At the minute, in this Shropshire location...

OT -0.2C 81%

IT 15.6C 60%

Clue? Wife who can't get her head around the benefit of keeping the kitchen door closed.

I'd be wary of letting the RH get below 30% if you can.

One of the projects in my last job was a big'ish lab/office build (it housed around 900 people and a lot of labs, some fairly specialist). For reasons associated with running cost, the humidifier modules were left out of the air handling units during the build and despite the building having no air conditioning systems (it was primarily passive ventilated with some fan assistance in the big MVHR units on the roof) the RH used to regularly drop down to the mid-20% range in cold weather.

The result was a marked increase in people suffering from dry skin and asthma attacks, plus a problem with static discharges giving people mild shocks when touching door handles etc. I was in that office for a year or so post-completion and although I didn't suffer from the skin or asthma problems I did have problems with dry eyes and a stuffy nose. Other than the low RH the air quality was pretty good.

The fix was an expensive retrofit of humidifier modules to the air handlers, to get the RH up towards 40% or so.

Which is why my wife starts to look like a muppet soon after walking into an airport! Amusing description, but the condition ain't funny. When we visited my brother in Montreal some years ago (mid-September, OT up around 93F), by the time she got off the aircraft she could hardly see. And then we got into an air-conditioned car, drove to their air-conditioned house, at least she got some relief by sitting outside on the porch, basking in all the humidity.

So Mr J S Harris,

With 900 people in the building, where did/does all that exhaled moisture go?

Jules, I won't be turning the MVHR on just yet then.

Damon, thanks, I'll send a bucket over.

Any more RH measurements?

<blockquote><cite>Posted By: dickster</cite>So Mr J S Harris,

With 900 people in the building, where did/does all that exhaled moisture go?

</blockquote>

Most of it was recovered as heat and used to warm the incoming fresh air, via the air handling system heat recovery modules. Being three storeys high, with two internal full height atria to allow natural internal convection ventilation, most of the air movement to the air handlers was natural, rather than driven by fans (although there were also big fans inside the air handlers to get the flow rates up through the ducting).

Household sized MVHR units do the same, and can even condense out moisture from the cooled exhaust air if the incoming air temperature cools the heat exchanger down below dew point, I believe. One or two here have mentioned problems with moisture in MVHR units I believe.

Posted By: dicksterwhether or not we've built a house that is too dry

There ain't no such thing as building a house that is too dry. If the air inside is too dry, just add water!

One of the easiest ways is to add some potted plants. In a dry atmosphere they'll suck up and vaporise water like nobody's business. The American architect Frank Lloyd Wright had this nice idea of a 'water fire' for buildings in warm, dry climates. It's basically a chimney over an indoor fountain. Water evaporates, increasing the humidity of the air and cooling it. It flows out into the room and more external air drops down the chimney. Rinse and repeat.

Posted By: julesWorth pointing out that comparing RH outside and inside is a bit misleading at the moment as the actual temps are so different. What really counts is absolute humidity

I'm not sure I understand your point. RH is what counts for biological effects, such as the effects on our bodies, or the growth of mould, rot and pests etc. I agree that it's usually most convenient to think in terms of absolute humidity or specific humidity when thinking about transport of vapour, although any possibility of condensation drags us back to RH again.

djh That's what we've started doing, already have house plants (too many) and last night left a pan of water on stove. Just a bit puzzled as to where all our moisture has gone. Maybe it's the accumulative drying effect of this high pressure system, hence the post to see if it's just us or a countrywide phenomenon.

Thanks and understood, anyone else noticed a marked decrease in internal relative humidity over the last week?

Posted By: dickster Just a bit puzzled as to where all our moisture has gone. Maybe it's the accumulative drying effect of this high pressure system, hence the post to see if it's just us or a countrywide phenomenon.

The air over the country at present originated in deepest Russia, where temps are about -30C. At that temp, as JSH says,the air contains very little moisture. Although it has picked some up as it travelled across eastern Europe and the North Sea it's still much drier than our normal feed of air, which arrives on a westerly having crossed thousands of miles of sea, and all the time at much higher temps too - so it can carry more moisture.

Yup. The suspense is killing me.

Pretty dry and cold (-7C last night) in Brussels last night (where I am this weekend), with frozen water falling out of the air!

Rgds

Damon

Well I have water condensing on the outside of my single glazed windows as the 6C outside is warmer than the inside! I can't wait to get my EWI on and a new roof