Hi all

A bit of background: I live in the Brecon Beacons surrounded by a lot of trees and a stream, all of which I presume lends to the high relative humidity (RH) in my house. It often sits in the 70% region and sometimes even creeps above 80%. In times of high RH, opening the window is counter productive and causes it to rise. We use a dehumidifier when it's at its worse.

We have a wood burner in one room, which does not have a direct (external) air feed. It's not been in long, but it does do a good job of reducing the RH. We'll soon be installing a further two wood stoves. I totally see the sense in direct air in as much as it will minimise drafts, and negate the need for a vent which would let copious amounts of cold air into the house...

But when it comes to humidity I can't get my head around it. No direct air to the stove means more air changes and air circulation in the house (I think?), which could be looked upon as a good thing. But then again, it also means more air coming from outside, where the humidity is higher, so perhaps it isn't such a good thing. And maybe that's all too basic a way too look at it anyhow.

I wondered if anyone out there has a better understanding of the science behind this and could explain which option is best and why, when it comes to humidity?

Many thanks

Given the option I would always connect a wood stove to an outside air source.

You need ventilation in a house, with an external air supply for the stove you can control the ventilation you want / need, without an external air supply you will have no control whenever the stove is in use.

All air will have some humidity. The amount of water that air can hold will depend upon its temperature, the warmer the air the more water vapour the air can hold.

The amount of moisture in the air can be measured in grams and this will be the absolute humidity. Air that is saturated to the point that it can hold no more AT A GIVEN TEMPERATURE is said to be at 100% relative humidity (RH) If the temperature rises then because warmer air can hold more moisture the RH will fall but the amount of moisture in the air remains the same.

When you bring in cold air from the outside at whatever the RH happens to be and the you heat it the RH will fall. The warmer you make the air the lower the RH will go.

If you cool the air the RH will rise and once you get to 100% RH the air can not hold any more moisture and further reductions in temperature will result in moisture falling out of the air. The point at which this happens is called dew point.

Whether or not you have external air supply for the stove does not alter the physics of falling RH with increasing temps. or rising RH with falling temps. Just that with an external air supply you have control over the amount of air coming in.

While effectively giving the right results in many common cases, PiH's description of air “holding” water vapour is wrong and in some less-common cases positively misleading. Unfortunately it's difficult to think of a way of phrasing a better description which is not a lot more wordy. Still, here's my rant on the subject: https://edavies.me.uk//2017/03/vapour/

More substantially, Greenfinger says that bringing in outdoor air increases the indoor RH with the implication that that's raising the indoor (absolute/specific) humidity. That's surprising as normally indoor air has a higher specific humidity (because of people/pets breathing and things drying) so maybe what's happening is that the outdoor air has slightly lower specific humidity and is actually drying the inside air but it's also raising the indoor *relative* humidity because of its cooling effect. So

Posted By: greenfinger: “We have a wood burner in one room, which does not have a direct (external) air feed. It's not been in long, but it does do a good job of reducing the RH.”

Fine, this is probably a combination of ejecting damper indoor air and bringing in somewhat drier outdoor air but mostly just a matter of warming the room sufficiently to bring the RH down for substantially the same specific humidity. But if you just had normal levels of ventilation, with an external air feed, the wood burner would presumably raise the indoor temperature a bit more and therefore bring the RH down even more. I.e., I'd think an external feed to the burner would do better overall at reducing indoor RH for a given amount of fuel burned.

(PS: cross-posted with djh, I went down a bit of a rabbit hole while writing this.)

In effect our traditional house gets a substantial proportion of outside air direct to the fires anyway.
The range is quite close to a badly sealed outside door, and I've left that to stop the stove sucking all the air out and suffocating us.

The open fire in the sitting room is over the only suspended floor, and the room is well-sealed. It can get very stuffy after a while, so I fitted a pair of sliding vents in the floor either side of the hearth. There is a terrific draught of fresh air if I open one, but it's close to the fire so it's virtually a direct pipe.

Overall the effect is beneficial:

warmer house reduces condensation
controlled air feed to the fire is safer and not stuffy
fire draws and burns better - more smoke goes up the chimney :)