Wood-walled basement?
From what I’ve read it appears one can build a basement’s walls out of wood. The wood is heavily pressure treated and specifically graded for use in foundations. Typical walls use 2×8 s on 16″ or 12″ centers and either 1/2″ or 3/4″ treated plywood. If the stud cavities are filled with air permeable insulation an interior air barrier must be used. The bottom of the walls is restrained by the basement floor which may also be made of PT wood or ,of course concrete and the top of the walls are braced by the first floor framing and sub floor.
I priced material cost for a 9 foot wall using 12″ OC & 3/4 plywood with cavities filled with Roxul and an inside sheathing of Thermoply. It comes to about $3.50 /sq ft of wall. This is what the concrete for an 8″ wall would be if concrete was $141/yd. Which it ain’t up to yet but any way to get the same r value (r21) with concrete will add enough to make the material for the concrete system more costly. To top it off the wood version is made of mostly renewables and has lower embodied energy ie it’s greener.
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Replies
Jerry,
You didn't ask a question, so maybe you've already got everything figured out.
Here's some feedback, however: be wary of the advice to use air-permeable insulation on a below-grade wall. The PT plywood used for the foundation will still be cold, and there is no interior air barrier yet invented that will prevent warm interior air from reaching the cold plywood.
Martin,
你肯定是正确的我没问一个问题。Sorry about that. Before making the post I searched here for wood foundation and wood basement and was surprised to find nothing. I guess I'm worried that what seems so good might have unintended consequences. Are you aware of any? Your point about condensation possibility I recognize but. I can't see a difference between above grade walls and below grade. In either case if permeable insulation is used an air barrier is vital to prevent damaging condensation within the wall. Shouldn't air barrier construction, details and materials that work above grade work just as well below grade? If I do use a wooden outer basement wall I'm fairly sure I'll still use an inner stud wall as well to support the floor framing. It seems to be the only way to reduce the thermal bridging in the "band joist" area. But with a wood exterior basement wall it seems quite practical and economically reasonable to have r40 basement walls as well by using mineral wool. A well detailed air barrier is certainly worth doing. No doubt some, hopefully very small, amount of condensation will occur whenever the outside temperature is below the indoor dew point at least in the below grade case it's on pressure treated lumber that is really toxic to mold etc. All an air/vapor barrier can do is dramatically slow the condensation process,because you are correct there is no ideal air/vapor barrier.
Jerry,
GBA discusses wood foundations inthe "Basements" article of the GBA Encyclopedia: "Foundations made from treated lumber can be less expensive than building with concrete or concrete block. Despite their novelty among many builders, these foundations have a long history of reliability. Regular carpentry crews can erect them, and wood foundation walls can be set on a gravel base rather than a concrete footing. All of that helps make construction simpler, and sometimes faster, than a concrete foundation. Also, framed walls are easier to insulate and wire than concrete. Structural insulated panels with a treated-wood exterior are a similar option."
Lots of information on these foundations is available on the Web:
From the NAHB research center:Wood Foundations.
From theForest Products Journal:Comparing in-place costs of wood and masonry foundations.
Wood foundations are code-accepted; seeR402.1.
You wrote, "I can't see a difference between above grade walls and below grade." Here's the big difference: above-grade walls can dry to the exterior. That's what happens every spring, when damp wall sheathing dries out after absorbing moisture all winter.
Martin,
假设我要使用由PT 2x8制成的木基墙系统。＆外部PT胶合板和我在这堵墙的内部隔离，并带有箔纸面对ISO。我有选择，我是否塞满了矿物质羊毛蝙蝠？添加蝙蝠的价格约为1.15美元/平方英尺，并将在组件中添加R21。要获得相同的R值，请使用ISO的价格约为2美元/平方英尺。与大多数地下室一样，某些墙壁的墙壁都会超过级别，但它可能可能会超过等级仅需墙高度的1/8。换句话说，任何绝缘材料同样需要空气密封！使空气可渗透材料的一部分隔热材料在水分问题中没有任何变化！我的结论是，我不妨利用矿物质羊毛的经济性，但是一旦添加了隔热材料，就需要使用空气屏障细节？此外，仅在螺柱腔中只能使用可渗透的空气绝缘材料，否则护套的下部可能永远不会变干。
If you don't let the air-permeable insulation dry toward the interior there's a good chance that it will have significant wetting and (temporary) loss of R-value during the colder seasons. Putting an air-barrier that is at least somewhat vapor retardent between the foundation & studwall is advisable, but there is still risk of condensation on the air barrier if there are any air leaks or high vapor permeance of the interior-side air barrier.
The better solution is to put a layer of air-impermeable semi-vapor-permeable insulation between the foundation and batt-insulated studwall that has sufficient R-value that the studwall-side of the insulating air barrier has limited condensing potential. A relatively cheap way to hit R21 whole-wall this way (that works for ALL of the lower 48 of the US) is to use 2.5" of EPS (seams sealed) between the foundation and a 2x4 studwall with unfaced R13 rock wool, with the only interior-side vapor retarder being latex paint on the gypsum. That way all of the wood is nearly half-way inside the thermal boundary, the only region with even a remote condensation risk is at center cavity on the above-grade portion, and the condensing surface is EPS, which isn't affected by the moisture. The foundation can still dry toward the interior through 2.5" of EPS (which runs about 1.5-2.5 perms at that thickness) so won't end up saturated with ground moisture. Even cheap low-density Type-I roofing EPS is just fine as the air-impermeable insulation in this stackup.
EPS increases in R-value as temperatures fall, and assuming the wintertime basement air has a dew-point under 45F (usually is unless the house is extremely air tight and under-ventilated.) When it's cold enough outside that the warm side of the EPS is 45F, a 2.5" layer of Type-I EPS will be approaching R11.5. A simple studwall with a framing fraction of about 15% with R13 batts come in at R10+, using R1 for the combined-R of the gypsum + foundation (on the other side of the EPS.), total stackup is ~R21 when it counts the most, a bit less in the summertime.
And that stackup is 2" thinner than the 2x8 studwall.
There are products out there that produce such a system and will give you a warenty to such . I have worked with this product before in cold wet climates and it was fantastic.http://www.enercept.com/enercept-basement-panels/