Avoiding a "step" with different block types in extension build

gimpotronitus

Hi,

I am building an extension at the moment. The extension is being built with 2 x 4" solids with insulation between them. The old house looks like it was built with a 9" cavity with feck all insulation.

Am I right in thinking that when the two walls "line up" that they wont be "flush" and will be out by about 3 inches or so, given that there will be 4 inches of insulation between the 4" bricks.

Assuming they wont be, what is the best practice here? The wall is not a party wall and is in the side entrance. The kitchen is against this wall so it needs to be flush all the way along.

Thanks a lot.

DOCARCH

Probably would have been best to consider at planning/design stage. Normally I would suggest external finishes should line through/be flush and on the inside, the 9" wall be studded out/dry-lined to build internal face of existing wall out to match internal face of new wall.

Photo would be good to see exactly what's happening?

De.Lite.Touch

gimpotronitus wrote: »

Hi,

I am building an extension at the moment. The extension is being built with 2 x 4" solids with insulation between them. The old house looks like it was built with a 9" cavity with feck all insulation.

Am I right in thinking that when the two walls "line up" that they wont be "flush" and will be out by about 3 inches or so, given that there will be 4 inches of insulation between the 4" bricks.

Assuming they wont be, what is the best practice here? The wall is not a party wall and is in the side entrance. The kitchen is against this wall so it needs to be flush all the way along.

Thanks a lot.

Perhaps you should have continued it in 9" hollows and externally insulated it all of it, with some high u-value blocks below DPC.

There's a lot of hoo-ha about the threshold cold bridge, but leaving a lot of uninsulated 9" hollow would be far worse IMO.

I think its possible you can take HD insulation below the DPC level and put insulation below the floor slab and turn it up at the outer wall.

Perhaps other technically minded people here will comment on that.

You need a U-Value of 1.5 for a floor with underfloor heating and some systems come with a foil underlay type insulation for a total 50mm build up.

Hey, its your call, but changing building technology to cavity still leaves you the existing 9" hollow, uninsulated to deal with.

As been said, a sketch or photo might tell a tale.

De.Lite.Touch.

FesterBeatty

I'd go with a drylined hollowblock wall instead of changing to cavity construction. P.s ensure the builder has installed starter packs between new/existing walls or you will end up with differential settlement and consequential vertical/diagonal cracking at the joint. (Most builders dont do it for some reason(!!) so its one to watch out for)

FB

RKQ

DOCARCH wrote: »

Probably would have been best to consider at planning/design stage. Normally I would suggest external finishes should line through/be flush and on the inside, the 9" wall be studded out/dry-lined to build internal face of existing wall out to match internal face of new wall.

Photo would be good to see exactly what's happening?

100%+
OP is it possible to line up both blockwork walls internally and insulate existing 9" wall externally?
Its worth considering especially if you might externally insulate your existing home at some future date.

I would not build the new extension with 9" hollow blocks :eek:
Tie in new extension blockwork to existing blockwork, with stainless steel straps to limit settlement.

Poor Uncle Tom

FesterBeatty wrote: »

I'd go with a drylined hollowblock wall instead of changing to cavity construction.

This is not acceptable construction for habitable purposes, imo.

De.Lite.Touch

Externally rendered hollowblock with a waterproof admixture can be used in less exposed places.
Hollowblock can be built with insulated plasterboard internally on mechanical fixings as opposed to dabs to keep the cavity as tight as possible.
This is because as the insulation rises, the cavity gets relatively colder and is that much more likely to be the place where interstitial condensation develops.
You want this kept in the blockwork, not the cavity - less likelihood of bad smells and damp patches developing.
The benefit of internal insulation is that it avoids cold bridging, given say 40mm warmboard reveals to opes and underfloor insulation returned up at the perimeter meeting the internal wall insulation.
Threshold details need a bit of work.

De.Lite.Touch