I've used it for trailer decks for years and it is very strong compared to OSB and it is totally rot proof. Cost is on the high side compared to OSB. About £70 for an 8x4 18mm sheet iirc but it's excellent stuff for external durability. (not fireproof though.....)
    • CommentAuthorborpin
    • CommentTimeAug 4th 2017
     
    Posted By: goodevansOSB appears to be only a little stronger than cement board in bending strength
    Maybe it is, but I used top grade (I should have used Marine) Ply (there is discussion elsewhere about classes of Ply). I would not use OSB for this. The cement board is more likely to crack as well. I used plasterboard on the inside.

    What I do suggest is getting some sadolins on the exterior exposed edge of the ply. I didn't and a bit of water started to delaminate it a little (before it all go sealed in). As soon as I noticed it I put some on and there was no further damage.

    I still do not understand the deflection issue. Any deflection is resisted by the sides of the box which is trying to be twisted in the plane of the sheet.
  2.  
    Bear in mind that just about anywhere in Ireland would be in a location subject to either severe or very severe exposure to wind-driven rain - http://www.servecommunity.ie/wp-content/uploads/2010/08/04-Joseph-Little-Insulating-safely.pdf (slides 5-7)
    Accordingly there wouldn’t be much enthusiasm for using even genuine ‘marine’ ply in any construction where moisture could get in and not get out. Not such a problem with a ventilated cavity. If anybody is using a cavity construction for a new build then structural (load bearing) cavity closers using the same Compacfoam are available from Cavalok and in my opinion are the best allround solution for fixing windows and doors.

    The method used in the video in the OP for fixing the doors was interesting even if the installers only appeared to arrive at it by chance. The first attempt using masonry nails (into what was clearly sub-optimal structure) and thin metal straps is poor practice for a heavy triple-glazed door. The fixing through the frame into the Compacfoam block is far more robust for the long term functionality of the door.

    The minimal fall of the pressed alu cills isn’t good – water will pool in front of the window and get blown back at the window and adjoining interfaces.
    • CommentAuthorborpin
    • CommentTimeAug 9th 2017
     
    Posted By: Monty GerhardyAccordingly there wouldn’t be much enthusiasm for using even genuine ‘marine’ ply in any construction where moisture could get in and not get out.
    In my case, the ply end up well inside the cavity, it was a problem while left exposed for a short while. I put a render stop bead in the ingles abutting the window frame, leaving just enough room for some expanding foam tape to go in so the seal is pretty good.
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