Question of the Day - #122 Ext basement wood frame mass wall

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Question of the Day - #122 Ext basement wood frame mass wall

Postby RDavidson » Tue Jan 06, 2015 1:24 pm

The new residential energy code defines “above-grade wall” as follows: A wall more than 50 percent above grade and enclosing conditioned space. This includes between-floor spandrels, peripheral edges of floors, roof and basement knee walls, dormer walls, gable end walls, walls enclosing a mansard roof and skylight shafts.

The same code defines “basement wall” as follows: A wall 50 percent or more below grade and enclosing conditioned space.

The same code defines “exterior wall” as follows: Walls including both above-grade walls and basement walls.

The term “mass wall” is not defined in the energy code but is in the IRC as follows: MASS WALL. Masonry or concrete walls having a mass greater than or equal to 30 pounds per square foot (146 kg/m2), solid wood walls having a mass greater than or equal to 20 pounds per square foot (98 kg/m2), and any other walls having a heat capacity greater than or equal to 6 Btu/ft2 • °F [266 J/(m2 • K)].

Energy Code table R402.1.1 requires a “basement wall” to have an R-value of 15. A “wood frame wall” must have an R-value of 20 or 13+5. A “mass wall” must have an R-value of 15/20.

If you are faced with a look-out basement design where grade at the front basement wall is within 12 inches of the top of foundation, the rear grade slopes to an elevation 7 feet less than the front with a sloping side elevation and the side walls are poured concrete from top to bottom with only the rear wall being wood framed, what are the insulation requirements for the side walls? What kind of detail would you expect to find at the back corner of the house where the concrete wall and the wood frame wall meet? Where should the waterproofing be placed on these side walls?
RDavidson
 
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