I am looking to add more insulation in the garage ceiling, so far from all the contractors I've talked to there are two options: one is to remove some sheetrocks from the ceiling and use spray foam, the second is injection foam. From convenience point of view, injection foam is better because I don't have to reinstall sheetrocks and repaint them. I think they both do a decent job at air seal (especially on the rim joist), and sound blocking. But what are the drawbacks for injection foam in the ceiling?

foam is tricky since it sets up instantly. I know nothing about injection foam except that you won't have any idea of the coverage until it gets cold or until you use an IR camera on it.

Injection foam does not expand or adhere (much), and will not air seal NEARLY as well as either open or closed-cell polyurethane. It also has a checkered history- if installed at the wrong mix or temperature it shrinks & cracks as it cures, and in the worst-case scenario it outgases formaldehyde. (They are all variations on the same urea-formaldehyde insulation that became the subject of lawsuits back in the 1970s.) Also, incomplete fills are common, especially when you need a vent space on the top side, as in a vented roof.

ANY high R/inch foam (most injection foam runs ~R5/inch) is wasted when installed between R1.2/inch joists/studs/rafters. Even at a fairly low 20% framing fraction means most of the conducted heat transfer is through the framing.

If this an unvented ceiling and the foam is going to be in contact with the roof deck, see my other thread. Injection foam is HIGHLY permeable, more permeable than open cell polyurethane at any thickness, and the lack of air-sealing at the edges makes it even more unsuitable.

If it's a vented roof with mini-attic you need the right balance permeance to keep it vapor open enough to dry seasonally, but not so vapor open that the cold roof deck & rafters take on mold/rot levels of moisture over a winter.

Is dense packing cellulose into the ceiling a good idea in terms of insulation and air sealing? I am worried about the weight since it's heavier than foam.

you cannot dense pack cellulose into a ceiling. You need a defined, enclosed space, such as a wall cavity (10" x 16" x 9' for instance) to create the density. An attic (26' x 40' x infinitely high) is vastly too big. Some installers have difficulty with 12" deep wall cavities.

I mean the ceiling of a garage, its made of 2 by 8 studs, 16'' apart.

OK; you might be able to dense pack that.

What is going to be the top-side barrier to dense-pack against, how is the mini-attic vented, and how are you going to ensure that there are no leaks where the cellulose piles up and blocks the venting??

Or is there a conditioned/heated room over that garage?

I have bedrooms above the garage.

Dense pack it then. 3.5lbs per cubic foot cellulose is about as tight as closed cell foam, and tighter than injection foam, as long as it has plywood subfloor on one side, and sheet rock on the other.

High R/inch high-cost foams are a total waste when applied to between joists that have ~5x the thermal conductivity of the foam. In a 7.25" 2x8 cavity, with 3/4" ply subfloor, 3/4" hardwood, and half-inch gypsum, with a ~20% framing fraction the whole-wall R of the assembly runs about

R20 with 3.5lb cellulose

R23 with injection foam

R25 with 2.5lb polyurethane.

If you really need/want the R25 performance, insulating the joist bays with cellulose and gluing an inch of fire-rated polyiso (Tuff-R) onto the ceiling gypsum gets you there.

Take a look at the fastener spacings on the ceiling gypsum- you may want to add some to get to 8" o.c. spacing (ring-shank nails are fine) to keep the pressure of dense-packing from pulling it back from the joists on the existing fasteners, especially if they are smooth-shank nails.

I was quoted $1900 for cellulose and $2000 for open cell foam. Both are expensive but I think the higher R value and better air seal should justify the $100 difference, right?

not necessarily. foam dries hard and tends to crack over time, especially if the wood moves or dries and shrinks. Cellulose will absorb those changes.

If I go with cellulose, will it find its way into the bedrooms above through small gaps in the plywood subfloor.

Probably as it is being installed, but not once you have the bottom side well air sealed. Consider a layer of taped Thermax on the bottom and also use tape to seal the top. Some have suggested stabilized cellulose to eliminate dust issues although I don't know if that can be dense packed.

Bedrooms above all have hardwood flooring, so there could potentially be more dust in the room after the cellulose installation.

I was doing a research for tripolymer injection foam and saw it contained formaldehyde, and a company named Retrofoam is under a class-action lawsuit. So that means a no no for injection foam then.

Almost all hardwood flooring installed over plank subfloors has rosin or kraft paper as an air barrier & slip surface between the subfloor & finish flooring. Plywood subflooring is already nearly air-tight. The only real leak points where you'd get any measurable dusting would be at electrical & plumbing penetrations, which can and should be caulked or can-foamed prior to installing the cellulose.

When I personally dense-packed the walls my horse-hair plaster on wood lath antique, the only place cellulose dust ever appeared inside the rooms were bays with un-sealed electrical outlets, and even then the quantities were miniscule. As I went along I learned that 2" clear packing tape over the outlets was sufficient to prevent even that minor issue. The dust that makes it through is primarily the fire retardents, during the initial stages of the installation, before achieving dense-pack density. The cellulose itself fills & clogs those small air escape paths pretty quickly. Toxicity of the fire retardents used in cellulose is extremely low. (Borate-only "stabilized formula" is the right stuff to use here, whcih has only borate fire retardents, no sulfates.)

All current injection foams are just minor tweaks on the urea-formaldehyde foams of the 1970s lawsuit infamy. The problems are fewer and there are applications where it's the best option (insulating masonry cavities is one), but it's near the bottom of the list of what makes sense here.

Going with half-pound pour slow-rise open cell carries a much higher risk, of both blowouts & bowing of the ceiling gypsum, or an incomplete fill, leading to a net loss of performance. Both are a real PITA to fix. Cellulose is much more straightforward, and the difference in "whole-assembly" thermal performance is less than R1, a 3% difference in steady-state R value, but the cellulose also has a more stable performance over temperature (not that you'd be seeing huge temperature swings in the garage if you keep the garage door closed.)

For cantilevered floor, do I need to install horizontal rigid foam board to seal off each cavity between floor joists before dense packing cellulose? The current cantilevered floor has no seal on the bottom so I have the option to do that, of course I will definitely seal the bottom with plywood before dense packing. And also should a lack of vapor barrier be a concern?

The lack of a vapor barrier on the cantilevered area isn't an issue in climate zones 5 or less, since there is no rain-wetting of the underside of the floor to worry about, and the drying capacity toward the exterior through plywood is high even in winter.

In zone 6 it would be better to use 5/8" exterior grade gypsum sheathing (eg Georgia Pacific DensGlas Gold) rather than plywood for sealing up the bottom. The gypsum is moisture tolerant and highly vapor permeable- it's fine if/when it takes on a bit of moisture, and dries extremely quickly (10x as fast as plywood.) If the joists extending to the cantilever are the same joists that are being insulated in the garage, there isn't any need to add blocking (foam otherwise).

In zone 7 or higher an interior side vapor retarder such as a flash-inch of closed cell on the underside of the sub-floor would be in order, as would the foam-board blocking between the joists.

Posted By seiyafan on 13 Nov 2013 11:19 PM
I was quoted $1900 for cellulose and $2000 for open cell foam. Both are expensive but I think the higher R value and better air seal should justify the $100 difference, right?

What are the ceilings measurements, length & width? Is it a simple rectangle?