27 Jul 2020
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United Kingdom
Hi, I would like a view on how best to close the cavities around window and door openings.

The wall is 7N blockwork on the internal leaf and rough stone on the outer, which results in a very undulating cavity size. I have 80mm tongue and groove insulation in a 150mm cavity.

The stone is at least 100mm and I'm expecting the windows to be recessed 80mm back from the face of the building, spanning the cavity (they are 90mm units).

I think that off-the-shelf cavity closers won't make a good tight fit to protect against wind driven rain or prevent air leakage, though maybe using something like easi-close along with foam would do the trick? Or, should I simply use more insulation board and try to plug the gaps alongside the vertical damp membrane?

You can see from the pictures that the gaps are very different sizes. Advice on how best to do this to achieve the best insulated result would be much appreciated.

Many thanks!


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Pack with more celotex, then fill with expanding foam, or depending on how good your packing is, mastic. You need to pull the DPC out further.

Is there a cavity tray above that opening?
Thanks for the advice, Woody.

There is a cavity tray, yes. It has been made using DPC layer above the opening, though I don't see any visible weep holes.

How far must the DPC wrap around the stone face? I am thinking that water would simply track between the DPC and mortar then go into the cavity... do I use mastic between the DPC and stone to prevent this from happening?
I don't understand the construction, why has the internal block work been partially returned, I can't see what purpose it has.

The vertical Dpc arrangement doesn't look too clever, I'd be nervous of leaks.
I'd be tempted to use 300mm wide dpc -push it the cavity 100mm, once insulation is packed in, fold dpc flat back to inside reveal, fit window, then fold dpc back against window, once reveals plastered cut dpc off flush.

Are the windows going to have mortar pointing to finish the outside?

How are you going to finish the inside? Usually the plastic on a closer acts as the base for bonding plaster to stick to. You could use plasterboard for the reveals -foil backed perhaps
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Bring the DPC out by about 20mm so that it meets and is bent around by the frame for a complete barrier. This includes the cill, not just the jambs.
Hi Notch, I understand that the internal block return is to provide a more secure fixing point for the windows and doors, rather than leave a fully open 150mm cavity.

Interesting point about the DPC. I like the idea, so if I understand you correctly, essentially wrapping around the insulation, folding inwards and then cutting off once windows are installed. Sounds sensible, though I thought that vertical DPM was supposed to fold outwards, against the stonework so that may cause issues?

The window installers say they will use expanding tapes which should account for a lot of variation. I had not really thought about options for the external finish, other than to use mortar.

On the inside, foil backed insulated plasterboard.
Hello Woody, my concern is that in some cases 20mm of DPC overlap could still leave gaps for water to go in to the cavity in between the joints in the stonework because it won't sit flush up against the stone.

As long as I bring the DPC out beyond the "weakest link" in the stonework, then would that be sufficient with mortar on the outer? I can't help but think there must be an accepted way of doing this but it is difficult to find examples.

Maybe I am worrying about this too much! Thanks again for the help!

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