Windows installed to too far away from DPC?

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I just realised I poted this in the Buildings section, so here it is the correct section as well.

Where should windows be in relation to brickwork and DPC? Should they be as close as possible to the DPC or away from it?

The windows in our ex council house were replaced by the original owners maybe 20 years ago. I have always thought that they were placed too far out as they are only 1cm from the outside line of rendering. Our neighbours either side are the same.

We continually get a cold area all around the inside reveal adajcent to the frame, which then turns to dampness and mould. The frame isn't cold or wet, so I have wondered where the dampness comes from. Recently in this bad weather we have a physically wet patch as big as your hand about 6 inches from the top on one side of a window. There is no break in the silicon on the outside.

As I'm in the middle of doing our kitchen, I had the opportunity to chop the plaster back from one of the windows. The cavity is clsed with a house brick on edge and slate to act as a DPC. The window is 2 1/2 inches away from the slate.

This 2 1/2 inches is the sort of area that we experience damp and mould around reveals.

I'm planning on replacing the windows as I go along and moving them back towards or straddling the DPC slightly. Is this correct?

Read more: //www.diynot.com/forums/buildi...r-away-from-dpc.393802/#3031363#ixzz2tZlaLwRe
 
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They are normally just fitted to where the old ones were when retro fitting but ideally want to be fitted at least 30mm back to protect from the elements. If you have cast concrete or tile cills it can effect where you need to fit them.

In new build they are fitted to close/span the cavity.
 
Hi Gazman. The old windows would have been thick wooden framed ones, so maybe they just put them where the front face of the old wood frame was?

The cills are cast concrete and have a raised platform incorporated, where the old window frame would have sat. This places the inner face of the old frame at the slate layer.

Is it likely that the cold is penetrating the outer leaf, going past the uPVC frame and surfacing on the inner reveal plasterwork? Afterall, this is still the same block.

Here's a pic of a kitchen window, but all the windows in the house are installed the same way. You can see the slate in the middle of the pic.
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Yes they would have fitted to the old outside line to save having to render or trim all around the windows on the outside.

Having the windows fitted spanning the DPC may help a bit with thermal bridging. Are they cavity walls that are just closed up with block or solid wall?

If you were to bring them back as far as you were planning would the window then sit behind the external concrete cill? You will have to consider how you will make the cill good and keep it water tight.
You could use pvc trims (wouldn't look very nice) or patch in with cement and paint it but this could be thin in places and crack up.

Before changing the windows it may be worth an experiment. Pick the worst window for condensation/damp and fit some insulation backed plaster board around the reveals and leave it for a few weeks to see if it makes any difference.
 
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FENSA recommend a 75mm set-back for the frame. Its not always possible.

Replacement frames should, if possible, be set back to touch the inner skin.

Again, its not always possible or convenient, say, for instance, that you have to match up to a render line.

The key details are:
that the sill must project to allow free moisture discharge from the sill throating channel.
and that an insulated cavity closer is used - cav closer's depend on the width of the cav and what, if any , cav insulation is in place?

I suspect that the use of a modern cav closer will do more to eliminate damp and cold bridging around the frame than anything else.

Perhaps, look at cav closers on ribaselector.co or cavitytrays.co
 
Thanks for the suggestions.

Gazman, the walls are cavity walls and the existing windows have narrow trim top and botom which indeed looks bad, but the reason for it at the bottom is due to the plinth on the concrete cill where the original frame sat. I've pulled the trim off here and it is filled with clear silicon behind it, which I've poked with a blade and it goes back 35mm to the plinth. The window is 70mm unit, 40mm of the plinth is showing on the inside edge, so the plinth must be 75mm.

This allows me to fix the new window almost at the inside edge of it, with 5mm of the plinth showing. The window will be a 70mm unit, so this still means that I won't be able to come up to the slate, by 20mm, or else I'll have a 25mm lip of plinth on the outside. I could chisel this off, as either way I'll have to use a trim, but it's rock hard.

Your idea of insulated plasterboard could be the solution here as I won't need to come right up to the slate if the insulated board stops the cold from coming through it.

ree, putting a cavity closer in now would mean completely knocking out the existing cavity closing brickwork. Possible, but a lot of added work in my limited timescale as the kitchen units, plasterer and sparky are all booked in.
 
With the new PVC windows you could use PVC cills too to give you a bit more.

Stub cills are 90mm over all which would give you an extra 20mm to bring the window back in.
All the other sizes of PVC cills have a lip at the front though that would make fitting and measuring a bit more tricky.

Stub cills don't look too bad on a concrete cill, If you look around you road you will probably see some windows fitted with them.
 
Not sure about having a cill on top a cill, might look a bit naff?

Looking at the cill/plinth again and as I will be making good the render around the outside anyway, I could disc cut any protruding concrete cill plinth back to the new window line. Then put a narrow trim over it to hide the plinth, as I would have to do anyway and as was done by the previous fitters.

That way, I can bring the window in as much as I wanted, up to the slate and not use any insulated plasterboard. I'm assuming that there is no benefit insulating beyond the slate inwards?

Regarding the insulated plasterboard. I have looked and the only types are quite thick, Kooltherm K17 for example. There is a thinner foil backed board, as I've used in my porch ceiling, but it can only be fixed with mechanical fixings. This would mean drilling into the cavity closing brickwork and I'm not sure how much meat is there to drill into. The redbrick that has been used for this, that I can see, is a sliver about 50mm wide, but no idea how deep.
 

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