I’ve been studying the R-value of wood. Specifically, how does an inch of wood compare to an inch of fiberglass or foam insulation? I ask because my brother is preparing to sell his “plank house.”

What is a plank house, you ask?

It’s old, for one thing. Gordon Bock, with whom I worked at Old-House Journal Publishing, tells me the plank house’s heyday was mid-to-late 19th Century, although settlers built them from day one, as did their European ancestors until they ran out of trees. Plank houses were quick’n'cheap, which is just what a cold’n'homeless Pilgrim needed.

For another thing, a plank house is simple. For settlers not schooled in the finer points of geometry and physics, a plank house was a reliable way of constructing a box that was unlikely to fall in on you.* (Plank houses are also known as box houses.)

And it’s… woody. Trees were more burden than blessing, once upon a time. Slabbing them out in three-inch planks, and then standing them shoulder-to-shoulder in solid walls, disposed of many irritating trees. The rough planks were covered with plaster inside, and clapboard outside, and voila: a solid house.

Like, solid. Musket-ball and arrow-proof.

When my bother polished up this single-family fortress, he added two layers of foam-board to the exterior, and clapboard over that. Inside, he re-plastered below the chair-rail, and left the planks visible above.

What, then, is the resulting R-value of these walls? I ask because I’m an e-freak — efficiency in all things.

Presumably the foam-board delivers at least the standard minimum for walls, R-13. (I’m not sure which version he used. Polyiso is R-7.2 per inch.) Wood clapboards add another 0.8. But those three inches of solid wood are harder to assess. And the low air-infiltration delivered by two solid materials should be substantial.

First, the solid wood: Hardwood flooring checks in at R-1.2 per inch. But that is young wood, still holding some of its earth-given water. The plank-house walls are perhaps 200 years old, and dry. Modern R-value tables simply don’t go there. After all, modern houses are simply not built with old-growth timber. But supposing an ancient 3-inch plank equals a modern, three-inch, tree-farmed plank, then it might deliver R-3.6.

(Is this getting long? I’m still TOTALLY interested, for whatever that’s worth.)

So: Perhaps the “wall system” of this updated plank house is in the 15 to 17 range.

But R measures only one thing: the amount of time it takes for heat to migrate through one inch of a material.

Heat that goes around that material is not factored in. If outdoor air can migrate through your soffit and the bajillion gaps in your vinyl siding, then through the rips and window gaps in your Tyvek, and past the joints of your fiberboard sheathing, then: It is freeeeeee as the breeze to proceed through your fiberglass (“filterglass,” efficiency freaks call it), then around an electrical box or window molding, and into your living room. Insulation is kind of pointless of you leave a million little ways for air to slip around it. R-values assume an outdoor wind speed of zero.

But the Plank House leaves fewer little ways for wind to enter. Its modern insulation is an air-proof variety; and its ancient insulation is, too. What’s more, its modern insulation eliminates the ookzillion “thermal bridges” that convey heat right past the insulation in modern construction.

(Thermal bridges: In most walls, the insulation is broken every 16 inches by a wooden stud. Hence those walls are R-13 for 15 inches, then R-5 at the stud; R-13 for another 15 inches, and then R-5 at a stud. Doorway and window framing makes thermal bridges you could drive a truck over. Someone slightly more maniacal than I could calculate what percentage of a building’s envelope is made up of these “thermal bridges,” and make us all feel quite dismal about our true R-values. My true hero would calculate the thermal bridging provided by steel nails, and contrast that with the wooden pegs that hold together the Plank House.)

And finally, we must — must — consider the issue of “thermal mass.” Heat, according to Physics, sinks into things; then begins to sink back out of them. The heavier the mass, the longer that cycle takes. A modern wall isn’t very massive. Heat sinks in and out of drywall and fiberglass and floppy pine boards pretty quickly.

But how quickly does heat sink into, and then radiate back from, a three-inch slab of slow-growth, dry, old wood? People who live in log cabins report that this process is slow, that the temperature is remarkably steady in those massive buildings.

Science isn’t so vocal on the subject, presumably because energy efficiency must always contend with time efficiency: Today, trees are scarce, and sawmills are surgical. Today, it’s more efficient to build an airy frame, and stuff it with lightweight materials.

But back when trees were plentiful and sawmills were crude, erecting a box of planks was the speediest way to build a shelter. Back when trees cast shade over your hungry cattle, you did not mind throwing a few more of them on the fire if the cold came creeping through the planks.

*Gordon Bock notes: “Believe it or not, there is also a horizontal-plank house construction method, where they simply stacked board after board on top of each other, flat side down, like logs. It’s a pretty inefficient use of wood (and not that stable), but where wood was cheap, it was done. I saw a garage built like this near Jackson, Maine.”