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Cellulose vs. fiberglass batt cavity insulation with rigid foam exterior in zone 5 central NJ

I'm looking at cavity insulation options for a new 2200SF house that will have 1-1/2" polyiso on exterior walls and 3" on the roof. Cavity insulation prices are coming in at roughly $4500 for fiberglass batt (R21 walls/R38 roof) and about 3 times that for cellulose. While the benefits of cellulose have been well covered on this blog, I'm having trouble justifying the added cost for cellulose and wonder to what extent the short comings of fiberglass are mitigated by the addition of the foam on the exterior. In short, does fiberglass batt cavity insulation perform better with rigid exterior insulation?


Asked by Scott Schaub
Posted Dec 12, 2012 4:11 PM ET


4 Answers

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Q. "Does fiberglass batt cavity insulation perform better with rigid exterior insulation?"

A. Of course. In your proposed wall assembly, the continuous layer of 1.5-inch-thick polyiso is doing most of the heavy lifting. The fiberglass batts (interrupted by studs and inevitable voids) won't be very effective.

If you are going to save $9,000 by choosing the fiberglass batt job, you might want to apply some of the savings to making the polyiso thicker.

Answered by Martin Holladay
Posted Dec 13, 2012 2:49 PM ET


I'm a bit shocked that cellulose would come in that high, even dense-packed! Wet-sprayed might be a 50% uptick over batt in my area, but never a 3x kind of deal.

Whatever you put in the cavities, a bead of caulk between the framing and sheathing in every cavity, and at the bottom-plate/subfloor interface, and between doubled-up top plates goes a long way for air tightness, and is one of the cheapest performance upgrades you'll ever buy.

Dropping the framing package down to 2x4 and bumping the foam up to 2" gives you the same whole-wall performance as the 2x6 +1.5" wall, but without the extra wall thickness, and guarantees the sheathing stays above the dew point of the interior air even at typical 99% outside design temps for central NJ, which makes it extremely resilient against mold/rot even if the gypsum leaks air (but seal the seams and any electrical/plumbing penetrations to the gypsum anyway, eh?)

Answered by Dana Dorsett
Posted Dec 13, 2012 3:30 PM ET


Dana, thanks for the feedback. I definitely have the sense that cellulose just isn't that prevalent in Central NJ which may account for the higher costs. Your suggestion of dropping to 2x4 and increasing the rigid to 2" sounds good as does the caulking.

Answered by Scott Schaub
Posted Dec 14, 2012 11:11 AM ET


If not cellulose, you might look into blown fiberglass rather than batts. It'll be more expensive, but Spider / Optima / Propink L77 at 1.8lbs or higher density are real improvements over batts- slightly better on total R, but a LOT better on minimizing performance robbing faults.

Blown insulation reliably fills all voids and conforms around plumbing, electrical, knotholes, etc. for near-perfect fit, no compressions or voids. While there are some batt installation virtuosi out there, "typical" installations still leave a lot to be desired. At 1.8lbs or higher the air-retardency of these products are comparable to dense-packed cellulose, but at 1.0lbs. (shich is a lower-cost but still rated standard installed density for some of them) they aren't very air retardent at all, and are subject to settling within the wall cavity over time.

Answered by Dana Dorsett
Posted Dec 14, 2012 12:25 PM ET

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