Exterior T&G and Relative Humidity

Hello,
I'm installing a T&G beadboard porch ceiling made from poplar 1x4s, priming both faces and painting the exterior face only (using acrylic latex). I'm wondering whether I can install the boards tight together, or if I need to leave a small gap between the boards to allow for expansion due to changes in relative humidity.
I understand that wood expands with increasing moisture content, up to the fiber saturation point. So will the latex paint slow the exchange of moisture sufficiently that the board is more or less always at the moisture content in equilibrium with the average relative humidity? If so, then as long as the boards are acclimated to these conditions, it should be OK to install the boards tightly together.
FWIW, my information is that the mean relative humidity here in Berkeley, CA is 73%, with a median of 77% and a standard deviation of 17%. 2/3 of the time the relative humidity is between 60% and 90%.
Thanks, Wayne
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On Wed, 17 Jan 2007 00:32:38 +0000, Wayne Whitney wrote:

When outside, wood moisture levels usually decrease down to whatever the equilibrium state is (12-15%). I'm an architect in NC, a very humid place during the summer, and although it is still common practice to leave a 16d nail spacing between decking boards, there is no reason to for expansion. I think it is a good idea to leave some space so that water can easily shed off the surface, but this is a consideration for the wood's protection, not expansion. (Completely different story on interior flooring--it is the opposite strategy since the wood starts ultra-dry and is put down over moisture containing concrete--but that is another post altogether.)
In fact, what usually happens (and there are homes everywhere around here, including my own, that testify to the fact) is that spacing GROWS due to shrinkage. This can happen even with dried lumber since drying is usually done by a fast mechanized process but the cells still contain some moisture that eventually migrates out of the wood, and thus, shrinks it. (Imagine an old, unused sponge. Even if you drop it in water, it won't expand to the size it was when new.)
In your case, the T&G won't even be directly exposed, and I assume it is already dried, either by kiln or air. So it is safe to assume that it won't expand. What you want to avoid is the rather large gaps that will appear over time. All this does not mean you should jamb them as closely as possible. But the whole idea of T&G in the first place is to manage board expansion and shinkage per board. You should install them with a good fit, no tighter or looser. It is a good idea to leave a trim-covered gap around the perimeter to manage any eventual motion either way, but this would likely be seasonally cyclical.

Nothing stops humidity movement, short of expensive man-made membranes, and that only for a time. Even the world's best resinous flooring systems will pop right off of concrete if the moisture pressures below it are not properly managed.
My $0.02 US.
--
Steve Hall [ digitect dancingpaper com ]

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Hi Steve,
Thanks for your thoughtful response, a few comments:

Are you suggesting that recently cut and kiln dried wood taken to a moisture content of, say, 12% may shrink over time even if the moisture content stays at 12%? Or just that kiln drying freshly cut lumber doesn't usually take the moisture content as low as its equilibrium moisture content in an unexposed exterior application?

Well that is my big concern. I don't know the history of the boards before I bought them, except that they were kiln dried at some point before milling. After I bought them I stored them in my unheated garage for 5 weeks, primed both sides, stored them inside at 30% RH for a week, and painted one side. Do I need to let them sit outside before installing, and if so how long?

I see how T&G will do that if you install it tightly at a moisture content at least as great as the highest moisture content it will see during service. But if the moisture content at installation is lower than it may see in service, a tight installation could cause buckling due to expansion, yes?
Thanks, Wayne
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I do not have an answer to your question but a thought. Wood installed on a floor in a dried in porch might do different things compared to the same wood installed on a ceiling. Heat from the attic or maybe infrared might dry things out.
Wayne Whitney wrote:

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On Wed, 17 Jan 2007 01:52:08 +0000, Wayne Whitney wrote:

