External insulation?

26 Jun 2004
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United Kingdom
I have a house with pebbledash but 9" solid walls which have a poor insulation value.

Rather than adding 50 mm internal insulation on the external walls is there any easy way to add useful insulation on the OUTSIDE?

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Don't think there's any easy way. It's quite expensive and things like window cills have to be changed. It is said to be effective and you don't lose any internal space. The rooms are slower to heat up than with internal insulation, but the walls become a bit like storage heaters so they hold the heat a bit longer.
The theory isn't difficult but I'm not sure if there are any unexpected snags that can crop up.

As already said, you have to decide what you will do about everything on the outside - cills, down-pipes etc. Then decide how much insulation you want -either PUR, xpanded poly or extruded poly - and buy battens to suit .

Screw battens to the wall fix insulation sheets between, securing by drilling walls and finishing with a large washer.

There is some kind of nylon scrim that can then be ( glued ?) onto the insulation to make render etc adhere better.

Be aware that the expanded poly damages easily and doesn't look good afterwards and also that vermin (mice usually) find it very cosy too.

Above is just from watching it being done a couple of times, so I'm not claiming that everything needed is included.
You need to be aware that putting the insulation on the external of the building is not as effective or efficient as putting it on the inside.

If you have problems with available space use a thinner section of insulation at window/door reveals, other than that its a relatively straight forward job - remove skirting boards (fill holes) and stick insulation to walls, then either plaster skim coat or tape and joint to finish, replace skirting and decorate. Assumes your using gyproc thermaline super or other similar system.

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If you have problems with available space use a thinner section of insulation at window/door reveals, other than that its a relatively straight forward job - remove skirting boards (fill holes) and stick insulation to walls
and light switches, wall sockets, radiators, architraves..........
@ alittlerespect

Would you explain why it is not as effective as I don't understand why this would be the case?
Well I'm not sure its not as effective per say, whilst it may take longer for the house to get warm it should stay warm for longer once the heatings gone off. So arguably you are spending money on heating the external walls.
It depends on what you require for your lifestyle. If you want the rooms to heat up quicker, internal is better. This is for people who spend less time at home. For people at home most of the day then external is better as the walls become storage heaters. You then have the benefit of radient heat which is more comfortable.
Dear mountainwalker

I was hoping to save a response for a later day, but I am not sure when that day will come, so here goe's:

At present the wall construction (including plaster and pebbledash loses heat at an approximate rate of 11kW/hr.m2.C - that is 11kW per hour per m2 of wall area per degree C tempertature differential between the inside and outside air temperatures.

Whereas adding 50mm PIR insulation will reduce the heat loss to just under 1kW/hr.m2.C - 986 Watts to be precise.

The point of the insulation is to keep the heat inside the habitable room, not throw away the heat in warming up a totally inefficient wall, also putting the insulation on the external face of the building means that the insulation will never retain sufficient heat to function at its best.

I don't have time just now, but I will put together a graph to show the difference in heat loss between the two options, but I have done some calcs and I can tell you now that the external application loses out to the internal application!

Internal insulation can cause damage to older properties with solid walls where the joists are in the wall and there are timber lintels. The walls rely on heat from the house to keep them dry, and without it tend to be much damper.
ALR, looking forward to the graph, can you do one of suspended timber and concrete floors if you get time?

Can't agree with your comments about old solid walls being damaged by internal insulation

If you have a solid wall of 60 cm ( U 1.0 ) , internal temp of 20 C and external temp of 0 C then your heat loss in Watts per m2 per hour is

1 x 20 = 20 Watts per m2 and in this case per 0.6 m3 ( taking thickness into account).

Do you really think that 20 W over one hour can put much heat into the best part of 1,000 kg of stone and mortar ?

I have to tell you I don't.

Insulation plus a vapour barrier will prevent the migration of humidity into the wall, where it will at some point condense and lower the insulation value of the wall even more.

Just absolutely wrong.

There was an error in my last thread - heat loss is correct, but please, please do not pay any attention to the bit about temperature differentials as this was covered in the calcs that I did - perhaps I should never had started the thread as I was tired and now it shows.

So as not to make the same mistake I will provide a graph showing the existing heat loss through the wall and the heat loss with the insulation on the external and one for the insulaton on the internal walls, but this is not going to happen tonight as I need to do some shopping!!

All the calcs are based on a worst case scenario. and I have set the internal temperature at 20C and the external temperature at -3C.

@ alittlerespect

Sorry, but your heatloss is also awry. Nowhere on this planet is anyone losing 11 kW ( that's 11,000 W ) per hour per m2.

Have you got your K in the wrong place and mean 11 W/K/m2 ? Where K is the difference in degrees between internal and external temps.

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