DIYbanter - View Single Post - Keeping damp at bay in an unheated garage

Andrew Gabriel

In article ,
(D.M. Procida) writes:
The garage is unheated and damp, and I've noticed mildew on things.

I have left the doors and windows open for a couple of days and that
seems to have helped, but I'd like to do a bit better.

There's a chest freezer in there, that collects a lot of condensation on
the sides and lid.

If the freezer has an anti-condensation case heater, you should switch
it on. However, those often operate only around the door seal, and
that's to prevent condensation there freezing and sticking the seal
to the case.

I don't want to buy or run a dehumidifier unless absolutely necessary,

They don't work at outdoor winter temperatures anyway.

but perhaps the warmer (therefore damper) air around the freezer could
be ducted outside with the help of an extractor fan - it seems a shame
to waste the heating and damp-collecting properties of the freezer.

That's a misunderstanding of the problem. The condensation forms on
the freezer where the cold is leaking out from the inside (at least,
that's the easiest way to think of it;-). The heat from the freezer
is actually very welcome, and the heated air will have a lower
relative humidity, not higher (unless the freezer is frost-free).

A friend has a large garden shed which they use for storage. Problem
was that many of the items in there would be damaged by damp, so I
designed a system in there to maintain the humidity at no more than
80%. This has been running for 15 months now, and it has worked
perfectly - there's been no damp in the shed over this period -
no new metalwork in there has gone rusty (which it used to before),
and there's no smell of damp timber or fabric. Whilst the outside
humidity often reaches 100%, the humidity in the shed has never
exceeded 81%.

The system uses a Raspberry Pi to continually monitor the humidity,
temperature and dewpoint inside the shed (and also outdoors for
comparison). It switches on a heater in the shed if the RH exceeds
80%. This has completely prevented any condensation forming over
the 15 month period. It has cost £55 in electricity over the 15
months (mostly in the winter), which is much less than the £200/month
they used to pay in storage rental space.

At some point when I have time, I will write up the system in a blog,
but basically, you need a low power heater (depending on thermal
properties of the garage) to kick in when relative humidity exceeds 80%.
One slight complication here is that most humidity sensors become wildly
inaccurate outside the 30-70%RH range. I use SHT75 sensors driven by
the Raspberry Pi directly, which are accurate from 0-100%RH.

If the air circulation and temperature uniformity are good inside
your garage, you might even get away with a 90%RH setpoint, but I
didn't have time to try that before the items arrived for storage.
(I did start with a 70%RH setpoint initially, and bumped it up.)

--
Andrew Gabriel
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