Seasonal wood movement

[Ateallthepies]

Evening all,

I have been having a long standing debate with a pal on the seasonal effects on wooden items and would like others views?

I believe that wood in the summer will swell/expand due to the higher humidity and will shrink in the winter because of the opposite humidity effect.

My pal disagrees and believes the opposite as his front door always swells in the winter and shrinks in the summer.

Who is right and is there other factors other than humidity that effects wood movement?

Thanks,

Steve.

 

[Blister]

Steve

I think your mate is right , when dry looses moisture and shrinks , when wet wood expands

Like old boats , they wet them to get the timbers to expand and make them water tight

 

[Ateallthepies]

Yes I understand about taking on moisture but his front door has been painted more times than I care to remember(he is a painter!) so should be reasonably water tight?

Steve.

 

[Blister]

So if you are saying moisture cant get in as its painted , how will humidity get in in the summer ? as its still painted

 

[OPJ]

The problem with doors (possibly) is that while one face is exposed to the elements and the seasons, the other face is pointing indoors. During the summer, I expect all internal woodwork to expand, due to increased levels of humidity, as you say. But, in the winter, once the central-heating comes on, I then expect the wood to shrink, as this creates a drier atmosphere.

Outdoors though, it's difficult to predict exactly what will happen, given the irregular patterns of the British weather! :-D Timber selection may play a part, as well. If the door's made from softwood then, it's more likely to expand and contract frequently, as opposed to something like European or English oak, which is inherently more stable and durable.

This year, I've found that our softwood back door has expanded slightly during the summer and, also, more recently, during the colder weather... :duno: ;-)

 

[Ateallthepies]

Good point, though I thought water vapour might be smaller than normal water and is getting in somewhere where the runny stuff can't?


Steve.

 

[Sgian Dubh]

I believe that wood in the summer will swell/expand due to the higher humidity and will shrink in the winter because of the opposite humidity effect.

Regarding indoor furniture made out of wood that averages between about 7% MC (winter) and 12% MC (late summer) your friend is wrong and you are right. I could blind you with science on the subject, but I won't.

But when it comes to architectural doors there are other forces that come into play as they are exposed to both internal and external conditions of relative humidity.

What a lot of people forget is that doors are surrounded by frames, and these frames aren't as immobile as they imagine. It's often the case that a contributory reason for doors sticking is movement of the frame due to a variety of causes, in addition to any movement in the door itself.

Someone mentioned paint and its ability to resist the ingress and egress of moisture-- Blister I think it was. Film finishes such as paint, varnish and pre-cat lacquers are not impervious to water at a molecular level. Water vapour passes through such finishes, but it depends on the ability of the finish to resist water vapour that determines how fast wood is affected by changes in RH. This ability to resist water vapour is a function of a variety of factors, such as the number of finish coats, the ability of the finish to slow down the movement of water vapour per coat, etc. Slainte.

 

[Hudson Carpentry]

One other factor, the more paint you put on the bigger the door gets

 

[Ateallthepies]

Ah I'm beginning to understand!!

The relationship between the outside elements and the indoor CH system is a sort of balancing act?

If a front door is protected from direct water then most likely it will swell in the summer humidity. In the winter, due to the lower humidity and the central heating being on it will most likely shrink.

If the door is not protected from direct water then it will most probably remain in some sort of swollen state all year round but depends on the weather and how hot and dry your front hallway is?

Steve.

 

[Jacob]

......
I believe that wood in the summer will swell/expand due to the higher humidity

T'other way round.
Absolute humidity (the amount of water vapour in the air) is higher in summer but the relative humidity (the amount of water the air could contain) is lower so the net effect is drying.
Vice versa in the winter.
So your external doors will expand in the winter.
But it's not that simple.
The winter air absolute humidity is lower than the summer figure, but relatively high at external winter temperatures. Warming this same air will lower the relative humidity whilst the absolute humidity stays the same. So, brought indoors to heated spaces, winter air will become drier than summer air, so your internal doors will dry, and shrink.

In my building this is really noticeable: in summer the sarking boards in the roof all shrink and you can see the Tyvek membrane in the gaps. Reason; lowered relative humidity in the roof space. But the basement suspended floor (over earth) does exactly the opposite and expands in the summer, due to the high relative humidity of the underfloor space. In winter the basement is heated so the relative humidity falls, and the floor boards dry out, whilst the sarking boards expand.

BTW all paint is porous, but some more so than others, so rates of drying or moisture absorbtion will be different.

 

[woodbloke]

Think of wood as similar to a sponge. If it's dry and the air is moist it soaks up moisture and expands causing a door (say) to stick. Conversely, if the wood is 'wetter' than the surrounding air it will give back moisture and shrink. This is the basis of air drying green timber and is also why timber that you may think is dry (around say 12% MC) will shrink in a CH home in the winter. Timber at that MC% however would have been fine in conditions where there was no CH, hence a lot of the problems associated with antique furniture in a modern CHed home - Rob

 

[tomatwark]

Has you mate painted the bottom of his door?

That is the normal cause the doors swelling as the moisture is sucked up the end grain of the door stile.

Tom

 

[Ateallthepies]

Yep all faces painted, probably the top and bottom has fewer coats. When I say the door swells/expands it is not so much the thing is sticking but the vertical joins within the door keep cracking I suppose it is sort of T&G with chamfered edges?

Steve.