Suggested temperature for spare room?

We keep a lot of spare belongings in a couple of loft rooms. I don't want them to get damp or mildewy, so I set the radiators in the rooms on low even though almost no one goes in the rooms.
What sort of air temperature would be sufficient to keep these belongings in order? Is it something like 6 degrees, 8 degrees, 12 degrees?
If it makes a difference, there's not much damp air in the house in general.
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On 21/01/2018 16:38, pamela wrote:

It all depends on the relative humidity and the inside surface temperature of the walls and windows. For a given RH you need the air temperature to be sufficient to keep the dew point below the surface temperatures.
Try a few different values in the dew point calculator at http://www.dpcalc.org/
--
Mike Clarke

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On Sunday, 21 January 2018 17:52:23 UTC, Mike Clarke wrote:

I doubt she knows the vapour content of the air. 12 at a minimum, probably more. Below that you're sure to get mildew, and at 12 it's likely.
It's cheaper to run a dehumidifier than heat, then you wouldn't need to heat it. But the type of DH must match the room temp, compressor ones aren't effective below about 10-12C.
NT
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On 18:25 21 Jan 2018, wrote:

I just posted some figures. The room gets an average of 12 C at 68% RH giving a dew point of 6 degrees.
It rises to 9 using more extreme values the room has been experiencing. The values may reflect limited variation from unnecessary heating.
I'm not sure if that is good or bad.
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On 21/01/18 18:25, snipped-for-privacy@gmail.com wrote:

We run a Dimplex DH in our conservatory, at around 10 - 12 deg C ambient. It works well, as I often have to empty 2 litres of water from it daily. Not tried it at a lower temp as the underfloor heating comes in at 10.
--

Jeff

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On 07:17 22 Jan 2018, Jeff Layman wrote:

I had an ordinary dehumidifier which could draw astonishing quantities of water from the air in a flat (65 sq.m, one person, not particularly humid) but it was drawing far more than necessary to keep humidity at a reasonable level.
Despite claims it was silent running, it made an unwanted noise. Also bit of a nuisance emptying it almost daily as there was nowhere for a hose to drain.
Impressive performance though. Entertaining watching how much water it drew.
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On 22/01/2018 13:27, pamela wrote:

Clothes and wet towels on radiators etc. If you eliminate this continuous source of humidity and battle away with the de-humidifier (running on a humidistat) after a number of weeks it will draw less and less.
Mine is set to 45% RH and I was emptying it daily for many weeks and having to switch it off at night due to it running almost continually but now it's on 24/7 and only ever comes on for about 5 or 10 minutes at a time and the tank gets emptied once a fortnight or longer and even then it's never full, so it's one of those things that if it's running all the time it needs to be running all the time (unless you're have a duff humidistat!)
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On Sunday, January 21, 2018 at 6:25:43 PM UTC, snipped-for-privacy@gmail.com wrote:

DEhumidifier will also provide a little heat. We run a smaiil Meaco dehumidifier on a boat in a Marina thats not being used. It works fine regardless of the temperature
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On 17:52 21 Jan 2018, Mike Clarke wrote:

It's the general damp penetrating into belongings that I want to avoid rather than outright condensation which we never get.
Max-min readings over the last 4 days show:
temperature: 11 to 13.5C rel.humidity: 66 to 71 %
There's little variation because I have been using a night time oil filled electric radiator to stop overnight loft temperature plummeting when the central heating is off. I feel the temperature figure may be unnecessarily warm.
Dew point may combine humidity with temperature but I don't have a minimum value to aim for. In the end, humidity may not be an issue because my radiator can only detect temperature and adjust itself based on that.
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On 21/01/2018 16:38, pamela wrote:

A dehumidifier might be an alternative, especially if you have economy 7.
It difficult to gauge damp. If you have wardrobes I would make sure there was ventilation around them, and not to back them against an external (cold) wall without a suitable gap.
Any heat in a room will ensure the room will be drier than outside, and below any dew point.
The issue here is, being a loft, is that warm damp air will rise and enter these rooms with the potential of cooling down and losing water to anything cold.
I did read somewhere that humidity should be kept below 50% for clothes.
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On 18:38 21 Jan 2018, Fredxx wrote:

Good point about damp air rising. The loft rooms are on the third floor but the house is reasonably large with no source of unduly damp air (such as a kitchen or bathroom) close to the stairway. So that should be a problem.
A dessicant type low temp dehimudifier is said to be power hungry. https://www.breathingspace.co.uk/dehumidifier-guide-i25
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On 19:01 21 Jan 2018, pamela wrote:

Oops. Err, "So that should NOT be a problem". :)

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On 21/01/2018 19:01, pamela wrote:

I had forgotten that a refrigerant type of dehumidifier had a minimum operating temperature.
Any inefficiency with a dessicant type would simply add that power as heat to the room. If on economy 7 it is cheaper to run than burning gas, with the extra dehumidifying effect.
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Warm air rises. Warm air is often humid (e.g. from cooking, bathrooms, etc), and when it hits cooler things high up, that moisture may be released.

