Today is the coldest day since I moved into my house.
It’s a 1961 self-built bungalow, It’s October and there is early morning sunshine, but it feels rather nippy. Like many of you, I am experimenting with how long I can go without turning the heating on in my house. I want to know how my house works. Unlike many people, I have instruments to measure what is going on around me.
This is my collection, from left to right (a) thermostat controlling the gas combi-boiler, (b) thermometer showing inside and outside temperatures (and what time it is in Germany!), (c) my newest toy – a digital thermo-hygrometer showing relative humidity (RH) and room temperature here, (but it can do so much more!), (d) my 2004 thermo-hygrometer, relocated from the earth sheltered house I built [link to blog], the indoor temperature still works, but the external sensor is built into the other house, and as you can see the relative humidity is inaccurate, (e) my smart meter (showing the 390Wh I used to boil an egg for this morning’s breakfast – after I checked out the window condensation).
At 7.43am outside temperature is 1.8°C and it is 14.4°C in my hall.
The perfect conditions to check for condensation.
This is my bedroom window with lots of condensation across most of the window. I’m pointing an infrared thermometer at the window which is reading 8.1°C. I’ve been comfortably warm in bed for 8 hours and myself and my bed have been giving off moisture into the air for all that time.
To help keep warm I have a blackout blind in the window recess and a thermally lined roman blind over the whole opening. This keeps the room warmer but keeps the air against the window colder.
Some of the warmer moister air in the bedroom moves into that colder space, water molecules that touch the cold window lose the energy they need to keep them apart as a gas, and they therefore move together, becoming liquid – they condense. When this condition happens outside, it is called dew. The temperature at which this occurs is called the dewpoint temperature. It varies with the amount of moisture (water vapour) in the air. There is no dew on the grass today, because although the grass is colder than my windowpane, the air touching it is not holding as much water in it as I have managed to put into my bedroom air.
I’m curious to know what is happening in the rest of the house.
In the second bedroom I left the window exposed all night and no one was sleeping in it. There is a tiny bit of condensation at the bottom edge of the glass.
When I point the infrared thermometer at the centre of the window, it reads 12.5°C. Warmer than in my bedroom. It will have had warmer air against it, and it faces east, not north and so the early sun is on it, warming it.
There hasn’t been a source of moisture in the room either, from people or clothes drying.
My next stop is the large dining room window facing west. It had its roller blind pulled down over night, but not the roman blinds. No condensation at all.
All the windows in the house are uPVC double glazed windows, but it, and 
this south facing one in the sitting room are newer than the rest. The difference is easy to spot. They have black warm edge spacers in the cavity between the two pieces of glass that are made of highly-advanced plastic and foam rubber. All my other windows have the earlier perforated aluminium spacer bars.
Metal conducts heat. So, although the window in the sitting room has a lower temperature (10.2°C) when I point my gadget at the centre of it, than the one in the second bedroom did, it doesn’t have condensation. This is because the edge of this window is not as cold as the edge of the second bedroom.
To stop condensation forming on my window, I could stop breathing moisture out, but I don’t see that as a viable solution, so I need to:
- keep moisture levels low by moving moist air out of the space with a good ventilation strategy
- ensure the air temperature in the room remains above the dewpoint temperature
- ensure the temperature of all the surfaces remain above the dewpoint temperature.
Next I’m going to investigate how much energy it takes to heat my new house.
