Dew point

[Sandyn]

The chart below shows the dew point for different relative humidities and air temperatures.

So if you had a room at 24C and relative humidity in the room was 35%, if it was cold enough outside to get the window glass down to 7C, water would condense on it. This would easily happen with single glazing.

 

[artie]

Useful post
Would 35% RH be typical for a UK room?

 

[Rorschach]

Useful post
Would 35% RH be typical for a UK room?

Not living in the South West it isn't, 75% here according to my hygrometer.

 

[Bm101]

If it's warmer in my (insulated) shed than outside at colder times of year when I finish I leave the door open for an hour then lock up later.
I do the same with my van. If the heater's been (rarely) on then I open the windows driving the last few roads home to equalise the temps before locking it for the night. I rarely get issues with condensation this way with 4am winter starts.
If everything is uniformly cold then there are no damp issues with metals condensing.

 

[Sgian Dubh]

Would 35% RH be typical for a UK room?

No. It's a bit on the low side, even for winter readings, which are typically somewhere between about 40-45% RH to about 50%+ RH. Summer readings in most houses tends to hover somewhere either side of about 65% RH. Slainte.

 

[artie]

Not living in the South West it isn't, 75% here according to my hygrometer.

I'm feeling better now, mines fifty at the moment.
According to that chart you could get condensation in July.

 

[artie]

If everything is uniformly cold then there are no damp issues with metals condensing.

It's white frost outside right now, I know I will have drips in the shap tomorrow.
If I go out and raise the roller door will that be ample prevention?

 

[Phil Pascoe]

It's a mere 83% here. It was quite often 100% back in the summer.

 

[Bm101]

I'm not Michael Fish Artie! Just found that equalising temperatures works for me in my shed. As far as I know condensation forms on metal that is warmer than the surrounding air temp. (in certain, enclosed shed like conditions, not in the middle of the pacific on a rowing boat disclaimer) .
Edit... I'm talking rust prevention rather than 'controlling' humidity.

 

[Rorschach]

I'm feeling better now, mines fifty at the moment.
According to that chart you could get condensation in July.

Actually condensation is sometimes seen (very lightly of course) even on double glazed windows in the summer if we have a very clear, cold night.

 

[Nigel Burden]

According to the Met Office it's currently 94% here. As Phil said it was often nearing 100% in the summer.

Nigel.

 

[Sandyn]

I've been monitoring one of my rooms with a de-humidifier.
Outside it was 93% and 0C, in the room, it was 69% and 10C at 2:30 and with the de-humidifier on since then, (6 hours) it has dropped to 61%, but the temp has gone up to 11.1C, so it's probably just heating the room, and removing a little water.

 

[artie]

It's a mere 83% here. It was quite often 100% back in the summer.

In your room?

 

[Rorschach]

I've been monitoring one of my rooms with a de-humidifier.
Outside it was 93% and 0C, in the room, it was 69% and 10C at 2:30 and with the de-humidifier on since then, (6 hours) it has dropped to 61%, but the temp has gone up to 11.1C, so it's probably just heating the room, and removing a little water.

What kind of dehumidifier is it? Compressor or dessicant?

 

[artie]

I'm not Michael Fish Artie! Just found that equalising temperatures works for me in my shed.

I don't get condensation on the metal roof because I used the non drip backing, I get it on the clear plastic sheets.
At this time of year it's a pain, but there is so much free light all year , I wouldn't change it.
Maybe some one has a solution?

 

[Bm101]

Edited because I took Artie's posts the wrong way inexplicably and thought he was taking the mick. I'd had a long day and night and was a bit grumpy. Although this doesn't excuse my curt reply (now edited) I hereby and honestly apologise and hope he has the goodwill to accept my out of character umbrage.
Apologies Artie. Reading through your replies again I can see I was totally mistaken. I got the Zig unreasonably and through no fault of yours. Long hours, lack of sleep and getting soaking wet in the freezing cold for HOURS didn't help my mood but is no excuse.
Best regards as usual,
Chris

 

[Sandyn]

I don't get condensation on the metal roof because I used the non drip backing, I get it on the clear plastic sheets.
At this time of year it's a pain, but there is so much free light all year , I wouldn't change it.
Maybe some one has a solution?

possibly just a small 50W tube heater would keep the plastic sheets clear? If you know the relative humidity inside and the temp, you will know the dew point. If the plastic is at or below the dew point water will condense on it.
What's the non drip backing? does that somehow stop the condensation or just stop the water forming drips and falling off?

 

[D_W]

I've been monitoring one of my rooms with a de-humidifier.
Outside it was 93% and 0C, in the room, it was 69% and 10C at 2:30 and with the de-humidifier on since then, (6 hours) it has dropped to 61%, but the temp has gone up to 11.1C, so it's probably just heating the room, and removing a little water.

You've changed the gap to increase it a little vs. the dewpoint, which changes relative humidity a lot.

In the states (where the humidity in the summer can be super nasty just because the dew point can reach into the mid 70s in a lot of areas, and touching 80 from time to time (that's F, which would be about 27C), folks will often say here that the humidity is 10% somewhere or 95% and 95 degrees.

The dew point with any heat is generally what we're sensitive to. When it's 75 degrees, it's pretty intolerable - paper is mushy and tears easily, even if it's 85 degrees in the house, and things like leather coats in closets will develop mold if unchecked. Btdt (and so do books left on shelves).

Temperature, Dewpoint, and Relative Humidity Calculator

Brian McNoldy's T/Td/RH Calculator.

bmcnoldy.rsmas.miami.edu

Nifty calculator. In the US, I'll convert this to C, I've seen even supposedly well educated people suggest it's near 100 degrees F (38C) where they are and 95% humdity and unbearable. To find that the dewpoint is 75 (24C) degrees where they are and the relative humidity is actually 45%. If they were close to 75 degrees outside, it would be close to 100 RH and feel less hot (which is about what the temperature would be if it started to rain where they are, otherwise the air wouldn't be saturated and there would be no rain). When you say that RH is probably about 45 percent at that, it usually incites an argument about how it couldn't possibly be (has anyone ever been somewhere that it's 95 degrees and raining heavily? I haven't. It could be 95 degrees before heavy rain (33.3C), but it's almost exactly the dewpoint during (24C) it)

 

[Sandyn]

What kind of dehumidifier is it? Compressor or dessicant?

It's just a compressor type, so I've been looking at getting a desiccant type, but I wonder if there would be any advantage. I know they work better at lower temperatures, but they consume more power, heat the room and drop the RH, hence increase the dew point. So I think if I use my compressor unit with an additional amount of heat, the end result may be the same.
I have a wee test running with a power resistor stuck to the window dissipating only 4 watts. It is effective at keeping a small area clear of water, but apart from curiosity, it's pretty useless, lol
I currently use the windows as condensers don't cost anything to run. I have one of those Karcher window vac's they work very well. The steamy windows don't cause any real problems, but its more a cosmetic thing.
I might still get a desiccant dehumidifier

 

[artie]

possibly just a small 50W tube heater would keep the plastic sheets clear? If you know the relative humidity inside and the temp, you will know the dew point. If the plastic is at or below the dew point water will condense on it.

You've reminded me , some one said before. that a light bulb would do the trick, but I was sceptical and never tried it.
Maybe it's time to prove the point.

What's the non drip backing? does that somehow stop the condensation or just stop the water forming drips and falling off?

It's a thin film pressed onto the back of the box profile when it's being formed. It costs a little extra but is worth way more than it costs.
It just soaks up the condensation and allows it to evaporate overtime. According to the specs, it absorbs a stupid amount of water per m2 but I can't remember exactly.