Shed ventilation

On Monday, January 26, 2015 at 6:13:11 PM UTC, harry wrote:

presumably a dehumidifier would also do the job and might be cheaper than heating as a way of getting the RH lowered.
Robert
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Depends on its behaviour between 0 and 10C, which is not where they are designed to operate. In practice, you need very little heating to make a significant difference to the relative humidity at these temperatures.
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Andrew Gabriel
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Worst case is fog after a long cold snap. Lumps of metal are slow to warm and the high humidity leads to condensation and rust.
All my machine tools get a coat of de-watering spray. Not actually WD40 but certainly a first cousin:-)
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Tim Lamb

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I do not have evidence to show that either way. It might prevent some of the cooling at night, particularly radiating out to a clear sky, but it would also prevent solar gain heating it during the day. I'm not sure how much you can improve things with a purely passive approach.
This shed has been lined with 50mm celotex under the roof and on 3.5 of the walls (the person doing it is currently busy with something else but intends to finish the insulating). A raspberry pi programmed by me is doing the monitoring and also controlling a heater in the shed to keep it below 80%RH and above 5C. The data shows that whilst the absolute humidity is usually higher outside, that can change quickly when the weather changes, and the shed could do with a fan to forcibly change the air when the outside air becomes drier and the shed is lagging behind still with more moist air inside.
One thing that surprised me was how little heat was required to maintain the RH and temperature within limits. I'm using a large (by today's standards) 1kW oil filled radiator, reduced to 500W by using a series diode. However, the thing operates so little that it doesn't even feel warm when you go and feel it, but that's still enough.
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Andrew Gabriel
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Thanks to all.
I'll paint the floor in the summer. Possibly add some vents and possibly an oil filled rad.
Regards
Richard
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