Insulating a wall

[The Medway Handyman]

Customer in a Victorian house has a bathroom with a single skin end wall

2.4m x 2.6m which gets very cold and attracts condensation like crazy.

They have asked me to build a false wall with insulation to keep the bathroom warmer & reduce the condensation. However, in the middle of the wall is a metal Crittal window, obviously single glazed, about 1m x 1m.

Don't want to do the job & then find it hasn't solved the problem. My feeling is they would be better for them to spend the money having the window changed to a DG jobby & perhaps having the wall insulated at a later date.

Any thoughts?

[Phil L]

white pvc windows are very cheap these days, simply tell them that it's a wasted exercise unless the frame is changed at the same time - it may look like you are touting for work, but why do the wall and then find that it's not made much difference?

[Cicero]

========================================== You could try this old standby as an interim measure and as a possible indicator of the best way ahead. The condensation issue would almost disappear:

[Invisible Man]

Is this expanded polystyrene or similar? We used it many years ago and it did make the wall warm to the touch. Snag is that if anything with an edge falls against it it rips. I don't recall it lifting even in the bathroom. Funnily enough we also had Crittall windows. Used to get ice on the inside of the metalwork in the winter.

[Andrew Gabriel]

Window is 1m2 Wall is 2.4 x 2.6 = 6.24m2, minus the window which leaves 5.24m2

By single skin wall, do you mean a half-brick 4" wall, or a full brick 9" wall?

Can't find a U value for a half brick (and plastered) wall, but I'll guess at 2.5.

U value for metal frame single glazed window is 5.8

So loss through wall is 5.24 x 2.5 = 13.1W/C Loss through window is 1 x 5 = 5.8W/C

So 69% of the heat loss is through the wall, and 31% through the window.

How much these reduce by will depend how much you insulate them.

A Part L window will be about 1.7 U value, so replacinging the window alone would reduce that to 1.7W/C, which would be only a 22% saving in total heat loss.

Can't be bothered to work out the celotex/kingspan value, because I can't find a website which will give me the U values of either without giving them an email address to spam me with, but even 25mm of this is going to have a much more drammatic effect than changing the window.

[OG]

Depending on when it was installed, Crittall windows can be very attractive and it would be a shame to lose them. Once you've completed the false wall, could you then fit openable secondary glazing in the window space?

[John Rumm]

Area of wall = 2.4 x 2.6 - 1 = 5.24m^2 Area of window = 1m^2

I presume when you say "single skin" wall you refer to a 8" or 9" thick wall with no cavity rather than 4 - 5" one?

If so, the u value for the wall is say 2.2, and the window could be as bad as 5.6

If you take and arbitrary temperature difference of say 20 degrees, that means loss from the wall at a rate of 20 x 5.24 x 2.2 = 230W and from the window at 20 x 1 x 5.6 = 112W

So you are loosing twice as much through the wall as the window. Insulating the wall will certainly make the room much warmer - however it will probably result in even more condensation on the window.

Personally I would recommend they change the window at the same time, or at least add secondary double glazing. (you could get a basic uPVC DG window for £100 - £200 probably). Obviously the cost doubles if you jump through the part L building notice hoops.

If you gave the wall a 50mm overcoat of celotex, taped the joints and then battened (fixed through the insulation) and boarded, you could drop the wall u value to 0.3, which would be heat loss rate of just 32W in this case! A uPVC or wood DG window with >=16mm cavity would probably lose at a similar rate.

[dom]

A rule of thumb, and one I would agree with, is that DG isn't really worth it if you have solid masonry walls.

Given the potential damp problems, this is an ideal job for either foil-faced celotex/kingspan with stud and plasterboard on top, or the insulation backed plasterboard applied directly.

50mm foil faced full sheets are about =A315-18.

[neverwas]

I have been contemplating dry lining on these lines for some time but am puzzled that you (and John Rumm also) envisage insulation on the bricks with battens over . I thought the conventional wisdom was to batten the walls and then apply the PIR - as eg in Kingspan's

Have I got that (as all too many other things) ass upwards?

[Owain]

Insulating the inside wall is a nice warm indoor job for you to do now. Tell them you'll come and change the window late next spring when it's warmer for outside work.

Owain

[John Rumm]

The page you link to shows two insulating products in use - one between battens, and another bonded to the plasterboard (i.e. the K18 Dry Lining Board) and indeed that is one way of doing it. The disadvantage of a solution like that is the timber itself acts as a cold bridge - lowering the overall insulating value. The additional layer of insulation over the top of the studs however helps. The advantage is ease of fixing the plasterboard.

A simpler way in some cases is just to cover the wall with insulation, and then the plasterboard - hence no thermal bridges. However fixing the plasterboard so that it is level is harder to do that way. Hence slim battens can be screwed onto the top of the insulation (fixed through it with long screws) to make mounting the plasterboard easier.

For a small wall area such a TMH described I would be tempted to simply "glue" the lot in place with expansing foam. You can get a "board fix" version of it designed for the purpose.

[dom]

I've only done it with the foil-faced insulation board directly on the walls, followed by stud, followed by plasterboard. Two advantages that I can see to this approach - stud over the top of the insulation provides somewhere for cable routing - and it's possible to screw through the plasterboard and into the stud to fix stuff to the walls (like radiators). AFAIK this is the manufacturers recommended approach when using separate insulation board and plasterboard - certainly that was my understanding from Celotex's website. It will take up more space though, but an additional upside may be the moisture barrier it provides against solid masonry that might be problematic.

I can't see any solution but that insulation backed plasterboard has to go over the stud/battens or be direct fixed (again AFAIK this is what the manufacturer recommends) - and in many situations it sounds like it could be a good bit less work!

[The Medway Handyman]

I don't 'do' windows, beyond my ken as they say.

[The Medway Handyman]

I don't do windows, but I like to give honest advice.

[Phil L]

You don't do windows? - yet you can do fancy joinery on decking etc??

Windows are the easiest job anyone can do - people with no previous joinery skills can do an excellent job, and those with even a rudimentary knowledge of joinery can do a far better job.

It's just a matter of removing the old frame (which is the hardest bit)

offer the new frame into the opening and level up both upright and laterally, wedge in place with whatever you have to hand, drill through frame into brickwork and drive home a few screws, foam around, glaze.

the following day, apply silicone sealant

[BigWallop]

Or, fix battens around the inside surround and fit secondary glazing in the form of a clear acrylic pane. The dead air gap between the two will be a good insulator.

Also, the wall could be insulated by fixing upright studs to the original surface and lining with foil backed plasterboard. As long as you create a dead air space between the outer skin and dry lining, the wall will reduce heat loss by huge amounts. The cavity doesn't have to be any more than 25 -

30 mm to give good insulating properties to the wall.

[The Natural Philosopher]

heatloss through the window will be similar (larger, but similar) to the wall, area for area. Tackle the biggest area.

Put foam strip in the crittal for starters if you leave it.

Youll need a fan...

And either secondary glazing or some mega thick curtains will help the window.

[The Natural Philosopher]

is way more than that. 3-4. for 4" brick.

Yup. BUT the main effect of insulating walls will be a massive condensation on the window, as the really cold spot.

[The Medway Handyman]

Decking is a POP. Joinery isn't the problem though.

What happens to the brickwork above the bugger?

I can sort that, but I still worry about the brickwork above.

[Appelation Controlee]

-------------------8><

Shouldn't there be a lintel? Window frames aren't load bearing.