Insulating an external wall from the inside.

[VisionSet]

The walls of my house have standard cavities, but where the walls form the bay windows, the cavity drops to zero, ie two stretcher bond leaves butted together. This obviously contributes to poor heat retention. I want to insulate inside with something thin (ie low mu value). I will strip of the plaster first. What is my best option?

I think it is common to put some sort of vapour barrier on top of the insulation. why? A normal wall does not need it. I'm thinking polystyrene, then plasterboard screwed to brick, then skimmed and painted. mositure does not penetrate a regular plastered and painted wall, does it? Why should it in this case?

Any how, what is my best approach? I don't really want to spend loads on state of the art insulation. A freind has suggested 'bubble wrap' as a possibility.

[IMM]

Kingspan or Celotex. Very thin high insulation qualities.

To prevent condensation occurring inside the wall.

Yep, it does,

Cellotex or Kingspan between batons poly vapour barrier over and then plasterboard. Quite easy.

Nah.

[Christian McArdle]

Because as the vapour penetrates the wall, it gets colder and condenses out within the insulation.

Think Kingspan/Celotex, which have twice the insulation value per thickness. The vapour barrier can simply be foil that is preattached to either the insulation or the plasterboard.

Because it always does, unless stopped by a vapour barrier. An acrylic or vinyl based emulsion and gypsum plaster have very little vapour resistance. Oil based paints have some, but it isn't great. The reason problems occur less frequently with a straight brick wall are (a) bricks don't care about being wet as much as insulation and (b) they're so permeable, the vapour often gets all the way through before condensing.

Don't bother with second rate alternatives. One 20 quid sheet of celotex will cover most of it, as much of the "wall" will be windows and not require insulation.

Christian.

[VisionSet]

Thanks, that all made total sense. I have looked at Celotex pdf before and found all the options pretty confusing. Do you know exactly which product I should use? I guess I can choose the correct thickness from some web based mu calculator given my other (cavity insulted) walls mu values. Though I do have a window sill lip to come up to. The windows are all ceiling height, so it is just the base.

-- Mike W

[Christian McArdle]

As for thickness, this is likely to be determined by the space available, rather than the insulation required. 50mm will kill any condensation problems. More just gives you better energy efficiency. On such a small area, the difference will be marginal.

Of course, if the BCO is involved, they might have more specific ideas.

Almost any Celotex/Kingspan product would be appropriate. Either the insulation or the plasterboard should have foil attached. It doesn't matter which. If space is tight, the insulation is available to special order with vapour barrier and paper covering, avoiding the need for a layer of plasterboard.

Christian.

[Robert]

Have you considered leaving the wall untouched and paying the very sligthly higher heating costs?

Seriously, how long will it take to reconver the costof doing this? How many kWhr/year will you actually save by doing this?

R

[Christian McArdle]

Whilst marginal on energy efficiency grounds, it can make a significant difference to comfort, convection current promoted draughts and, in particular, condensation.

Christian.

[VisionSet]

Absolutely. I have a very inefficient house, external walls on 3 sides of almost every room. Huge bay windows and single glazing.

Though I should do the glazing - that is extra expensive and I have nice original georgian bar windows in good state of repair that I like.

The bays form running condensation with mould when unattended, no where else does this.

-- Mike W

[Hugo Nebula]

On Fri, 26 Nov 2004 09:30:52 -0000, a particular chimpanzee named "Christian McArdle" randomly hit the keyboard and produced:

If your aim is to make a poor situation better, I wouldn't be too worried about matching U-vaues. It's not going to make more than a few Watt-hours difference per year.

As long as he doesn't make the heat loss any worse, the BCO wouldn't be interested.

[IMM]

25mm of high performing Celotex is the equivalent of a solid stone wall of over 1 metre thick.

[N. Thornton]

dehumidifier

NT

[Ed Sirett]

Medieval Lord : How doth the north turret progress? Medieval Mason: With new-fangled Celotex, it will fly up, sire! ML : Bravo my man! Will it stand against all manner of siege engines? MM : The makers say its equivalent, my Lord. Thy clerk read unto me the parchment of specification in the market place.

[VisionSet]

Sealed units.

Cure the disease not the symptoms

-- Mike W