A basement for my double wall dream house, second try
I’ve now pretty well settled that a poured concrete basement with exterior insulation and aligned under the inner main floor wall which carry s the roof load is the simplest and most ” buildable” if the floor framing is simply extended (cantilevered) beyond the basement . With the floor truss extension, if I don’t use a classical rim board but use blocking located under the upper wall’s inner face and insulate between the joists with the same ‘stuff’ (r45 cavity, r1.3 sheathing and r0.63 plywood) I’ll use in the double stud walls the the r of the joist area alone is r40.4, almost no worse than the wall above. Good enough! Now onto the real problem. How to get ” outsulation on those concrete basement walls at reasonable cost. The TF system ‘Transform system looks interesting, it can be had with 5 1/2″ foam on the outside and 3/4″ inside an r value of 25+ and it has plastic studs 16″ OC on the inside and plastic flanges covered by 1 1/2″ foam on the outside for mounting cladding. Well it was interesting till I got a quote, they are really proud of that stuff!! About 4x my budget! So here is my alternative that fits the budget for materials.
Basically surround a classic poured wall basement with a non structural (except for soil pressure) wood basement. But do it with 2 layers of 2×4, wood basement pressure treated, framing. The inner is vertical 16″ OC and attached at the bottom to 2×4 bottom plate sitting on the same footing as the poured walls and attached to the concrete about 2′ below the cantilever floor above with angles and tapcons. These studs upper end is trimmed to land 14 1/2″ below the floor truss’s bottom plane. The cavities, including the horizontal one above the studs, are filled with r15 bats Then a horizontal ” strapping layer is created with horizontal 2×4 s, the upper most screwed to the bottom of the floor truss s, the rest to the vertical studs, 16″ OC for the top 3 spaces then switching to 12″ OC for he bottom 4′. After filling the spaces with more r15 1/2″ wood basement plywood is attached to the horizontal framing. After seam sealing and waterproofing as if it was a wood basement it’s back filled following the wood basement rules.
An r28+ ,7 1/2″ thick basement insulator for under $3.50/wall sq ft. excluding labor (I work cheap). Your thoughts?
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Replies
Jerry,
It sounds like a lot of work and expense to me. Easier approaches include exterior XPS or exterior Roxul (without any pressure-treated studs, needless to say).
You wrote, "The cavities ... are filled with R-15 batts." I don't know what you had in mind, but you certainly can't use fiberglass. If you are using Roxul, why not skip the studs and plywood?
Don't forget that if you are building a pressure-treated wood foundation, you need to install peel-and-stick on the outside of the plywood. That's expensive.
Martin,
First off, the only insulation material I intended in the "matrix" is Roxul comfort bat. The reason for the less than simple approach is to give a good substrate to mount thin brick to on the above grade portions.. It's simple not all Roxul is created equal or costs the same. Roxul comfort bats are very economical @ about $0.04/r sq ft But they need to be protected from soil pressure. OTH Roxul Drain board can be used directly underground, it's sufficiently strong but it is only available in 1" thickness @r4.3 to I'd need 6" and it would be a little less r. The drain board needs to be mechanically fastened and I still don't have a way to attach my thin brick on the above grade. But the real issue with Roxul drain board is it costs $0.20/r sq ft even more than poliso!
As to the need for peel and stick, Why do you say that? The ANSI/AF&PA PWF 2007 "Wood foundation design specs."and Southern Forest Products Assn "Permanent wood foundation design and construction guide" both make no mention of 'peel and stick' but specify a waterproofing layer of 6 mil poly.
Jerry,
I wasn't aware that 6 mil poly could protect buried plywood. That may be approved by the spec, but let's just say I'm skeptical that it would do anything at all.
Martin,
The "specs" like most codes are the minimum allowed. I have not decided how to waterproof but 6 mill poly is the minimum acceptable level of waterproofing. There is a huge variety of basement waterproofing products at widely varying costs with various "features" and side effects. It's a 'daunting jungle' indeed! My unconventional thought is 45 mill EDPM 'pond liner'. which can be bought for about $0.80/sq ft.
Jerry,
Why not insulate the basement exterior with extruded polystyrene with no related wood structure?
Ron,
How do I hang thin brick over XPS? I will be covering the above grade portions with thin brick attached to "Loxon Panels" that require a fastener per sq ft. The resulting cladding system weighs about 7 #/ sq ft.. With the PT plywood I can continue the brick below grade for a "normal" brick look, but with foam I need a 4" or more 'termite inspection strip' of exposed foam. Then there is the raw material cost. While XPS is often reffered to as r 5/", the aged value is less,some sources show as low as r4.2/" At about r 4.3/"he material cost of the XPS is higher without an answer to how to attach the brick. I actually would like to find a a less labor intense alternative.
I covered my XPS above grade with 3/4" foundation grade treated plywood attached to the concrete through the XPS with tapcon screws. Then I stainded the exposed plywood. The plywood extends about 12-18" below grade.
Ron,
Hopefully you are in a termite free area!
Ron,
IF I forget the plywood and just use tapcons to mount the brick panels that is $1.75/sq ft in stainless steel tapcons for 4" foam OUCH!