re-enforcing and insulating a single skin wall

21 Feb 2017
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United Kingdom
Hi All

We have an old single story single skin block built extension to the rear of the house. It's generally fine but i'd like to improve the appearance and insulation a little and just for my peace of mind, re-enforce it.

It's currently rendered to the outside. I've pulled off the internal plasterboard and there was 25mm polystyrene insulation underneath. It's like a sun room which is open to the house -been there about 30yrs according to a piece of newspaper I found stuffed in the wall. My plan is:

1) Horizontal 25x50mm battening to the outside, insulation, vapour barrier and vertical larch cladding

2) Inside, 50mm square vertical battening, then 50mm insulated plasterboard over it.

I have a few questions:
1) Should I also have a vapour barrier inside?
2) Should I also insulate between the 50mm internal battens?
2) Should this lattice of internal and externals battens and cladding give a good amount of extra strength?
3) Should I Instead leave a 50mm cavity and build an internal stud wall out of 2x4, and insulate the cavity and/or between studs? This is the way I started off thinking but the cavity means the stud wall doesn't offer any actual strength to the existing wall.

Or am I going about it the wrong way completely? I want a simple a cheap way to achieve it really - not rebuilding the walls...

I'm in the UK by the way!

Thanks for any help
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It's essentially a garage conversion. There are lots of threads regarding this.

The walls don't need reinforcing.
Thanks but that's kind of why I posted. There are dozens of ways to do it. Treating it as a garage conversion doesn't change the fact you can read something that confirms one method and another directly contradicting it.

For example, some say leave the cavity so it can breathe in between, others say pack it tight with more polystyrene insulation so there is no airflow and moisture can't gather. Then someone else will say it wall gather in the tiny voids still - interstatial condensation??

Looking at the celotex installation manual it suggests leaving the cavity. But then an older kingspan one says sandwich the insulation to the wall with the batton and leave the void behind the plasterboard...

So whats one to do?? Concentrating on the internal, I feel leaving the cavity (or at most using a wool insulation so there is still airflow) is best? Then 50mm rigid insulation & plasterboard or insulated board (either one with vapour check).

If you saw the quality of the brickwork you might feel they need some "help" o_O
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Polythene up the wall. Make your frame fixed floor to roof not to wall. Insulate with foil backed insulation complete, no air gaps, no cavity. Min 25mm insulation across the whole lot. Vapour check layer, or tape the foil insulation, then plasterboard.

You can't strengthen a loose wall without rebuilding it.
Thanks for your reply woody, i'm tackling this at the weekend.

I'm planning on using 2x4 studs. between the studs I will use EPS, then over the whole lot i'll use 50mm insulated plasterboard. I'll also double up the headers and pack against the roof joists for a tight fit.

One wall has 2 patio door openings 2.4m wide. What size timber header should i use over that, and shoud that sit on one or two shortened studs either side?
Make the beam out of the 4x2

Some 3 or 6 mm ply facing would be better

Or 3x2 and 9mm ply on each side if it's taking any load.

foto_no_exif (2).jpg
Wow awesome picture, i didn't consider that, and it allows insulation between too. I was planning on packing it against the ceiling joists there too so will assume it takes some of the load, but there is already a twinned 6x2 over the opening and bridging the external (single skin) walls.

Would still only a single supoprting stud be needed each side?
1) Horizontal 25x50mm battening to the outside, insulation, vapour barrier and vertical larch cladding
Important point: behind the external cladding, you need a breathable membrane (Tyvek or similar), not a vapour barrier. The membrane will prevent water ingress but allow any moisture within to escape.
Thanks, yes since original post I realised this.

So my buildup Fromm outside to inside will be:

2.Batten (no eps infill else wall can't breath??)
3.Breathable Membrane
4.Block wall (existing)
5.DPM where battens are & 1m up from floor as additional rising damp protection (continuing under studs and jointing to new floor dpm)
6.Studwork with 90mm eps infill
7. 50mm Insualted plasterboard with built in vapour barrier (or seperate insulation / vapour barrier / plasterboard in that order)

As I understand this means condensation, if any, only has the ability to form on the inside of block wall surface itself, the coldest part, where it is able to escape again trough the breathable membrane.

Does that sound right?
If you are putting cladding no why not treat this as external wall insulation and put 100mm of kooltherm, breathable membrane, screw batons through then clad. This will keep your internal wall dry and warm. You might have to alter your roof/soffets however.
Yeah I don't want to play with the roof, I have 50mm to play with really. If I do EWI that thick it messes up how this extended part joins to the rest of the house, would look a bit odd.
Make the beam out of the 4x2

Some 3 or 6 mm ply facing would be better

Or 3x2 and 9mm ply on each side if it's taking any load.

View attachment 121054

Hey Woody

Can I pick your brains again. I've basically done all the framing apart from over door openings.

Due to the ceiling height, door height and leaving room for the 50mm insulation, the gap over the door is only about about 5.5 inches. If I do the 3x2 with 9mm ply my cripples will end up only 1.5 inches tall. I'm thinking it's a bit much effort to create this web joist thing for such a small head height, there will barely be any air space for insulation.

Would I be better off getting 2 x 3m 6x2, bolting them together and cutting them down to fit exactly?

If there are other benefits of constructing it that way then I'll still do it!

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