Insulated From the Real World

As part of a major renovation to our house in Seattle, my wife and I are going through the process of evaluating many major system components, from heating to plumbing to flooring. One part of this evaluation is to look closely at insulation and trying to separate what "contractor default" from the "This Old House" behavior. In contractor default behavior, the contractor specifies <foo> because <foo> is what s/he's always worked with. In "This Old House" behavior, I specify <bar> because I saw <bar> on a $10M remodel on "This Old House" and thought I'd like it in my own house.

Insulation is one of those things. We have a ~1200-foot 1920 Craftsman-style house in Seattle. As is typical of those houses, the framing is 2x4 studs on the exterior walls with ship-lap sheathing under cedar siding on the outside and lath and plaster on the inside. No insulation (who needs insulation in Seattle?). Our plan is to add a full second story to the house -- about another 1300 feet of space. Code requires us to insulate the second story, and engineering requires us to remove nearly all the ship lap and replace it with skip sheathing (basically 4x8 sheets of OSB or plywood) to incease the structural integrity of the house. The first part means we need to choose an insulation material. The second part means that we can install just about any kind of insulation because the house is going to be totally open from the outside.

The contractor default behavior is to use fiberglass batts. The This Old House behavior is to use expanding foam. The medium behavior is to use blown cellulose. We've gone through a bidding process on each of these materials and, as expected, the costs are about foam = ~2x cellulose = ~3x fiberglass. In other words, foam is about 3x more expensive than fiberglass. So the question is, is it really that much better?

So first a quick lesson. We’re probably all familiar with fiberglass batts – the orange or yellow stuff you see crammed into walls and ceilings or stacked to the tops of the aisles in Home Depot. It’s good stuff – pretty inexpensive, doesn’t require much expertise or special tools to install, good insulator, reasonably resistant to fire, water, etc. You may be less familiar with blown cellulose, a gray, odorless, fluffy loose fill insulation usually made from recycled newspaper often treated with a variety of chemicals to help it resist fire and insects. Unlike fiberglass batts, cellulose is blown into cavities using what looks like a big fire hose and is held in place on walls with a thin membrane.

Icynene expanding foam insulation is rather like the stuff that comes in a can at the hardware store, only… well, only different. Instead of being a single compound, it’s actually a mixture of two compounds that use water as a propellant. You spray it into a cavity, wait until it expands until about 100x its size, then you shave it off with a really big hacksaw.

In evaluating each of these, there are several things to consider. One gross factor to consider is the R value of insulation per inch. It's not a perfect measure of insulation performance by any means, but it will give a rough guideline on what you get and in a house with 2x4 construction downstairs, getting in a bit more insulation in the constrained space can make a difference. The R values vary depending on a host of factors, but it came out roughly that the open cell foam we were looking at (Icynene) boasts about an R value of 3.6/inch. Cellulose is about 3.4/inch. Fiberglass is about 3/inch. Interesting side note: the data on the Internet vary hugely (up to 20%) in some of these numbers, and a lot has to do with the specific type of material and how it's installed. But in a nutshell, fiberglass isn’t very good and cellulose and Icynene are very good.

Other factors include how well they plug air leaks. Expanding foam is basically going to fill every nook and cranny. Cellulose does an OK job. Again, fiberglass isn’t very good. There’s also how well the insulation does when it gets wet. Again fiberglass doesn’t fare particularly well.

When everything nets out, the cost of the expanding foam is just too high based on what we get. If I had another $10,000, I’m not sure even then if I’d spend it on the foam. Instead, it’s probably going to be blown cellulose.

Comments (10)
  1. Xepol says:

    Pink fiberglass on the inside, highdensity pink foam sheeting on the outside with siding of yer choice over that.

    Cellulose is just tinder, unless you add the toxic fire retardants to it.  Oh, and it SETTLES over time, leaving the tops of the walls uninsulated.

