Gap for Floor Panels

[Jan Bianchi]

The bottom of the traditional country hutch I am building consists of two panels of cherry 15" wide and that are separated by a center style and framed around by rails and stiles. Because it is the bottom of a china cabinet, the panels are 3/4" thick. I have attached a picture of the detail that was in Fine Woodworking January/February 2007. Because there is a lip on the face side of the panels, presumably so the bottom will be flat and flush with the height of the frame, they will not float like a normal panel would. While the diagram shows leaving a slight gap for wood movement, it does not appear nearly large enough to me. According to the Shrinkulator I should be allowing for .55" of expansion in a house where to relative humidity varies from 50% to 94%. But that means I would have to leave a 1/4 inch gap between the lip of the panel and the frame on both sides which not only unsightly but not very safe for the dishes if the wood shrinks. I am reading the Shrinkulator wrong?

[Todd Burch]

I hope your house does not reach 94% humidity! Or even 50%! Along the Texas Gulf Coast, interior EMC is about 13% in the summer.

It's winter, so your wood will be drier than in the summer. If you left, IMO, 1/8" total for a 15" panel, you would be fine. (1/16" on either side). If you wanted to fudge up to 3/16", you could do that too. That's only 3/32" on either side.

And I don't understand your statement about the panel not floating. As shown, it would float. Frame and panel 101.

[Jan Bianchi]

Well, yes it does float a little as shown, but not nearly as much as a panel without a lip. Maybe I don't understand what humidity reading to use. I thought the Shrinkulator suggests we use the extremes of external humidity in the geographical region where one lives. I have no idea what the humidity is in my house is in the summer, but 100% outside humidity is not uncommon. The monitor in my shop reads 50% humidity right now, internal temperature 65 degree, outside temperature 50 degrees .

But thanks for the advice. 1/8 to 3/32" is much more appealing.

[Todd Burch]

I understand your confusion, and I didn't help it with an incorrect statement. While it might be 50% relative humidity in your house (and I was wrong saying it would not be) the moisture content of the wood in your house is not at 50% "relative humidity". EMC stands for equilibrium moisture content, and that's the moisture content the wood will settle at in your region. What you need to be concerned with is the upper and lower ranges of EMC in your house. This link shows the EMC for Seattle: http://www.woodfloorsonline.com/tech...sture_map.html

So, I was wrong again. It is wetter in Seattle in the winter than in the summer. 16.5% EMC in December and 12.2% in July. So, if one can assume your boards are currently at the highest moisture content right now (and they would be if you acclimated them in your house before cutting to final size), then you could get by with 1/8" combined just fine.

Sorry for the incorrect statements.

Todd

[Jamie Buxton]

... What you need to be concerned with is the upper and lower ranges of EMC in your house. This link shows the EMC for Seattle: http://www.woodfloorsonline.com/tech...sture_map.html

So, I was wrong again. It is wetter in Seattle in the winter than in the summer. 16.5% EMC in December and 12.2% in July. So, if one can assume your boards are currently at the highest moisture content right now (and they would be if you acclimated them in your house before cutting to final size), then you could get by with 1/8" combined just fine. ...

EMC is indeed the issue, but it is difficult to find solid data on it. The link goes to a table of EMC of wood outdoors. Interior wood is likely to be lower EMC, particularly in winter when home heating reduces the humidity of the air inside the building. Bruce Hoadley in his book Understanding Wood has a map which says that the EMC of interior wood in Seattle is 8% in January, and 10% in July. So according to him, Todd, you were right the first time, and you were wrong only in thinking that you were wrong.

[Jan Bianchi]

EMC is indeed the issue, but it is difficult to find solid data on it. The link goes to a table of EMC of wood outdoors. Interior wood is likely to be lower EMC, particularly in winter when home heating reduces the humidity of the air inside the building. Bruce Hoadley in his book Understanding Wood has a map which says that the EMC of interior wood in Seattle is 8% in January, and 10% in July. So according to him, Todd, you were right the first time, and you were wrong only in thinking that you were wrong.

