General: How to repair bad window tape installation and finish reveals

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- CommentAuthorShevek
- CommentTimeMar 20th 2022 edited
You might have seen that I had some windows installed in Portugal last year:
http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16834

I made the mistake of leaving out of their package the making good of the reveals and this resulted in a few problems:

A. I insisted they tape the reveals but they used a tape that isn't fully adhered (only a 20 mm edge of the tape is adhered), so it can't be plastered over. There's an air pocket behind the tape and it's kind of taut from window frame to masonry reveal, instead of running at a right-angle to allow for some plaster.
B. The windows are inward opening and the hinge is very close to the edge of the with frame (3-4 mm), so this makes it difficult to run any reveal finishes into the window. (I'm used to external opening windows in the UK and so I hadn't considered this problem).
C. At the head of the window there's no lintel, just the existing plaster (which had/has some reinforcing bars across it) and some insulation foam board behind that. So not much to plaster on to.

JAMBS
After staring at these reveals for months trying to determine how I could fit aluminium or wood reveals, I've decided instead that I should just cut the tape back, redo it with Pro Clima Contega Solida SL and then plaster the reveals. I'll tape onto the 3-4 mm of window frame to the side of the hinge + perhaps a little lap of existing tape across the shim gap that I could leave when I remove the existing tape. There's spray foam in the shim gap but I should perhaps squeeze a polyethylene backer rod in there too, to make sure I have something to push the Contega against.

1. There are a couple of window brackets on each jamb, which means there'll be a zone of plaster that's only a few mm thick. Do I need to put some mesh in there perhaps? Is there any type of plaster particularly suitable to such a thin application?
2. Should I use a corner profile where the reveal meets the main wall, or can I just plaster around that corner without any problems.

HEAD
At the head of the window, where you can see the tape has actually come away from the foam board and plaster, I was thinking about spraying some adhesive in there, push the tape back up, clip a metal lath strip or corner strip over the tape into the foam board or plaster, then plaster over it. Although I'm having a little trouble finding expanded metal lath strips in Portugal.

3. What would be suitable spray adhesive for this?

The alternative would be to do much the same thing but instead of spraying adhesive, cut the existing tape back and add some Contega again. Although how much adhesive I get into the foam board I don't know.
- CommentAuthorPeter_in_Hungary
- CommentTimeMar 20th 2022
Posted By: Shevek. There are a couple of window brackets on each jamb, which means there'll be a zone of plaster that's only a few mm thick. Do I need to put some mesh in there perhaps? Is there any type of plaster particularly suitable to such a thin application?
2. Should I use a corner profile where the reveal meets the main wall, or can I just plaster around that corner without any problems.

1 over here there is a product called glett (generic name not trade name) It is a finishing plaster that comes in grades for different thicknesses of application. The finest grade is suitable for 0 - 6 mm thickness going up to 6 -30mm application. Once mixed it has a work time of about 20 mins.
2 Use corner profiles. The Corner profiles are put on during the glett application.

The way glett is used over here is walls are rendered by the builder, usually with sand/lime/cement mixture then later the decorator comes in and goes over the wall with glett prior to painting. The grade of glett used will depend upon the quality of the wall and the rendering.
- CommentAuthordjh
- CommentTimeMar 20th 2022
Posted By: ShevekI should perhaps squeeze a polyethylene backer rod in there too, to make sure I have something to push the Contega against.
Where tapes or membranes go around a corner you want to allow a little slack, so the tape won't pull away or tear if there is some movement, so I'd be tempted not to put a backing rod in and make a small loop in the tape by pushing it in a bit further instead.
- CommentAuthorShevek
- CommentTimeMar 20th 2022 edited
Use corner profiles. The Corner profiles are put on during the glett application.

Okay, so I'll scrape off a few millimetres around the perimeter of the reveal first, so I can embed them properly.
- CommentAuthorShevek
- CommentTimeMar 20th 2022 edited
I'd be tempted not to put a backing rod in and make a small loop in the tape by pushing it in a bit further instead.

Okay, thanks, that's one this thing to worry about too.
- CommentAuthorShevek
- CommentTimeMar 21st 2022 edited
I was wondering if Tescon Profil (+ Tescon Primer) might be more appropriate than Contega Solido SL?

At 80mm the Contega Solido SL is going to extend most of the way across the reveal, whereas 60 mm of Tescon Profile has a thinner release strip (12 mm) and will leave more masonry surface for the plaster to adhere to.

But it sounds like Contega Solido SL has superior adhesion to the masonry/plaster substrates:

Tescon Profil datasheet:
Pretreatment with TESCON PRIMER is required in the case of adhesion to wood-fibre underlay panels and smooth mineral subsurfaces. Concrete or plaster subsurfaces must not be sandy or crumbling.

Contega Solido SL datasheet:
Reliable joints: waterproof SOLID adhesive has extremely strong adhesion on mineral subsurfaces

I guess I could always cut the Contega Solido SL down to reduce its width to 60/70 mm before using. In fact I could cut it down on the release strip side to give myself a more appropriate release strip width.
- CommentAuthorjfb
- CommentTimeMar 21st 2022
id definitely go with the Solido - you can plaster onto it but not the tescon.

next time - measure up for windows and then allow another 50mm or whatever thickness you need for reveal insulation/finishing!
- CommentAuthorShevek
- CommentTimeMar 30th 2022 edited
Posted By: jfbid definitely go with the Solido - you can plaster onto it but not the tescon.

ðŸ‘ðŸ¼

next time - measure up for windows and then allow another 50mm or whatever thickness you need for reveal insulation/finishing!

Yes, I made the mistake of leaving it up to the window fitter/manufacturer to measure up and make the correct allowances.

This was compounded by their inexperience dealing with airtightness tapes and expanding foam tapes (this is in Portugal). They were used to using mastic sealant externally and just expanding foam to the gap, no tape. So they were used to a small gap between brickwork and window (for the mastic joint) and they used a tape that wasn't appropriate for plastering over (it wasn't fully adhesive). They also used an expanding tape that didn't expand enough and had to mastic the external joints anyway. The other thing I didn't like is that they used window brackets without a kink in them.

50 mm allowance would have been too much in this situation too. It would have reduced the glazing area significantly without improving insulation very much because the wall cavity here has a spray foam insulation layer of about 30 mm to the outside face of the inner leaf.

The big bang for buck here was making the windows airtight. It's made a huge difference to internal comfort.

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