I have new door and windows for back of the house arriving, single storey 1980’s extension to a 1970’s house. Very rough render/harled finish externally, solid plaster internal.

As part of the work I am moving a stud wall internally, and this will reduce the back door from 2.6m width of sliding door to 1.6m of french doors.

The existing wall appears to be solid blockwork – I have taken off some render, and can see solid block at front, past frame and pick it up again internally as solid.

280mm in total for wall depth. 10/15mm of render and 10/15mm of plaster internally. This suggests a 250mm depth block.

I plan on infilling the 1m wide x 2m high gap left by reducing door size. I am concerned that the render needs to be on similar substrate to prevent cracking, and that he thermal properties are pretty crap!

My suggestion is still 250mm, solid wall blocks, but use modern light/thermal blocks, and add internal insulation via an insulated plasterboard straight onto said solid wall.

Thoughts and issues I may have missed?

Just to make things awkward, 1m beyond the door the other way the wall construction changes to 1990’s timber frame cavity wall….

Are you sure it’s not two skins of 100mm block with a 50mm cavity?
That’s the construction I would expect to see in an extension of that age.

250mm Is not a standard thickness of block. In the eighties you could get “grunters” which were 215mm thick and bloody heavy, hence the name. They weren’t commonly used in domestic applications though.
Is the section of render you’ve hacked off in the ingoe of the external door? If so, it might be block returned to close the cavity.

It *could* be the 100mm/50mm/100m, that was what I was expecting.

The wood doors helpfully are blocking my view, and I dont want to take them out until morning of fitment / check before the new doors are correct size. They are 120mm overall, so could hide a cavity. Door is fitted from outside – the render has been re-done around them.

I can get a fingers round window edge, that is block returned from reveal, but I can see the inside of wall and it looks like solid block….

I cannot see any weep holes or cavity vents either…

I am placing bets on a bodge job when I get stuck in…

Can you get access to the top of the wall where it meets the roof? A loft space perhaps? Might be visible there.
I assume you’re doing the work yourself?
I’d infill the gap in 100mm block with an insulated cavity regardless. Assuming you’re retaining the existing lintel and the infill panel Isn’t structural.

Can you get access to the top of the wall where it meets the roof? A loft space perhaps? Might be visible there

Nope, sealed loft space without even ventilation in…

Assuming you’re retaining the existing lintel and the infill panel Isn’t structural.

I am keeping the lintel, so yes this would work well.

I assume you’re doing the work yourself?

Yessir.

I would drill a 6mm hole with a masonry bit, from the inside or outside depending where it’s easiest to make good, and poke a bent wire down it to see if there’s a cavity.

Ahh, good call.

I would drill a bigger hole, but I just want to see you drilling big holes in your house.

I have a sledge hammer…?

Solid wall with internal insulation is a really bad idea. It’s not thick enough to prevent damp coming through and or you may get condensation in the wall depth.

1m wide by two high is really small. If it’s in block you wil need 2 leaves with wall ties between the leaves and back to the existing wall. You will need to put Dpm between the leaves at the door junction or the damp will jump the gap. If you only have a 50mm cavity there’s no room for insulation.

I would do a 100mm timber inner leaf fixed back to the brickwork. Face in 9mm min osb/ply. Face with a breather membrane (tyvek) then build a wall in 100mm block. Remove circa 300mm external render, metal lath the joint then put render over. It may still crack but less likely, or just put a mastic joint there.

Internally insulate between the timbers, then finish with a foil backed plasterboard.

Or just get a blank door panel beside the French door to make up the width….

I can’t really be arsed tonight but yes you can insulate a 50mm cavity. Google celotex to start with..

I can’t really be arsed tonight but yes you can insulate a 50mm cavity. Google celotex to start with..

The timber wall would work well. hmm.
There would be big wastage (a roll of breather for example, for 2m Sq)

Actually edit all that, 2 leaves of 100mm block, rendered inside and outside to match the existing will be the best approach and more stable. Don’t bother with insulation if you don’t have any in the wall elsewhere as it will make next to know difference.

You could sneak 25mm high density insulation in there but if you bridge the cavity it can cause real issues.

Celotex won’t bridge the cavity. Expamet the joint as above, plenty of fixings both sides ie nails. Make sure you use a furfix strip or similar up the existing 2 leafs of masonry, at this point and at the reveal your cavity ties want to be in at 225 centers. I’d then reboard that whole wall with a 25 mm insulation backed plasterboard dabbed on, this will help with any internal movement and subsequent cracking and also give you a second layer of insulation. If you don’t put an insulated board on the reveal I guarantee you will get mold at the bottom of the door on the reveal within a year.

It’s built without celotex in the gap. Tbf, the rest of the thermal insulation is shocking, so lots of new stuff in roof and overboarding walls happening….the floor however 😕

What could possibly go wrong?
Hidden behind some plasterboard above what was the old back door I found this… 😐

[url=https://flic.kr/p/E9Z99T]IMG_20160310_082858110[/url] by Matt Robinson, on Flickr