# Insulating Exterior Wall Internally

diminishing returns

figures are made up but reasonable

let's suppose you have a renovation house with no loft insulation.

Let's suppose your bedroom ceiling is 10 m2 and loses 2kW of energy when 20C inside and 0C outside.

Let's suppose you add a meagre 50mm of glassfibre to the loft. The heat loss from that room halves to 1kW (hooray) and maybe saves you a pound a day on cold winter nights.

You then add another 50mm glassfibre. The loss halves again, to 500W

You then add another 50mm and it halves to 250W.

So the greatest benefit, and the best financial return, was on your first 50mm

If you carry on adding more thickness, the incremental value becomes trivial.

This is especially important if you are insulating walls internally, and reducing the room size.

In Switzerland I have seen some kind of shiny white foam board added externally to old stone buildings, then a cement board added and a render-look paint added. They just do 100mm. Some of their winters can be long and cold, but they have calculated that this gives a worthwhile economic return.

diminishing returns

figures are made up but reasonable

let's suppose you have a renovation house with no loft insulation.

Let's suppose your bedroom ceiling is 10 m2 and loses 2kW of energy when 20C inside and 0C outside.

Let's suppose you add a meagre 50mm of glassfibre to the loft. The heat loss from that room halves to 1kW (hooray) and maybe saves you a pound a day on cold winter nights.

You then add another 50mm glassfibre. The loss halves again, to 500W

OK

You then add another 50mm and it halves to 250W.

No, you need to add 100mm to halve it to 250 W.

So the greatest benefit, and the best financial return, was on your first 50mm

Absolutely true.

Hi all

Thanks for the replies, looks like I will be specifying 50mm celotex.

Just out of interest,is it safe to use Jablite in walls within the house?. I recently had a brick shed built where EPS was used just wondering if it's OK to use in the house.

Thanks

is it safe to use Jablite in walls within the house?.

Yes.

Is it the fire safety you're concerned about?

You'd need to cover it in plasterboard or something, and cabling would need protecting. I think the approved documents talk about surface spread of flame etc.
Although people cover their walls in that thin polystyrene wallpaper to keep the condensation off which doesn't sound a great idea to me.

Hi guys

Thanks for the reply. It will be covered with 12.5mm plasterboard and then skimmed. My main concern is, in the case of a fire will this release toxic fumes as it's plastic burning? I know the plasterboard should give around half an hour of fire protection.

With regards to electric cables, why would these need covering? Due to heat?

Thanks

See this post
EPs works OK, about half as good as celotex but a lot cheaper. The only issue is it can't have cables touching it as it destroys the cable insulation.
You'd need to derate for any insulation though.

Just out of interest, how does the cable get destroyed? Is it due to the heat being trapped?

in the case of a fire will this release toxic fumes as it's plastic burning? I know the plasterboard should give around half an hour of fire protection.

The protection provided by the plasterboard is considered sufficient.

With regards to electric cables, why would these need covering? Due to heat?

There is a chemical reaction between an ingredient in polystyrene and the PVC cable insulation.
You also need to worry about the cables overheating due to being surrounded by insulation, but that's true for all types of insulation.

Thanks for the reply. Will need to speak to my previous builder who built a brick shed using EPS and running cables on all 4 walls! And sealed with plasterboard on top. Twin and earth cable was used for the sockets (Prysmian I believe). Will this be an issue for me in the future? It's been over a year and haven't had any issues yet...should I be expecting the worst?

Yes pvc cable should last for ever unless you leave it exposed to something damaging. The bad thing about it being hidden is you can't see the deterioration.
Notice that when you buy new electrical equipment eg microwave, the cable is always neatly folded into a plastic bag to protect it from the polystyrene.

Thanks for the reply. Will need to speak to my previous builder who built a brick shed using EPS and running cables on all 4 walls! And sealed with plasterboard on top. Twin and earth cable was used for the sockets (Prysmian I believe). Will this be an issue for me in the future? It's been over a year and haven't had any issues yet...should I be expecting the worst?

The reaction happens slowly, and if the cable is undisturbed you may not actually notice anything even when the cable insulation is quite seriously degraded.

Is it possible that the cables run in a gap behind the insulation, or have been protected with some sort of conduit or capping?

I tried to find some pictures and discovered this:
http://www.esfrs.org/black-museum/a...-cable-insulation-and-polystyrene-insulation/

If you buy the Celotex (or equivalent) pre-bonded with plasterboard, the foil on the back is reversed, so the paper side is facing outwards. This allows dot'n'dab adhesive to stick. Normal boards have the foiled side facing out so don't really stick.

We needed a room doing, and opted to pay the bit extra and get the pre-bonded boards, 3 sheets were installed within an hour

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