I've got Bruce Hoadley's book, but it's hard to decipher. I've assumed the Shrinkulator just has formulas that Hoadley uses. So why does the Shrinkulator ask for either the range of relative humidity in a geographical area or the moisture content of wood? Is it not a reliable source?

[Jamie Buxton]

I've got Bruce Hoadley's book, but it's hard to decipher. I've assumed the Shrinkulator just has formulas that Hoadley uses. So why does the Shrinkulator ask for either the range of relative humidity in a geographical area or the moisture content of wood? Is it not a reliable source?

I don't know this Shrinkulator you speak of, but any calculation of wood hygroscopic expansion is driven by the humidity in the air around the wood. When the air has more water in it than the wood, the wood absorbs water, and expands. If the air has less water than the wood, the wood gives up water, and shrinks.

A starting point is to look for data on the atmospheric humidity. Weather stations measure it all the time. Unfortunately, that's only the starting point. In most of the US we heat our houses in winter. In some of the US we cool our houses in the summer. Heated air can absorb more water than the same air if it were colder. That is, it is dryer. That's why buildings in New York feel so dry in winter; they actually are drier. And when air-conditioners cool air, the reverse holds -- the humidity of the air increases. That's why air-conditioned homes feel and sometimes smell clammy; they are actually are more humid.
That is, to do a proper calculation of interior humidity you need to know the humidity and temperature outside, and then you make some guesses about what the interior temperature is. From that, you can calculate the humidity of the air inside your home. Not too many folks go to all that trouble.

One place these calculations may have been done is in Hoadley's book. There's two maps at the beginning of chapter 5. They show the EMC of interior wood in the lower 48 in January and in July. Unfortunately, he never really tells the reader that he has done the calculations I outline above. I think he did the work, but I don't have proof. At any rate, you can read the maps to get EMC numbers to feed into that shrinkulator thing.

One more issue.. Hoadley's maps will tell you how the wood is going to move after it has acclimated to your home. Instead, you might want to think about the move from your shop to your home. The shop may be not heated or cooled, so it is more like the outdoors. You might want to pull outdoors data from that link which Todd cited.

[Jan Bianchi]

I don't know this Shrinkulator you speak of, but any calculation of wood hygroscopic expansion is driven by the humidity in the air around the wood. When the air has more water in it than the wood, the wood absorbs water, and expands. If the air has less water than the wood, the wood gives up water, and shrinks.

A starting point is to look for data on the atmospheric humidity. Weather stations measure it all the time. Unfortunately, that's only the starting point. In most of the US we heat our houses in winter. In some of the US we cool our houses in the summer. Heated air can absorb more water than the same air if it were colder. That is, it is dryer. That's why buildings in New York feel so dry in winter; they actually are drier. And when air-conditioners cool air, the reverse holds -- the humidity of the air increases. That's why air-conditioned homes feel and sometimes smell clammy; they are actually are more humid.
That is, to do a proper calculation of interior humidity you need to know the humidity and temperature outside, and then you make some guesses about what the interior temperature is. From that, you can calculate the humidity of the air inside your home. Not too many folks go to all that trouble.

One place these calculations may have been done is in Hoadley's book. There's two maps at the beginning of chapter 5. They show the EMC of interior wood in the lower 48 in January and in July. Unfortunately, he never really tells the reader that he has done the calculations I outline above. I think he did the work, but I don't have proof. At any rate, you can read the maps to get EMC numbers to feed into that shrinkulator thing.

One more issue.. Hoadley's maps will tell you how the wood is going to move after it has acclimated to your home. Instead, you might want to think about the move from your shop to your home. The shop may be not heated or cooled, so it is more like the outdoors. You might want to pull outdoors data from that link which Todd cited.

Thank you. This is helpful. The Shrinkulator is on online calculator of wood shrinkage/expansion at http://woodbin.com/calcs/shrinkulator/htm It cites Hoadley as a source. Using 8% and 10% EMC instead of relative humidity, the movement is only .09" vr 15 " which seems much more managable.

[Frank Drew]

Jan

If you're orienting the floor panels' grain so that the expansion will be front to back, can you "float" the rear frame rails, or even eliminate the rear rails, allowing the panel (without the stub tenon) to expand and contract out the back of the cabinet?