cavity wall insulation

[Stephen]

Hello again,

Whilst searching the internet to learn about celotex for my other post about floors and ceilings, I came across a web site that said a house built today would need 100mm of celotex in its cavity. It also said that houses built before 1975 had much smaller cavities, so to get to today's standards of insulation, you would have to add 50mm of celotex to the walls. I am told that my house was built in the 1930s. I haven't yet removed a brick to see what insulation, if any, I have. I suppose I should do this.

As it happens, another job I'm about to get round to, is redoing the bathroom. It's only a small room but it does have two external walls. I think I could add 25mm celotex to the walls without problems. I know that's only half what the web site recommends, but it's better than nothing.

I could use 50mm but I might need to move the door frame across, as it is close to the external wall. I do wonder whether all this work is worth it, but I suppose I have not lived through a winter here yet to see how cold it is, and it would mean I would use my heating less, so I suppose it would save fuel in the long term. I guess I will have to do the calculations again and see what the savings are with both thicknesses.

Would it be best to put battens on the wall at 600mm centres and insulate between those, or would it be better to have no wood and glue the insulation to the wall with some sort of "no more nails" product. The advantage with the latter being there would be no gaps in the insulation? I suppose battens would allow me to get the wall straight if it was not already.

I have an irrational fear of the wall falling down if I glue it; I'm thinking I would be gluing plasterboard with tiles on to celotex to the wall. But I suppose that's only because I am used to mechanical fixings and have not glued before.

I would need to fix a radiator to the wall and a toilet cistern. What would be the best way to do this? Use long fixings into the masonry wall behind the celotex or should I cut out the celotex behind these and fit a wooden batten to the wall and screw into that?

Thanks, Stephen.

[tabbypurr]

Anything beats nothing. If you're rerendering then yes, do it. Its easy to check cavity depth, and easy to CWI with poly bead. Fix sheets with mechanical fixings, battens aren't normally used now.

NT

[Tim Watts]

Given the usual air gap that is needed, that would be a 5-6" cavity!

I don't think Part L requires anything like that but I'd have to check...

Often 2", maybe 3".

If you have a cavity, just get it filled with the best material you can (blown beads are one option, foam is another). Blown fibre is still around and is OK but can settle over time.

It's a *lot* better than nothing and can help in bathrooms to reduce the condensation on the walls. You can also get plasterboard with an integrated celotex/kingspan layer to make this sort of job really easy.

However, I would not combine that with cavity wall insulation, unless you batten the inner insulation off the walls or the cavity fill is breathable (blow fibre is so some extent, blown beads probably are, foam fill is not). Otherwise you'll risk a damp inner wall leaf that will track the damp to the floor and ceiling joists (interstitial condensation).

Your building inspector will probably be able to advise (if you don't mind declaring the job under a building notice as technically you are supposed to for insulation mods[1]).

[1] However noone really cares, least of all the BCOs.

No. Even 12mm would make a big difference to the comfort factor in the room. But I'd just get the cavities filled if they aren't already.

I went though all this and in a typical house with a filled 50mm cavity, the biggest source of heat loss tends to be the double glazed windows. In other words fixing the walls "more" often is irrelevant in the overall scheme, unless you are trying to solve a peculiar problem.

Also, from experience, if you insulate inside the walls, you lose a huge advantage in summer of the walls acting as a cold sink. I find new build houses stifling in summer compared to a proper brick and plastered building.

[Stephen]

Hello again,

Whilst searching the internet to learn about celotex for my other post about floors and ceilings, I came across a web site that said a house built today would need 100mm of celotex in its cavity. It also said that houses built before 1975 had much smaller cavities, so to get to today's standards of insulation, you would have to add 50mm of celotex to the walls. I am told that my house was built in the 1930s. I haven't yet removed a brick to see what insulation, if any, I have. I suppose I should do this.

As it happens, another job I'm about to get round to, is redoing the bathroom. It's only a small room but it does have two external walls. I think I could add 25mm celotex to the walls without problems. I know that's only half what the web site recommends, but it's better than nothing.

I could use 50mm but I might need to move the door frame across, as it is close to the external wall. I do wonder whether all this work is worth it, but I suppose I have not lived through a winter here yet to see how cold it is, and it would mean I would use my heating less, so I suppose it would save fuel in the long term. I guess I will have to do the calculations again and see what the savings are with both thicknesses.

Would it be best to put battens on the wall at 600mm centres and insulate between those, or would it be better to have no wood and glue the insulation to the wall with some sort of "no more nails" product. The advantage with the latter being there would be no gaps in the insulation? I suppose battens would allow me to get the wall straight if it was not already.