Having repaired a number at Repair Cafe's, I would never leave a dessicant disk type dehimudifier running unattended. If the disk stops going around (a common failure), the heater may manage to set light to it.
--
Andrew Gabriel
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I have developed climate control for a friend's large shed, so they can store furnishings and tools without the furnishings starting to smell damp, or the tools going rusty.
With some experimentation, I have found that keeping the relative humidity below 80% solves the issue. So I monitor humidity, and use a 500W heater to increase the temperature so the relative humidity drops when it reaches 80%. This has now been running for 4 years, and is working very well. I also set a minimum temperature of 5C, so he can store things which mustn't freeze.
In practice, when it's 100%RH outside (which is it for much of the winter), you only need to keep the shed a couple of degrees warmer to drop the humidity in there to 80%RH. The heater is an old oil filled electric radiator, and the level of heat it needs to output means it doesn't even feel warm if you touch it, given the duty cycle it operates just to drop to 80%RH. It's actually a 1kW radiator which I drop the max power by having a half-wave rectifying diode in series. (An option exists to output the full 1kW if the shed is to be heated up warmer for someone working in there.)
One important thing is that everything is stored in the shed with plenty of airflow around the walls and floor - old pallets are used to raise things off the floor and space things away from the walls, with airflow behind. Otherwise the things you are storing will form a thermal insulation layer which will encourage condensation to form in them over time.
So in a house, an important factor will be is the air in the store area from the rest of the house, or from outside? If it's from the house but the area is significantly cooler than the rest of the house, you may get high humidity and condensation. If the air is from outside but the area is warmer than outside, then you are likely to stay below 80% humidity and never have any condensation. The same rules apply about keeping the stored items well clear of external walls/roof surfaces, so that the stored items do not end up forming an insulating layer which slowly collects condensation.
--
Andrew Gabriel
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On Sunday, January 21, 2018 at 10:35:24 PM UTC, Andrew Gabriel wrote:

Very helpful. Also if the room is on the north side of the house the moist air will migrate there
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On 22:35 21 Jan 2018, Andrew Gabriel wrote:

Are you monitoring humidity in person by looking at a meter and then switching on the heater by hand?
Or do you have some electronic humidity meter gizmo which starts the heater when the RH drops below a preset value?
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I am using electronic control, which is based on a raspberry pi computer. Initially I had it just monitoring the humidity and temperature inside and outside the shed, to understand what the humidity typically was, and how the shed came to have condensation forming in it. Once I understood that, I designed a control system to switch a heater on at > 80%RH or < 5C. This probably doesn't need a raspberry pi, but I already had that built with accurate humidity sensing. The Pi also records the energy the heater uses (by logging the on/off times and knowing the power consumption), so the system costs can be monitored, provides a web interface so the current state/costs can be seen, and logs remotely back to my own systems so I can see it working remotely (and control it if necessary).
The electricity costs worked out at about 1/50th of the previous cost of paying for the storage space in a warehouse. It was a pleasant surprise how little heating is required to take outside air at 100%RH and raise temperature just enough to drop it to 80%RH.
This could be done more simply with a humidistat, although I have no idea how accurate they are near extremes or at low temperature. However, if you vent the space to the outside rather than the house, and let some heat into it so it's always warmer than outside, that pretty much guarantees you are always below 80%RH even when outside is 100%RH.
--
Andrew Gabriel
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On 18:02 22 Jan 2018, Andrew Gabriel wrote:

Very impressive! It's rather more elaborate than I am inclined to attempt.
I guess I'll simply set my radiator TRV to keep the room temperature at around 10 to 12 degrees (with a Dimplex to kick in on very cold nights) while remembering to open the windows every now and then.
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On 22/01/2018 18:22, pamela wrote:

This might be simpler than a Raspberry PI: (Amazon.com product link shortened)
Though I might have thought 80% RH might have been on the high side, but Andrew has the experience.
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