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Odor from Roxul Comfortbatt?

I'm replacing the insulation in my partial height basement stud walls (concrete to 42", 2x6 to 92.5). I hate fiberglass and the old batts were poorly installed. More details if you care but suffice it to say this is the time to replace the wall insulation.

Fast forward to finding Roxul ComfortBatt finally easily available on their website for special order (pro desk at Home Depot can get it as well). Ordered a decent quantity of it.

Received my first part of the shipment, opened a bag, and installed 4 of the 5 batts. Great. Not scratchy, doesn't immediately cast off fibers, cuts easily, does conform to the bays a little better (it isn't magic however), fast to install. Labor savings alone if batts need cutting would pay for the difference between the mw batt and the fb batt. It does seem like it doesn't flatten against the mid bay blocking as well and it seems like it doesn't want to be handled a lot. It also needs to be picked up in a non-plank fashion or it might 'crack' or tear.

Install 5th roxul batt. Whoa. I can smell a distinct catty odor. Ask my helper to smell it. Helper describes it as 'doggy'. To me, it seems to be coming from the batt. I could only detect it within a few feet, such as by holding the batt or sniffing it directly. The 5th batt was from the bottom of the pile sitting on the basement concrete slab (which was damp due to other construction activities). Perhaps that's why it kicked off an odor and the others didn't?

Now I'm concerned. I got this stuff because it was GreenGuard rated and the formaldehyde binder is supposed to be cooked off during production. I'm not sure what I smelled but I doubt I want to breathe it. I see elsewhere on this site that people suggest a catty odor being from ammonium sulfate potentially.

After awhile, we returned to the offending batt and sniffed it again. No remaining odor.

I'm thinking I'll return to the batt tomorrow and mist it to see if it kicks off new odor.

For the community, any sense of:
- What the odor is?
- Whether it is safe?
- Whether this odor is typical of Roxul ComfortBatt
- Whether Knauf experiences this problems (and if it doesn't, where I could get it).
- Will the odor/compound blow off in a reasonable (a few days) period of time? Would misting or ventilation help speed up that process?

Asked by Keith H
Posted Mon, 12/16/2013 - 05:20


12 Answers

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I should add that time has proven that I have a fairly receptive nose, at least on the human spectrum.

Answered by Keith H
Posted Mon, 12/16/2013 - 05:22

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I suspect that the odor will gradually dissipate, but you might want to call Roxul to get a technical opinion from the manufacturer. Here are the phone numbers: 905-878-8474 and 800-265-6878.

A more important issue is the fact that you have chosen to install an air-permeable insulation on the interior side of your basement walls. This type of insulation is not recommended, because it allows humid interior air to contact the cold concrete wall, leading to condensation and mold. You should choose to install rigid foam insulation or closed-cell spray polyurethane foam insulation instead.

For more information, see How to Insulate a Basement Wall.

Answered by Martin Holladay, GBA Advisor
Posted Mon, 12/16/2013 - 09:57

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Any guess at what is being off-gassed? In choosing comfortbatt, I thought I was avoiding the off-gassing associated with foam and the hazardous fibers associated with fiberglass.

I didn't explain my system very well. I have a 'daylight' basement': concrete foundation wall to 42" without studs, fir, or insulation. Above that I have a conventional above grade 2x6 stud wall that was insulated with fiberglass batts that I am planning to replace with Roxul ComfortBatt.

Unless I misunderstood the dozen articles on this site and FHB (including the excellent article that you linked) that I read about insulating and finishing basements, there should be no problem with using permeable batts in the upper above grade portion of the wall.

I'm actually going to post a Q&A about how to deal with the bottom portion of the wall but will keep this to the comfortbatt odor.


Answered by Keith H
Posted Mon, 12/16/2013 - 13:02

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You mentioned that the odor was "catty." In a recent article on another topic, John Straube implied that formaldehyde can smell like urine.

Straube said, "We have seen problems ... in floors over garages. The humidity is driven inward, and the humidity releases formaldehyde in the OSB. In hot, humid climates, those risk factors go up. When this happens, the smell of hot pee in the house can be quite noticeable. That’s the urea or uric acid from the damp OSB."

