External wall insulation and new windows

[phatboy]

Hi,

I am looking at buying a house that has cavity walls, but the cavity is very narrow (Wall is just over 9" thick).

I am considering external wall insulation as some other houses on the estate have done, but also new windows at the same time.

I have read a couple of articles that suggest moving the windows to the front edge of the current walls, so that when insulation is installed there is no 'cold bridging' between the windows and the blockwork.

Can anyone in the know advise if this is a sensible approach, or completely wrong?

Also I am aware that the window sills will need capping to be longer. Is it also reasonable to form new sills that are 100mm deep, after removing the old ones?

Thanks
Tim

 

[AronSearle]

All good.

You will need to insulated the cavity as well though, or air movement inside the cavity can have a big impact.,

 

[phatboy]

Thanks Aron,

Is it possible to insulate the cavity with it being so narrow? I hadn't thought of doing that tbh, I thought unless you could get a DPC into the cavity insulating it can cause it to bridge damp from outside?

So I am correct in thinking that moving the windows to the edge of the blockwork is the right thing to do? I assume then I would need cavity closers and then plaster it all up? This will be my first experience of a building over 30 years old!

Thanks

 

[crank39]

Not sure you'll get a cavity closer in, they're generally used on new builds where you can build around them

 

[AronSearle]

The external insulated rendar will provide another weatherproofing layer, ensure that openings are properly sealed and maintained (like seals around window frames).

Loose polystyrene beads will fit, and should not cause an issue of moisture bridging as they allow some drainage.

At the very least that cavity needs to be sealed, or you will just loose heat through convection within that cavity, reducing the effect of outer wall insulation.

So I am correct in thinking that moving the windows to the edge of the blockwork is the right thing to do?

Yes.

 

[SmileyDan]

If you insulate over the roof at the same time, going the whole-tea-cosy-hog I *think* you don't need to fill the cavity.

It's been known to fill the cavity with concrete (more thermal mass) but bead is easiest and is a DIY job if you want to do it. EPS is not wicking AFAIK.

Ideally the windows should be in the insulation layer, not on the masonry. But you need EWI thick enough. But you were going to do that anyway, right?

 

[AronSearle]

If you insulate over the roof at the same time, going the whole-tea-cosy-hog I *think* you don't need to fill the cavity.

Yes, as the cavity then becomes an internal area. Whilst it might still act as a 'heat chimney' it is still only moving warm air within the insulated building.

It's a question of how you seal up the 'tea cosy' so that the wall/roof junction does not become a cold bridge/air leakage point.

As to thermal mass, it is generally over-rated. In our climate thermal mass normally causes more problems than it solves (stores heat in the day when you don't need it and releases it at night = summer overheating at night).

 

[phatboy]

I can't think how the windows would be fixed into the EWI, as it will surely be soft?

To 'tea'cosy' the place I would somehow need to push insulation over the cavity, which would block the air circulation through the roof space? Any suggestions in how to work around this?

 

[AronSearle]

The 'tea cosy' idea requires a heck of a lot more effort, not saying it is not worth it, but you need to be doing more than just putting insulation all around.

The roof does not need to be ventillated if it has a suitable membrane (not the black felt you typically see in lofts).

A lot of houses are going to be hard to effectively 'tea cosy' as the junction between the walls and roofs would normally need some opening up to do it properly, otherwise you end up with insulation gaps at that location.

I can't think how the windows would be fixed into the EWI, as it will surely be soft?

Treated softwood batten support frame.

Still present a small thermal bridge, but much less than the brick.

 

[SmileyDan]

There shouldn't be a bridge at the windows, at least not via the supports, because you return the insulation onto the frames.

Plywood is most often used to support the windows.