What temp/humidity does water condensate in loft?

[ahlondon]

I am going to run a dehumidifier in the loft when the weather is at its coldest and would like to know if anyone knows the science that would help me set the humidity detection to the right amount. Example; operate to dehumidify to 60% and operate when 5 degrees celsius or lower. I want to reduce the moisture enough so that water doesn't drip from roof underlay.

Background
I know creating more vents is the best thing to do; this a temporary quick fix.
When it's extremely cold, condensation drips from the underlay (it's 50 years old and not breathable) creating huge puddles
I would only run humidifier when it's very cold (perhaps 0/1 degree outside)
I have maxed out the overrun extraction times on the powerful centrifugal inline extraction fans that are pretty much above the showers to remove as much everyday moisture as possible

 

[Notch7]

Condensation forms at a range of temperatures….it’s the result of warm moist air hitting a cold surface.

So it’s going to happen when the external temperature is low, say during a chilly night and morning.

When and how it happens might depend on whether it’s a ventilation issue between roof space and outside or an insulation / air tightness issue between house and roof space. If you have warm moist air getting in from bathrooms or bedrooms that might be the cause.

Does your loft have plenty of loft roll insulation….sometimes it’s pushed into the eves and blocking soffit / fascia vents

 

[ahlondon]

I don't think my style of pitched room created vents through eaves! There is lots of insulation but it is not near the gap where eaves would be. I can try sealing off the 4 downlights in bathroom perhaps but the overall moisture from the whole will still reach the loft eventually....

 

[foxhole]

I don't think my style of pitched room created vents through eaves! There is lots of insulation but it is not near the gap where eaves would be. I can try sealing off the 4 downlights in bathroom perhaps but the overall moisture from the whole will still reach the loft eventually....

You are mistaken .

 

[mrrusty]

This is how you work it out. From the table above say take a temp of 20 degrees. The saturated vapour pressure @20 degrees is 17.54. If the relative humidity in your loft at 20 degrees is 50%, then the vapour pressure is 17.54 * 50% = 8.77

8.77 is the saturated vapour pressure of air at about 9 degrees, so any surface colder than that will attract condensation if still air at 20degrees/50%RH comes in to contact with it and has an opportunity to cool to 9 degrees.

However, that air at 20 degrees/50% RH has the capacity to absorb more water, so if it is moving over a wet surface without an opportunity to cool, it will evaporate water, increasing its humidity.

By extracting moisture, a dehumidifier decreases the RH of the surrounding air, and hence lowers the dew point.

 

[foxhole]

I am going to run a dehumidifier in the loft when the weather is at its coldest and would like to know if anyone knows the science that would help me set the humidity detection to the right amount. Example; operate to dehumidify to 60% and operate when 5 degrees celsius or lower. I want to reduce the moisture enough so that water doesn't drip from roof underlay.

Background
I know creating more vents is the best thing to do; this a temporary quick fix.
When it's extremely cold, condensation drips from the underlay (it's 50 years old and not breathable) creating huge puddles
I would only run humidifier when it's very cold (perhaps 0/1 degree outside)
I have maxed out the overrun extraction times on the powerful centrifugal inline extraction fans that are pretty much above the showers to remove as much everyday moisture as possible

You haven’t said why?

 

[SFK]

Note that many dehumidifiers do not work at cold temperatures.