Puzzling over my blow in insulation options
Hello!
I have a wonderful 1897 house in Portland, Oregon (zone 8b – not so cold, not so hot, so very rainy!).
[Editor’s note: Portland, Oregon is in Climate Zone 4C.]
I don’t have to worry so much about the temperature extremes but it rains here, famously, all the time. I would like to insulate my exterior walls and need to do blow in because I’m not opening up the walls. I am also going to insulate the attic which is currently open (looking to do along the roof line probably with spray in foam).
From what I’ve read, there seems to be quite a bit of discussion about rock wool vs. cellulose. I am certainly no expert so I’ve been doing my best to read up about the past several months. I am almost positive I have no vapor barrier so cellulose just seems like a bad idea. My house hasn’t shown any leaking issues, but banking on something not getting wet seems like a losing battle here. Additionally, the idea of cotton/paper in walls just sounds like a recipe for mice or rats (ick). My house has stood for so long, I can’t imagine cotton or paper will be pest resistant for another 100+ years. (ref: https://www.treehugger.com/sustainable-product-design/what-greenest-insulation-case-mineral-wool.html)
Rock wool also sounds like it has some great noise insulation which is very appealing as I live in a noisy area. Also from what I read, it provides heat insulation almost as good as cellulose. I plan on staying in my house indefinitely so I’d like to use the best materials. (pro mineral wool article: http://blog.lamidesign.com/2012/01/what-you-don-know-about-mineral-wool.html)
Looking around for folks who do blow in rock wool has been impossible however! I swear I’ve called upwards of 15 companies and no one does it. I finally found a great insulation guy who would even talk to me about rock wool or foam in my open attic, and now he’s fallen off (assuming he’s caught the wave of building happening here).
My questions are:
1) Do my concerns about cellulose make sense to folks here? It would be so much easier to use it, I just feel really antsy about how it handles water as well as long term potential for pests.
2) From what I’ve read, rock wool is one of 3 blow in options – why can’t I find anyone in the Portland area?
Thank you!
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Replies
Mr. Miller,
You're right: few residential insulation contractors offer blown-in mineral wool.
By far the best type of insulation for homes like yours is blown-in cellulose, installed according to the dense-pack method. This approach has been used by weatherization contractors in New England for about 40 years.
You don't need an interior vapor barrier to install cellulose insulation in your walls. I don't know who told you that -- but they are wrong.
An interior vapor retarder can't hurt, but it really isn't required. Ordinary vapor-retarder paint can be used for this purpose if you are worried about vapor diffusion. (You shouldn't be.)
If for some reason you are having trouble finding a contractor to install dense-packed cellulose insulation in your empty stud cavities, blown-in fiberglass would be the second-best option.
For more information, see How to Install Cellulose Insulation.
Mr. Miller,
Q. "Banking on something not getting wet seems like a losing battle here. Additionally, the idea of cotton/paper in walls just sounds like a recipe for mice or rats (ick)."
A. If your walls are getting wet, that would be due to a problem with your exterior flashing or siding details, not your choice of insulation. No matter what type of insulation you install, you need to pay attention to details that exclude rain.
Mice and rats aren't known to nest in cellulose. (They do nest in fiberglass.) The main reason that rodents (mostly) avoid cellulose appears to be the borates that are routinely added to cellulose for fire resistance.
The upsides to rock wool is a high R/inch, and high resistance to fire. But it isn't a rodent-repellent by any means!
The upside to cellulose is high air redardency (not an air barrier, but damned close when installed at 3.5lbs density in walls!) high recycled content (85% by weight), and a large capacity for buffering/managing moisture content, "sharing" the wintertime moisture burden with the structural wood, and a modest but measurable amount of thermal mass.
The slightly lower R/inch of cellulose is compensated for somewhat by the thermal mass and air-tightening aspects.
If this is strictly for the attic, not the walls, go for the higher air-retardency and thermal mass of cellulose, which is sufficiently air retardent that it performs at it's rated R even without a top-side air barrier. Be sure to air seal at the attic floor/top-floor ceiling before insulating. Be sure to specify "sulfate free, borate only" for the fire retardents. Should sulfated fire retardents become wet (say, from a undetected roof leak) they stink, and they corrode metals, whereas borates are fairly inert, and won't cause further damage (beyond the bulk water incursion itself.)
If looking to maximize R-value in walls there's an argument for high-density (1.8lbs per cubic foot or higher) blown fiberglass. At those densities fiberglass is as air retardent as 3.5lb cellulose, but whether the higher R is still thermally bridged by the framing, it's not nearly as big a gain as the center-cavity R value implies, and the higher thermal mass of cellulose will bring the fuel use numbers to near parity in a PNW climate.
I've personally never found a rodent nest in cellulose. The closest I've seen is when a curious squirrel chewed in to a bag of cellulose stored in a garage removing a baseball sized amount of material and making a mess, but he didn't nest in it. I've seen mouse-poop on the top of attic cellulose, but no burrows, but I have found mouse burrows in the 1950s vintage blown rock wool in the attic of my house. Rodents seem to find low density fiberglass very attractive as a nesting material/nesting place, and have rarely seen a fiberglass batt insulated house that didn't have at least some mouse-nests in the insulation a decade or three after installation.
Don't buy into Greg Verdara's (mostly unsubstantiated) opinions on what happens to cellulose over time & moisture cycling. Cellulose is good stuff when properly installed- I've personally seen 30+ year old cellulose in walls in a full-gut rehab that looked as if it had been installed the prior week. But I've also seen cellulose that was installed at such a low density that it sagged several feet (in a 2 story balloon framed house) in 15 years. The so-called "fluffing" scams of installing insulation are now mostly in the past (seemed like a "thing" in the late '80s, to mid '90s, and more common with fiberglass than with other insulation) but no matter what the material, choose the installer carefully.
Thank you so much Martin and Dana! I appreciate the professional advice. I'm still just puzzled by why blow in rock wool is so hard to find but maybe it's just simply out of favor because cellulose is less expensive and sounds like it offers much of the same benefits.
Seems like these things run in phases, and vary in both time and location. In the 1940s & 1950s blown or batt rock wool were pretty common in the US, and blown cellulose was still pretty rare, while fiberglass was starting to take over the market, clearly dominant by the 1960s. In thel late 1970s or early 1980s cellulose started to come on strong for retrofits and attic insulation, but still well behind fiberglass. At that time blown rock wool was still available- I know of at least one house in my area that had rock wool blown into the walls in 1983, but far more in that era retrofitted with blown cellulose or blown fiberglass, and none recently.
Fast forward a few decades, now that higher-density higher-performance rock wool batts are available rock wool is developing a following for new construction (Greg has apparently drunk the cool-aid!), and it has become the go-to standard for insulating framed walls in Europe. But it still has but a tiny US market share compared to fiberglass despite being a superior product by several measures. Blown rock wool seems to have been relegated to "specialty item" status- still necessary for insulating between flue liners in masonry chimneys for high temperature appliances such as wood stoves, but now only rarely used as attic or wall insulation.