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Is this insulation plan any good?

Here's what my builder is proposing:
R-21 High Density batts covered with .003 plastic for all 2x6 sidewalls.
R- 49 17” fiberglass blown in house ceiling.
R-15 for garage stud walls.
R-19 7’’ blown in garage ceiling.
R-21 Kraft batts for basement stud walls.
Cell foam around all penetrations and box seal.
Tyvek building wrap exterior walls.

I think the 3 mil plastic on the exterior walls is a vapor barrier required by code.

I really have no idea if this is excellent, crap, or somewhere in between (the builder is telling me that this seals the place up tight, but I'd prefer to not just take him at his word). I've heard cellulose is better than fiberglass, but I'm not sure what the tradeoffs are. Overall having trouble finding some good resources to learn about this stuff.

I'm building in central Illinois. Hot, humid summers (typically between 80-100 F with >80% humidity), and pretty cold winters (typically between 0-20F).

Thanks in advance!

Asked by tim evans
Posted May 9, 2014 3:07 PM ET


2 Answers

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R21 in the walls is IRC 2009 basically code min for the climate zone-5 part if IL- if you're in zone 4 it would be slightly over code min.:


If you have vented siding (vinyl siding, or a1/4-3/4" ventilation gap between the siding & sheathing established by furring or a mesh product), the 3 mil plastic would be more likely to create a problem than solve one. With vented siding it meets code without interior side vapor barriers in all IL climates:


R23 rock wool would be a modest performance uptick on both thermal & moisture transfer than R21 fg, and is sometimes cost-comparable. Certainteed MemBrain variable-permeance vapor retarder would make the assembly more moisture resilient than 3-mil poly, or vented siding/latex paint alone. Damp sprayed cellulose (with or without the MemBrain) would be an uptick too. Rock wool is completely fireproof, and won't melt durign a fire the way fiberglass would, making R15 rock wool preferable in the garage area. Cellulose will eventually catch on fire when it's fire-retardents all vaporize, but if it ever got to that point the house is already gone.

Blown fiberglass attic insulation is second-rate, since it loses performance to convection in the winter, and being semi-translucent to infra-red absorbs some of the radiating heat coming off the hot roof at an interior layer, making the temperature an inch or two into the insulation hotter than the attic air in a typical vented attic. Cellulose is a better option, since it's 10x more air-retardent and doesn't lose performance to convection during the winter, and is opaque to IR radiation.

R15 is fine for an unheated garage, as long as the partition wall AND exterior walls are insulated.

Basements should not be insulated with studwalls & batts only, since that puts the stud edges and fiberglass in contact with the concrete, which can only dry toward the exterior on the above-grade section. Below grade it can saturate with ground moisture and wick into the wood. With kraft facers you limit the ability of that below grade section to dry toward the interior. A better solution is to use 1-2" (R5-R8) of unfaced EPS trapped to the concrete with a 2x5/R15 studwall, with NO interior vapor retarder. In the zone-5 portion of IL 1.5" (about R6)would be better than 1", for limiting wintertime condensation. Seal the seams & edges of the EPS with can-foam before insulating the studwall, and put 1" of EPS under the bottom plate of the studwall and slab unless you already have both insulation AND a vapor barrier under the slab.

The "whole wall" thermal performance of R15 2x4 + 1" EPS is about the same as R21 2x6 construction, and R15 +1.5" would be a modest thermal improvement. But the key aspects of the stackup is that the EPS slows the transfer of ground moisture into the stud wall, and insulates the studs to keep them above the dew point of the entrained air in the cavity, and since the assembly can dry at reasonable rates toward the interior through latex-paint-only, the wood & fiber stay dry and mold-free year round.

Answered by Dana Dorsett
Posted May 9, 2014 4:10 PM ET


Here's the short version:

1. You don't want polyethylene on your walls: Do I Need a Vapor Retarder?

2. The plan for the basement walls is very bad: How to Insulate a Basement Wall.

3. Cellulose is preferable to blown-in fiberglass for attic floors: Blown Insulation for Attics: Fiberglass vs. Cellulose. See also: How to Install Cellulose Insulation.

Answered by Martin Holladay
Posted May 11, 2014 5:21 AM ET

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