External insulation?


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.
I've been a bricklayer since leaving school in 65 and have worked on many old buildings. I can assure you that internal insulation can cause timber rot in old buildings. This is based on experience and fact, not on calculations from a book. The balance in some old buildings will only need a slight change to cause a problem. For example a 9 inch exterior wall with the joist ends in the wall by 4 inches can easily become too damp with the loss of heat from the house. Even on a cavity wall, CWI has caused the bricks to spall in certain instances.
This does not apply to every old building, but is something that has to be considered.
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Hello Stuart

I'd just like to start by saying that I am 5 years or so younger than you so it is not really the case "it is on the screen so that's the answer".

I'm not a builder but have been renovating my place for a few years - this not to say I have any experience but some serious interest I can't and truthfully speaking would not want to put your experience in doubt - but have to say I don't understand it.

I have laid out my ideas as best I understand them.

Moisture causes damp and rot so if you prevent the moisture from getting to somewhere it can condense won't that prevent the rot ?

Introducing a sub-topic here, can you tell me as someone with experience when rot starts in joists and how quickly

if there is condensation how many years/decades will it be before the joists are weakened ?

I find it difficult to understand how condensation - as apposed to significant water ingress - can cause joist rot.

Thanks MW
The instances I am talking about are due to water ingress rather than condensation. A 9 inch solid wall will normally allow rainwater penetration for about 4 inches and then the water will evaporate out. This will vary on things like the type and condition of the bricks/mortar and the exposure.
In cases where there is a fine line between rot forming and not forming internal insulation could tip the balance.
If the moisture content of the joist ends reaches 20% or more, then dry rot could start. If it reaches 30% or more then wet rot is more likely.
There is no real time limit, as sometimes the timber will rot quickly.
In many cases changing the fabric of the building with things like double glazing and insulation won't affect the timber enough, but in some will upset the balance enough to trigger a timber rot attack.
External insulation will protect the fabric of the building, but on old buildings will ruin the looks and can't usually be used. With external insulation condensation isn't really a problem as the walls will usually be warm enough to prevent it.
Regards Stuart
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Opps done it again - looks like it was the right thread but did not realise there was more than one page to the discussion!!

Well, while I'm here I may as well comment that heat loss is calculated in seconds (W/s) to obtain the heat loss for one hour it is necessary to multiply by 3600s/hr. MountainWalker - I would comment that your idea of heat loss is not correct, and there is a lot more to working out heat losses, and if your up for a bit of learning a good book to start with is 'environmental science - isbn 0582416205.
In the temperature range we are talking about C's are as good as K's (or maybe even better as there is less confusion).

Where the dew point line on the previously attached charts cuts into the room temperature lines this will indicate where condensation is most likely to occur through the building elements.

Otherwise happy interpretations on the charts!
Two forms of rot - Wet Rot - due to continuous water ingress - once water is stopped wet rot stops and its a case of repairing the damage.

Dry Rot - Dry rot starts with a combination of lack of ventilation and high humidity and warm/stagnant air which encourages a fungus to grow which gives off a musky/mushroom kind of smell. Unfortunately Dry Rot can simply survive on the moisture in the air, once established it destroys the cellulose fibres of the timber and although the timber may look ok it is like polystyrene and breaks easily leaving the timber with no structural strength. Once established dry rot is an absolute nightmare to get rid of/control. If this is diagnosed at an early stage get it sorted by a professional company as it is capable of traveling between the interface of the brickwork and plaster and it will even grow through brickwork and travel across pipework. Normal place to find dry rot is in basement properties which do not have adequate ventilation and/or in ground floor suspended floors where people have blocked up the perimeter air bricks or again in old basement properties where insufficient air bricks were provided and there is an underlying damp problem.



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