I have an irrational fear of the wall falling down if I glue it; I'm thinking I would be gluing plasterboard with tiles on to celotex to the wall. But I suppose that's only because I am used to mechanical fixings and have not glued before.

I would need to fix a radiator to the wall and a toilet cistern. What would be the best way to do this? Use long fixings into the masonry wall behind the celotex or should I cut out the celotex behind these and fit a wooden batten to the wall and screw into that?

Thanks, Stephen.

[harry]

You can buy plasterboard with insulation already attached. Consider hacking off existing plaster and substituting this stuff. Stuck on with special glue.

But check your cavity. You may be able to have cavity wall insulation fitted for free by your energy supplier. Some government scheme.

Google free cavity wall insulation UK

[Dave Liquorice]

If it has cavity to speak of, parents 1930's semi had solid 9" brick walls.

Have look around the walls on the outside, most retrofit cavity insulation requires the drilling of holes every so often to inject the insulation.

Well worth it IMHO. Lads bedroom sounds similar. It was always a cool room. When decorating I put just two sheets of 12 mm PB bonded to 25 mm celotex on the end wall. Made a tremendous difference to the comfort of that room.

[Brian-Gaff]

Are we saying then that this treatment of non cavity walls makes a significant difference fro outside walls? Some years ago I seem to recall that the view was that this sort of thing was not going to save one much money, and you also would not know if condensation affected the inside of the brick either. Brian

[tabbypurr]

You can do the holes on the inside if you prefer. Tidying it is slower, but no final disfigurement.

NT

[Dave Liquorice]

Yes, most certainly. Just the two sheets of 25 mm backed PB on one wall of the lads room made a heck of difference. The wall is 15" random stone/rubble infill. The house was done with 50 mm backed PB on galvanised frame work with a breathable membrane behind the framework. The place needs almost no heat compared to what it needed before.

It's not cheap or quick and is very disruptive compared to squirting a bit of something into a cavity. I guess this is why cavity wall insulation is subsidised and insulation dry lining isn't.

The insulation should be spaced away from the wall and a tiny amount of ventilation will prevent the build up of damp.

[Stephen]

Sorry for the late reply. I have my news reader (agent) display by date and for some reason Agent appended my post and your replies to an old post from 2003, where I did not see them!

[Stephen]

I did think it sounded large.

Yes, there seem to be a lot of companies offering to do this for free at the moment, eg British Gas.

I saw some plasterboard in Wickes with insulation on the back but it was (IIRC) extruded polystyrene rather than whatever it is that celotex and kingspan et al are made from. I wondered whether the u values were much different? Also, it was less than an inch think, so I wondered whether 25mm glued separately would be better?

Having removed some of the plaster when I removed the tiles, it seems the gaps around the frame were not filled with foam, so yes, there's certainly a draught coming from the window.

Thanks, Stephen.

[Stephen]

Thanks for the encouragement; I'll certainly give it a go. My main concern is not getting the sheets on straight to give a plumb wall.

[Martin Bonner]

Celotex and Kingspan are about twice as good as polystyrene. (lambda is

0.022-0.028 rather than 0.035-0.050 for polystyrene - low is better). That means >I went though all this and in a typical house with a filled 50mm cavity,

Well fix that first! Draughts *really* kill the warmth of a house. However, what Tim means is that the heat flowing through the double-glazing dominates. The U value of a double glazed window which meets current regs is about 2.0. The U value of an old double glazed window is 3.0. The U-value of a filled cavity wall is about 0.5 - which means that actually I think the walls are still leaking more heat than the windows (but it's much closer).

If you want to save money/feel more comfortable, go for 25mm. If you are a paid up Green Party member (like me), I'd go for at least

50mm - it doesn't cost that much more, and it doesn't take that much more space.

[Dave Liquorice]

You ensure the battening that you attach the boards to is plumb and flat(*) by packing out as required.

(*) But this is the real world, walls are rarely plumb and flat. In a recently built place they be close enough not to worry but worth a quick check. The worst wall here buldges into the room by about 2" relative to the ceiling at about 3' from the floor which is an inch back... I only noticed when putting on verticals for wall papering.

[The Natural Philosopher]

what you do is set a vertical batten plumb at each side of a room. and then stretch strings between then and ensure every batten just touches the strings by planing or packing.

If you think the problem may exist, its easy to ensure that it is sorted. It's when you assume the wall is straight...