Yes, most woods shrink over time simply due to the microscopic cellular breakdown of the material. Moisture can make freshly cut wood expand and contract but as the structures decompose over time, they are no longer as flexible or as ideal a place for moisture to be retained. Air pressure dictates this migration. This is why pressure is usually used in treating wood, it forces the preservative into cells that couldn't normally be contacted by the chemicals under atmospheric (or even submerged) conditions.
In my experience, another significant factor in the disparity in moisture percentages is due to the imprecise methods of measuring vast quantities of construction materials in an automated process. If you could scientifically measure more and smaller samples, the error (and shrinkage) would be reduced. We specify wood for some construction purposes to be kiln dried to, say, 12-15%. But I guarantee that the wood that shows up on site will rarely meet that specification, although some measurement technique during production registered that it did.
Wood moisture content will eventually equalize at some rough point based on the species and the environment. I call that equilibrium, but it is actually a narrower range of fluctuation from surrounding conditions. Granted, this could be more extreme in situations such as a piano from Louisiana moved to a barn in Arizona. Or it could be more significant in the case of a 200 year old violin simply moved about the country in a concert tour without being in a sophisticated moisture controlled case. Neither compare to the difference from freshly cut lumber to the first time it is cut down to a workable size.

Quite the contrary, it is possible to "over-dry" wood in a kiln. It can then still take on water and expand a little. But over-drying also damages the wood cells, the very property of the material we want to preserve. It also prevents vast expansion comparable to the shrinkage I usually am more worried about.

You're not going to have a problem. I don't know the size, but if they are 2x6 or bigger, it is likely that the wood still hasn't come down to the equilibrium range. In any case, I would expect that after kiln drying and your significant storage time it should be very close, and if for some reason they are below it, it can't be by much. Remember, these are construction materials, not anything as hyper-sensitive as a musical instrument.

Tight is a relative term. If you somehow clamped each piece into the next before nailing it, you might be able to cause problems. But if you use a reasonable construction method, expansion won't be an issue. Just realize that those tongues and grooves are DESIGNED to maintain a joint with expansion and contraction over time. Don't try to pinch them together or leave too much room for expansion, the idea is to let groups of two or three can work together. I'll close by reiterating that my chief concern for outside materials is always shrinkage, never expansion.
--
Steve Hall [ digitect dancingpaper com ]

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Hi Steve,
Thanks for all your comments. It sounds like my concerns are theoretically possible, but in practice only occur under extreme circumstances. I'll give the boards stored inside a day outside to reacclimate, and then I'll install them snug but not super tight. I think that should take care of everything.
Cheers, Wayne
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Wayne, you're getting some misinformation. Wood will adsorb and release moisture to get into equilibrium with the relative humidity of its surroundings. Period, end of sentence. Makes no difference how it was dried, to what point it was dried, or what finish you put on it. Its shape will modify accordingly. Of course, if you heat its surface in sunlight, as on a deck, or place it over a crawl space with no vapor barrier, the weather report is not going to determine the relative humidity.
Since you have lots of room to shrink with no risk of gapping, and it's pretty humid right now, sticker the stuff on the porch a _week or two_ to acclimate to the higher, and put it up without pushing, but without deliberate gapping and let the law of averages take care of you. Nails will allow movement, as you know, so leave a gap under molding at the edge to take up any movement of the giant board you've made that can't be taken up internally.
Information on wood, why, how, and how much it moves, as well as a lot of other things. http://www.fpl.fs.fed.us/ GOOD information.
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OK, this is certainly true if the ambient relative humidity is constant. But given that the ambient relative humidity is changing, the wood is always trying to "catch up". So it seems like it becomes a rate-limited effect. The moisture content of the wood would fluctuate within a band corresponding to a band of relative humidity fluctuations that is narrower than the full ambient fluctuations, because the extreme ambient fluctuations don't last long enough for the board to reach equilibrium. It would seem, moreover, that anything retarding moisture exchange with the environment, like a paint finish, would make the wood's fluctuation band narrower.
Is that analysis correct? Also, what about the statement that a freshly cut piece of wood dried to a given moisture content, say 12%, will be wider than that same piece of wood at 12% moisture content several years later when it has aged? The idea, presumably, would be that aged wood responds less to fluctuations in moisture content while retaining the same oven-dry dimensions.

Right, so the wood was outside 5 weeks, and inside 1 week at 30% RH. Seems like 1 week outside ought to be sufficient to reverse whatever shrinkage keeping it inside allowed?

This I didn't know, actually. Are we talking about toe nails only, or also face nails? I'm rebuilding the porch, and the way it was constructed is that each board was toe nailed through the tongue into one joist at center span, and then the ends of the boards were supported by a 1x8 frieze board, with a face nail from above through the board into the frieze board. The span is roughly 6', so I'm rebuilding the porch with two joists instead of one.
Thanks, Wayne
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