When replacing or moving windows outbound of their current position into an EWI layer, what are the options for internal finish?
The trouble is some step-by-step retrofits is that internal finishing may, confusingly, be an early task. To get “family acceptance factor” (FAF - I’ll claim that acronym) it may be necessary to “finish” a refurb/retrofit with replastering, painting etc early.
If there’s no budget for EWI and windows early, and it must be done later, how can the windows be moved outbound without ruining the early finishing work? Or is that impossible?
Thoughts:
Is replastering just the internal reveals really possible? Every plasterer I asked about this did the familiar teeth-suck thing and muttered about cracks and joins.
This is actually for a hypothetical house. We’ve been down this road before, but although the end result was good it was tough during the process for some in the household. I was posting to try to work out the feasibility of trying this again, and maintaining healthy relationships with the rest of my family.
We’re currently looking to purchase a new house but I’m going to need a solid plan if I’m to convince others.
Likelihood is it will be a post war cavity-walled house as those are the styles we tend to prefer.
I didn’t actually realise the plywood box approach had a maximum size, although I suppose that makes sense. There’s always the compacfoam, timber or angled straps approach to fall back on.
What do you mean by:
You would need to stop at least some plaster to fit the boxes to the brick/structure
?
And by:
Oh dear!
I assume that means there aren’t any common answers…
I guess I’m trying to tap what others have done, but although the above techniques have been written about a lot, finishing seems to not be as well documented.
Oops, I’m not sure what I actually typed, after spell checker got to it!
What I mean is that the box needs attachment to the reveal for a greater distance than the depth of the current frame, therefore at least some of the existing plaster will need to be removed in order to attach the box.
Actually, for the nearest finish I would remove all the plaster and line the whole reveal.
If your hypothetical house has an insulated cavity then you could ensure the window is lined up with that and extend the EWI into the reveal
By this I mean that I wouldn’t try it. Any movement between old and new could dislodge or break tiles. A plain matt or silk finish is what you need.
Where I have insulated window reveals I have left them all white. This makes a feature of the window, doesn’t need to match existing decor and allows more light to enter the room. This last is a particular concern where the reveal is significantly deeper, as otherwise less light will reach the room. A common side effect of thick wall insulation is dark(er) rooms.
Is it possible to line with plasterboard and skim to meet existing plaster - would that crack?
If your hypothetical house has an insulated cavity then you could ensure the window is lined up with that and extend the EWI into the reveal
The intention here would be to have EWI and move the windows into that layer. The cavity would also be filled. There would also be some sort of consideration to the air tightness of the cavity, assuming the internal plaster layer is the AT layer. If a window box fitted from the internal wall, over the cavity and into the EWI layer we’d need to ensure no air ingress from the cavity into the thermal envelope.
You can save money on quality ply by fixing a sheathing of thin ply over cheaper board, so long as the base isn’t too porous. Ideally you need a base board that is air tight but water vapour open.
I agree that airtightness is a major consideration, particularly when there is a cavity. I would go for overkill and make the empty reveal totally airtight to, from and between leaves and then make the ply box airtight to the reveal. If you have read this forum avidly before posting this thread you will know that I’m in love with Blowerproof, so much so that I should ask for a commission! I would paint the entire reveal with 2 coats.
It’s much easier to handle than taping sheets of plastic into place and sealing the edges.
If plastering it makes an excellent primer too.
Yes, it is possible to line up old and new plaster and skim over. If one side is plasterboard and the other is wet plaster then you will get a hairline crack. In such a case you need to remove the plasterboard to a less obvious location and wet plaster the remaining space. Generally the corners of the reveal are handy as they are normally reinforced and often hidden by curtains. This ties in with my airtightness regime.
Try to avoid plasterboard. If you can. Air moves uncontrolled behind the boards. Wet plaster, by contrast adds to airtightness.
I used low density fibreboard for my first ever refurb. Retrofits weren’t invented then. I treated a wet cellar wall with epoxy paint, fixed acoustic fibreboard and skimmed, to keep the structure permeable. That was 1978.
So I’d say if moving windows out early, it would depend on how you’re moving them out.
If it’s ply boxes, then ply is airtight so you’d just tape from frame to ply with standard airtight tape, as well as the corners of the box, then plasterable tape from ply to inner wall face before plastering wall (use primer first before taping to masonry). Then plasterboard in front of ply.
If you’d be using brackets externally (take care to keep about 20mm clear between end of bracket and outside of frame so that insulation can cover it), then I’d just do a wet plaster finish internally (primer on masonry, then plasterable airtight tape from frame to masonry, then plaster.
In either case, no need to insulate internal reveal as you won’t get any advantage from doing that once the EWI is done (you would if you’re doing internal insulation, or perhaps in the scenario where EWI is only being done many years later…what I mean is that when EWI is in place, no need to insulate internal reveal…).
Or in the scenario you mention where windows are going to be done later, I think if it was me I’d then just leave the windows where they are and finish the airtightness works properly. Then when EWI is done, don’t move windows but insulate the external reveal from EWI back to window. I’ve done thermal models for both scenarios, and found that the lion’s share of heat loss at this junction is dealt with by insulating the external reveal. I’ve just looked up an example project, and where windows remained but external reveal was not insulated, psi-value was 0.32 W/mK. Then we could reduce that by 91% to 0.03 W/mK by insulating the external reveals back to window, and then if we would move the windows into the EWI layer it would become zero.
The second scenario is what I was alluding to earlier. Thanks for the figures @Eric_Fewster.
If the psi values are a concern then the internal reveals can be insulated. That is my plan. As there is no rush I would insulate as each room came up for redecoration.
It could be argued that insulating the reveals will not completely resolve the psi value issue, and it won’t. Remember that the thermal thickness of walls at window frames is just the thickness of the frame. By insulating the reveal you are increasing the thermal thickness to that of the whole wall and removing any relative cold spots in the reveal. This is particularly important in high humidity areas.
Windows are done early because they’re wanted, but then to avoid later disruption shouldn’t be moved later when EWI goes in.
The solution of returning EWI in the external reveals works here.
Another possibility which might work in some scenarios: spec the windows 20mm smaller in height and width. Encase the window in insulated strips (probably a high performance insulant given the feeble depth) like we see with the Compacfoam solution, except to sit in the reveal rather than proud of the external face. Weather proof on the outside and air tighten on the inside. Then when EWI goes in, the external reveal EWI meets both the insulation and the window.
Not sure how the fixings would work. They could go through the frame, or U/J shaped brackets used? If you were using Compacfoam you could attach to the foam then the foam attaches with a “normal” plate bracket to the reveal.
Has anyone seen this approach documented? I almost think it could be a product option for the window manufacturers.
It is important that the insulation meets and covers as much of the frame as practicable as the frame normally has a poorer U value than the panes. By including insulation in the window installation you allow a thicker insulation layer in the external reveal.
Only the insulation below the frame (and sill) need to support any weight, so you could specify lower U value, lower strength insulation for the other three sides.
I would really encourage a push for EWI to be completed with windows. Especially if full scaffold is being set up, as many window installers and EWI folks will not work from scaffold towers! The cost saving can be significant.
