Dehumidifiers

[transatlantic]

Ok, so I bought a dehumidifier for my house, as during the very cold days (say 3 degrees or lower), I get condensation on the windows over night. Not the whole window, just the lower 2-4 inches, like below (not my photo).

Without the dehumidifier, the relative humidity is about 60 to 65%, which from my research is on the high end of whats recommended (they say between 50 and 60%). When I run the dehumidifer, it drops to about 50%, sometimes lower. Yet I still get the same condensation? perhaps a tad less, but still very noticeable and needs wiping down.

What's going on? I know the dehumidier works as there is water in the tank, and I have 4 different meters of different brands all giving the same relative humidity reading, so I know it's removing the moisture.

The temperature over night (inside the house) on the colder days the house is around 16 degrees.

 

[llangatwgnedd]

Have you got cavity wall insulation?

 

[Beau]

We can rarely get below 60-65% RH but then we do live on Dartmoor. Tonight we are at 60% RH with the heating cranked up. If you are getting condensation at lower RH I would suggest that the glazing is poor but on very cold mornings think it cold be expected. To help do the basics like using extractors when cooking and showering and let them run on if necessary.

 

[sunnybob]

keep your room thermostat set to 22c minimum throughout the whole 24 hour day.
If the window glass temp falls below 18c then you will get condensation (unless the humidity is down to single figures)

 

[Beau]

There is a fair bit on Wikipedia about due points but I think this chart should give you an idea about why you are getting condensation.

1 You either need to raise the air temperature which will lower the RH

2 Reduce the RH in the air either with dehumidification or at source ie Kitchens and bathrooms

3 Raise the temperature of the surface where the condensate forms



I am no expert on this but living in one of soggiest places in the UK gives you a bit of first hand experience.

 

[transatlantic]

Have you got cavity wall insulation?

Nope, no cavity wall insulation. Thought about it at one point, but decided it wasn't worth the risk. I'd rather just pay a few hundred more a year in heating rather than risk thousands of pounds worth if it goes wrong.

We can rarely get below 60-65% RH but then we do live on Dartmoor. Tonight we are at 60% RH with the heating cranked up. If you are getting condensation at lower RH I would suggest that the glazing is poor but on very cold mornings think it cold be expected. To help do the basics like using extractors when cooking and showering and let them run on if necessary.

Yes - I think the double glazed UPVC windows are very old, at least 15 years at a guess.

keep your room thermostat set to 22c minimum throughout the whole 24 hour day.
If the window glass temp falls below 18c then you will get condensation (unless the humidity is down to single figures)

There is a fair bit on Wikipedia about due points but I think this chart should give you an idea about why you are getting condensation.

1 You either need to raise the air temperature which will lower the RH

2 Reduce the RH in the air either with dehumidification or at source ie Kitchens and bathrooms

3 Raise the temperature of the surface where the condensate forms

I am no expert on this but living in one of soggiest places in the UK gives you a bit of first hand experience.

Raising the air temperature will bring the dew point up? surely I want to bring it down?

http://www.dpcalc.org/

 

[sunnybob]

You made a big mistake not fitting cavity wall insulation.
Over 30 years ago I had my then house done with rockwool. fantastic increase in internal heat.
About 20 years ago I had my then house done with polystyrene beads because at that time I could not afford the rockwool. Another marked increase in warmth indoors.
If I was still in the UK and had to do another house, it would be rockwool without hesitation.

The dew point temperature isnt your room temp, its the temp of the window glass. you need that glass above 18 and preferably 22c. radiators should be fitted below windows for that very reason alone and you need to keep the rads on long enough for that glass to stay above 18 all through the night.

Last winter i had this conversation with my daughter while i was staying at her house in Somerset. Her windows were running with condensation all day long. Upon inspection i found she had all her air vents open above the windows because her landlord had told her that would reduce the moisture.
W.T.F.?
She argued with me untill I one day closed all the vents and put the wall thermostat to 24 (her landlord had also told her to reduce the temp to 18). That evening every window was clear.

 

[transatlantic]

You made a big mistake not fitting cavity wall insulation.
Over 30 years ago I had my then house done with rockwool. fantastic increase in internal heat.
About 20 years ago I had my then house done with polystyrene beads because at that time I could not afford the rockwool. Another marked increase in warmth indoors.
If I was still in the UK and had to do another house, it would be rockwool without hesitation.

There are also many other people who have had issues. But that is a dicussion for another day!

The dew point temperature isnt your room temp, its the temp of the window glass. you need that glass above 18 and preferably 22c

Hmmmm ... but I thought that condensation on the window is caused by a difference in temperature from the inside glass to the outside glass? ....so if I increase the temperature of the room, thus increasing the temperature of the window, there will be an even bigger difference?

 

[RobinBHM]

You could consider replacing the double glazing.

It looks like the condensation is forming due to the thermal bridging created by aluminium spacer bars.

If you replaced with warm edge spacer bars and Saint Gobain planitherm total soft coat low E coating, you might find the inner glass may stay warm enough around the perimeter to stop the condensation forming.

I dont know if it will overcome the condensation, but it may well help or minimise it -and since upvc windows are dry glazed its not too difficult to do. Your problem will be finding a trade price for double glazing.

 

[stuartpaul]

keep your room thermostat set to 22c minimum throughout the whole 24 hour day. ....

I'll tell you what Bob, - I'll increase my thermostat to 22 and run it 24 hours a day, you make a (significant!) donation to my (significantly!) increased fuel bill!

I'd rather spend 10 minutes every day 'desmisting' than the several hundred pound increase in fuel bills.

You pays your money .......

 

[Beau]

Hmmmm ... but I thought that condensation on the window is caused by a difference in temperature from the inside glass to the outside glass? ....so if I increase the temperature of the room, thus increasing the temperature of the window, there will be an even bigger difference?

By increasing the room temperature you will lower the RH as the warm air has more capacity to hold water in suspension.

 

[transatlantic]

keep your room thermostat set to 22c minimum throughout the whole 24 hour day. ....

I'll tell you what Bob, - I'll increase my thermostat to 22 and run it 24 hours a day, you make a (significant!) donation to my (significantly!) increased fuel bill!

I'd rather spend 10 minutes every day 'desmisting' than the several hundred pound increase in fuel bills.

You pays your money .......

This is why I was trying to solve the problem with the dehumidifer. As keeping that on for ~6 hours a day (I have found that in total it's only on for that amount of time to maintain ~50% RH) is much cheaper than keeping your central heating on 24/7.

 

[sunnybob]

I can only offer a solution, sometimes a solution is more expensive than the problem, thats for you to decide.
But if the cavity walls were insulated, and the loft had 100 mm of insulation, there would be a very small increase in fuel bills.
I know that.
I've proved that to myself.