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That means the walls and structure have to be kept above the Dew Point - the temperature at which condensation occurs. Its not something that just forms on surfaces - it forms within the walls, which is what that old fraud of rising damp is about - moisture condensing in brickwork or stone.
This is really interesting, since a common recommendation is to fit internal insulation. For example, this is being recommended to us because the cavity in our walls is pretty nominal (~50mm). The downside is that the inner leaf of blockwork then doesn't warm up so the dewpoint moves inwards. I've read people on here saying how good they felt that interior insulation is, but I'm interested in whether this effect can cause an issue and reduce the benefit of the existing cavity.
ffm-
There's a couple of things - one is efficiency of running the home eg damp walls conduct more heat to the outside, the other is how temperature, humidity and damp (condensation) are connected. These affect ambient temps and humidity, which are really important. Thermal comfort (how you feel) is also influenced by other things - a really significant one is radiant heat - if the surfaces in your line of sight (your body's thermal line of sight, really) are cold, you will feel colder. If they are warm you will feel warmer. Internal insulation provides a warm surface. It also improves overall thermal efficiency (less heat lost to the outside), so whatever heat you are inputting leads to warmer temps. The big thing to watch out for is interstitial condensation between the warm insulation and the cold wall. The existing cavity will still be doing a job to prevent damp from the outside getting to the inner leaf, and it will still be doing a job of reducing heat loss but much less significant compared with the new insulation.
Edit to add: In the summer, another thing to consider is that internal insulation covers up your thermal mass, so overheating is more likely to be a problem - not because of the insulation but because you lose that exposed thermal mass which acts to buffer heat gains.
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External is much better but more spendy.
hoefla
Howard
I read something recently on another forum and saved it for reference as we’ve just moved into a new place that I suspect will be expensive to heat -
“This is a tricky topic - but something I've been very involved with over the years - we run a Chartered Survey company, and I've written the book on dampness in homes - and the effect that temperature has on the building fabric. I'm sure Martin won't mind me mentioning the name of the book - its out on the 17th Sept, and pre-orders are being taken - called The Warm Dry Home and available on our website - just look for the Heritage House website.. (I'm not sure about the admin rules for posting links) So, back to the topic.
A warm house is a dry one. For a house to be dry, it needs to be warm enough that condensation doesn't form within the building fabric. That means the walls and structure have to be kept above the Dew Point - the temperature at which condensation occurs. Its not something that just forms on surfaces - it forms within the walls, which is what that old fraud of rising damp is about - moisture condensing in brickwork or stone. Where does the moisture come from? Mostly we humans - living, breathing, cooking, showering. It has to go somewhere, and most houses are NOT well enough ventilated - so moisture is able to settle within the fabric - it passes through it as a gas, but if that gas is cooled down below Dew Point, it condenses, and is held in the walls and fabric as water.
A damp wall transmits heat much quicker than a dry wall, so house gets colder and damper. How do we stop this happening?
We make sure the building stays above a critical temperature - we have a guideline of 15 degrees C., at an RH of 50% - and total moisture content of 7 grams water per cubic metre of air. That's about a teaspoon. Given those parameters, the Dew Point is about 4 degrees C. So you wont get damp, and the house stays dry.
So - how does this relate to the OP? When to turn heating on? You turn it on in such a way that you don't allow the FABRIC of the building to drop below 15 degrees C. You keep it on - constant low temperature heating is cheap to run, and ensures you keep a DRY house, so long as it is well ventilated. I can't emphasise enough that for all this to work you need to ventilate - humidity controlled extraction in kitchens and bathrooms is a must.
So when night time temps start to dip below 15 degrees, you need to get the heating just ticking over, to maintain temps at the right level. Its not about when, so much as 'what are the temperatures'.. In ALL rooms as well ! Don't leave some rooms with heating off - they'll just get mouldy and manky.
Make sure you also check the building - guttering and downpipes work, drains are clear, there are no cracks allowing water into the ground and soaking the soil at low level - ground levels externally are 150mm below internal floor levels.
All of this promotes dry walls, and thus best chance of maintaining a warm home - which wont lose heat nearly as quickly, and cost less to heat. Remember - it costs a lot more to heat damp air than dry air. “