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swampwiz_gw

Does the slab wall need to be widened if using extra sheathing?

swampwiz
14 years ago

Here is my situation. I will have a generally long, skinny home that will be optimally placed with the long axis along a latitude, so that the east and west walls are short. There are also long 2' eaves for the south side, so inSOLation there is minimal. I will be using open cell spray foam inSULation, especially as it not so expensive (a marginal cost of only $.16/in-ft^2, with a marginal increase in net R of about 2.8/in.) The walls are only 2x4, and the cost to increase to 2x6 for the sole reason of allowing more thickness of insulation is not worth the reduction in expected energy savings. Likewise, the use of closed cell is not worth it, as it is much more expensive ($.60/in-ft^2), with only a small net increase in R (about 3.8/in.) NOTE: the net R value takes into account the low R value of the studs!

So, the only alternative left is to add insulated sheathing. The big benefit of sheathing insulation is that there is no detrimental effect of having a low R in parallel, as the sheathing adds R in series. Although because of the high cost of insulated sheathing, the net numbers still do not quite work out to use such sheathing all along the house (although very close!), because of the insulation for the west and east sides, and the fact that those rooms may get warmer than the rest of the house, it makes sense to use the sheathing there.

Unfortunately, those east and west rooms are my bedroom and den respectively, so I plan to be in those rooms a lot during the worst times. Thus, I will be putting in an extra 2" of insulated sheathing on the west and east sides.

My question is whether this would require the slab wall to be widened by 2", or whether the sheathing would just stick out the extra 2". I tend to think that it would not be any big deal, but I just wanted to check.

Also, it would seem that this would not have any effect on the installation of siding around the corners, as the base is simply enlarged. There are some 45 degree corners, and the worst case is that the extra sheathing would need to be trimmed to a chamfer, which would seem to be a relatively trivial operation.

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