    Expanding foam offgases for years, you really don’t want it inside your house.

    Or maybe you do, who I am to judge other people’s inhaling preferences? <grin>

  2. johnmont says:

    Pink fiberglass can grow mold too easily if it gets wet.

    The "old" expanding foam (closed cell, high density) outgasses for years. The water-based stuff doesn’t — about 24-48 hours max.

    The other alternative I’ve examined is blown fiberglass.

  3. M3 Sweatt says:

    We had a local contractor spray water-based expanding foam in our bath remodel; we found in our research that is relatively inert. It was more expensive (just under 2x) than fibreglass but the acoustic and insualtion qualities sold us on the premium.

    If we were doing a larger area (such as all the exterior walls of our home) we found another product we would use: ThermoForm Wall-Spray. It has better binding properties to open wall cavities and is relatively non-settling.

    Good luck!

  4. johnmont says:

    We met with our contractor (actually framing carpenter — we don’t have a general contractor) this morning and walked him through a few things including the choice of blown cellulose. He looked at me oddly and pointed out that we live in a pretty temperate climate and that fiberglass is probably enough, but shrugged it off. In his world, all insulation is about the same. But I’m still a big fan of foam — it just seems to do what insulation should do.

  5. Xepol says:

    I helped my father and grandfather blow a few extra feed of fiberglass into our attic.  Nasty job that.

    Again, take perfectly good fiberglass bundles and tear them apart and blow them… It lends itself to settling and bunching again over time.  After all, you’ve already compromised the physical integrity of the fiberglass bundle to blow it up there.

    Oh, the endless static electicy shocks that day, and the WEEKS of itching (it doesn’t matter how well you dress, those damn glass fibers get EVERYWHERE!)

  6. Xepol says:

    Oh, and fiberglass does NOT mold, pink or otherwise.  Any organic matter that gets trapped in it can mold tho.

    Cellulose, being organic, would mold beautifully however unless you added even MORE toxic chemicals.

    Personnaly, I would have to research any expanding foam that claimed it didn’t offgas for more than 2 days before I would lend any credibility to the statement.  Ah well, off to google!

  7. johnmont says:

    I’ve definitely see mold in fiberglass batts before and had to remove it from several houses when I worked construction in high school and college. If you search for fiberglass insulation mold on MSN (or Google) you’ll find that it’s not uncommon. By contrast, search for cellulose insulation mold ( and you’ll find that it’s unusual. The reason is that most cellulose insulation is treated with borax, an inert cleaning agent ( So cellulose isn’t loaded with dangerous chemicals, it has borax in it.

  8. johnmont says:

    As for settling of blown cellulose, it’s definitely true that it will settle — anywhere from 5-20%, depending on the installer. That’s why it’s pushed into cavities under pressure that reduces settling as much as feasible. It’s also why many insulators add water to the cellulose to get it to stick (which leads to a host of other problems). In theory, you can pack cellulose to roughly the density of wood, though you’d get an R value about equal to that of wood as well. Settling is probably the single most worrisome aspect of cellulose to me.

  9. mheller says:

    Here in northern Massachusetts, where we can face -10 F with 50 MPH winds, insulation really matters. We did blown cellulose in the walls and attic of our last house, which dated from the Civil War, and also had to add fiberglass batts in parts of the attic. The biggest reason to blow cellulose was the labor cost versus ripping out walls to install fiberglass.

    In this house, built in 1910, we went with fiberglass batts: the cellulose just wasn’t cost-effective by comparison, we were ripping out walls anyway, and we needed the money for tuitions, remodeling a kitchen, and remodeling a bathroom.

    I can’t imagine why you and Amy would fuss over insulation in Seattle. Some simple measures to reduce mold formation — moisture barriers and the like — should mediate the risk of using fiberglass, and you’re already facing bare wood.

  10. johnmont says:

    Budget review this evening. With the cellulose premium over fiberglass, it’s probably not happening.

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