Loft humid in the summer time

frozen_wastes

So I poked my head into the loftspace of my bungalow.

As well as being hot, it felt quite humid. This I thought was a bit strange. Summertime lofts should be hot, but they should have the same humidity as the outside air temperature - unless of course there's a buildup of moisture underneath the roofing felt.

So my loftspace has vents at each of the gable ends, plus continuous ventilation at the soffit level. Last year I also added tile vents. So in theory, that should be sufficient ventilation to allow moisture to disperse. I have no tanks or pipes in my loft space.

So I decided to quantify my personal perception. I rigged up a raspberry pi to a temperature and humidity sensor in the loft and started logging data last night.

Temperature outside was 18 degrees, with a dewpoint at 10 degrees according to local weather station.

Early Evening loft temp: 31 degrees, 45% relative humidity, 18 degrees dew point. So it was definitely much more humid than the outside air. That dew point caught my eye - it was the same as the outside air temperature.

My expectation would have been that as the evening progressed, the temperature would drop while the dew point remained the same leading to higher relative humidity. Not so! As the temperature dropped, so did the relative humidity and consequently the dew point. My last data point was 22 degrees, 40% relative humidity leading to a dewpoint of 7 degrees. I guess the moist air was being dispersed.

I'll have a full 24 hours of data logged tonight, and my expectation is that the humidity will climb back up again during the daytime.

It's as though there's a store of water in my loft that's evaporating during the daytime heat, and then dispersing in the night time.

For good measure I had a good look around and only saw historical signs of rainwater ingress (occasional and very minor water stains on some fibreboard underneath the roofing felt). I also measured the moisture levels of the trusses and rafters and it was coming in at around 10%, 15% for plywood sheathing at the gable ends.

So where is this moisture coming from? My downstairs area is not the source, it's cool and dry.

Edit: No! I do not have any plants of any description being cultivated in my loft!

coffeehound

When you say the living area is dry, how are you quantifying that — just the absence of condensation, or measured with a meter?

Bungalows seem to suffer more problems with humidity, perhaps due to underfloor humidity rising up with a smaller living space volume compared to a multistorey home. Some of that humidity would eventually end up in the loft space as humid air rises. And there has been a lot of rain lately.

naf123

Moisture most likely coming from you! As humidity rises it has to end up somewehre - the loft!

Thats why with loft conversions, its advisable to have a/c installed.

frozen_wastes

coffeehound wrote: »

When you say the living area is dry, how are you quantifying that — just the absence of condensation, or measured with a meter?

In this instance I haven't quantified it, but it feels far less clammy. I'm a single person occupier, so the amount of moisture that I generate will be very small compared to a fully occupied property.

I feel it unlikely that I would be the source of humidity in the loft - not in the summer time anyway. It would strike me as odd that water vapour would migrate from a cool dry location to a hot humid location. Normally water vapour will go the other direction.

naf123

frozen_wastes wrote: »

In this instance I haven't quantified it, but it feels far less clammy. I'm a single person occupier, so the amount of moisture that I generate will be very small compared to a fully occupied property.

I feel it unlikely that I would be the source of humidity in the loft - not in the summer time anyway. It would strike me as odd that water vapour would migrate from a cool dry location to a hot humid location. Normally water vapour will go the other direction.

You have a shower. Bath - etc lots of water vapour - where does that go?

You have cooking in kitchen - where does that go

you also breathe out vapour constantly. Where does that go.

frozen_wastes

naf123 wrote: »

where does that go?

Water vapour will migrate to the place of lowest vapour pressure if diffusion is the only mechanism at hand.

However in the case of cooking and showering, that water vapour is expunged to the outside air by mechanical air extraction.

coffeehound

frozen_wastes wrote: »

... in the case of cooking and showering, that water vapour is expunged to the outside air by mechanical air extraction.

When running extractors, the make-up air will often be drawn in from the damp underfloor area. I suppose another factor with bungalows, as well as having a large floor area compared to house internal volume, they also have a large ceiling area, so making it easier for humidity to rise to the loft through the gypsum.

A PIV would reduce the amount of rising vapour from floor level in the first place, plus assist the extractors in doing their job, if you felt it was a problem.

frozen_wastes

The solumn area underneath my bungalow is ventilated and dry. There's a damp proof membrane underneath the gravel in the solumn area.

Anyway, I got back home tonight to inspect the data. Shame I can't paste a graph on here as it's quite interesting.

Relative humidity gradually rose through the night from 40% to 43%. This is what I'd expect a body of air to do as it cools down.

As the loft warmed up during the morning so did the humidity, rising from a minum of 11 degrees at 6:15am to 38degrees this afternoon.

Relative humidity hit a peak of 54% mid morning then it started to drop off. It's now down to 33% which is still a dew point of 18 degrees.

I think what I'm seeing here is a process of drying out of the building fabric. Quite how much moisture the fabric normally holds I think is an inverse function of temperature. I'd expect wood to hold more moisture at lower temperatures than at high temperatures (essentially as it bakes, then it releases moisture into the atmosphere). In the evening time the baking process is completed, and humidity returns to more atmospheric conditions.

If I collect data for tomorrow, I'm guessing that I'll either see that the loft is dry and has atmospheric humidity, or the building fabric absorbed moisture during the night and re-releases it during the day.

Just for info, I'm not home during the daytime so that somewhat discounts the idea that I'm the source of the humidity.

ComicGeek

frozen_wastes wrote: »

Just for info, I'm not home during the daytime so that somewhat discounts the idea that I'm the source of the humidity.

There's also a time delay in transfer of moisture through the building, so very likely that the mid-morning peak humidity is the delayed effect of you showering etc in the early morning. Just like there is a time delay in heat from the external surface of the roof penetrating into the roof space.

markin

What was the reason for adding tile vents?

frozen_wastes

I added tile vents because of the fact that it did feel humid in the loft and I wanted to control that.

There were historical problems - some of the fibreboard was sagging. And indeed when my wood burner was installed 8 years ago, the chimney fitter remarked that the underside of the felt was wet even though there was no rain.

Now the previous owners of the house had installed recessed down lights, and indeed the sagging fibreboard was right above this area. I've long since removed these downlights and filled in the holes (I do think recessed downlights are a terrible idea in many different ways!).

So last year when I saw that there was a tiny slither of what looked like rot (a bit of darkened wood) at the top of one of the rafters my thoughts turned to adding tile vents, and also getting the ridge tiles all rebedded (most could be lifted without any struggle and the mortar was in a poor state). I wanted to eliminate whatever the source of moisture was for causing that.

That was last year however. This year though I've gone through an exercise of filling in most of the ceiling cable transits with expanding foam, as well as filling gaps around some of the air extraction ducts. Together all these holes add up to the size of a 4 inch duct in the ceiling.

I think all this work puts my loft environment in better shape for the winter time. By eliminating air movement, the only way moisture can get through the ceiling is by diffusion through the plasterboard which is a much slower process. I'm still convinced that the source of the humidity is external to the building envelope (e.g. rain penetration), rather than within it.

By the way, I have my shower in the evening not the morning. This delayed effect hypothesis I don't think is too credible. I've now had four days of dry weather so I'll check out the data tonight and see how it turns out. If there is a drying trend with the air, then it would prove my conjecture about the moisture source.