Answered by Martin Holladay, GBA Advisor
Posted Mon, 12/16/2013 - 13:56

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Odors described as "catty" or often "fishy" are typically associated with the fact that the binders used to hold the fibers together use phenolic formaldehyde compounds. Slight deviations (although still remaining within the manufacturing spec) during the manufacturing process can result in off gassing of the described odors. Sometimes increased moisture levels can exasscerbate the odor as well, as was desbribed in one of the previous comments that was associated with OSB. Typically, the odors do dissipate with time. They are mostly noticable at the time the batts are removed from the bag or first installed. They can and sometimes do linger with elevated levels of humidity.
The manufacture of any building product containing these phenolic binders can be subject to this situation. Almost all domestic manufacturers of fiber glass insulation products have moved away from phenolic binders and are now incorporating organic (plant based) binders. To my knowledge, manufacturers of rock or mineral wool insulation batts have not made this transition. However, the levels of formaldehyde remaining at the time of use in those products manufactured with phenolic resigns is insignificant. Typically it is lower than the levels existing in the normal atmosphere.
Since you have noticed that the odor has dissipated, I would not be too concerned. As a 45 year veteran of the insulation industry (both manufacturing and contracting), I can tell you that I have been exposed to similar odors many times over the years. It can even vary by piece within the same bag of insulation at times. Although I am now almost completey bald, I can say with all confidence that my hair had not turned green prior to its demise from my scalp!
Chris Brown: Director of Training and Field Support
Knauf Insulation North America

Answered by Chris Brown
Posted Mon, 12/16/2013 - 14:41

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Thanks for the detailed answer. I did suspect it was a formaldehyde odor and that it was released by that batt being exposed to some moisture on the concrete (previous wet grind of the slab).

I take it Knauf also has not switched to biobinders?

What I find baffling is the GreenGuard rating in this context. Surely if I can smell it with the human noise we are not talking the low level of emissions I would have expected.

Answered by Keith H
Posted Mon, 12/16/2013 - 14:56

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Knauf not only has switched to bio-based binders, but we actually pioneered the technology when we introduced our EcoBatts with ECOSE technology about 5 years ago. The rest of the fiberglass industry followed that development. (

Greenguard (now part of UL Environment) has more than one level of certification. Those are Greenguard approved, Greenguard Gold (those two deal primarily with VOC emissions, with Gold being more stringent) and the 3rd is Greenguard Validated, which validates among other things that a product is formaldehyde free. Greenguard is a positive thing. You made no error in purchasing the product that you purchased. A short-lived odor issue does not translate to elevated levels of formaldehyde directly. Unless the odor does not dissipate for some strange reason, I would say that it is a non issue.

Answered by Chris Brown
Posted Mon, 12/16/2013 - 15:41

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Might be the wrong format but people reading this thread would hopefully appreciate it.

I clearly see on the Knauf site that your glass insulation products are formaldehyde free.

However, I don't see anything about your rock wool insulation products on the USA site nor do I see anything about Knauf rock wool insulation products being formaldehyde free. I have a call in to Rob at Knauf (Technical Services) to see what he can tell me.

While it may not be a long term concern, I certainly would prefer a bio-binder rock wool if one is available.

Answered by Keith H
Posted Mon, 12/16/2013 - 15:49

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Keith, you are correct. In the North American market, all of our manufacturing facilities are fiber glass (glass mineral wool) facilities. We do manufacture rock mineral wool products in other markets. Rob, (actually it is Bob) actually works for me and would tell you the same thing. Although similar in concept, there is a difference between the manufacture of rock vs. glass fibers. While we have made the transition to plant based binders across our line of fiberglass products, we aren't there yet with the
rock mineral products.
By the way, I wasnt trying to do anything more than answer your question. Your preference for a rockwool type product is simply that, your preference. I'm good with it.

Answered by Chris Brown
Posted Mon, 12/16/2013 - 16:43

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Aging question but I wanted to provide an update re my first use of Roxul comfortbatt

First, the good:
I love the installation of this stuff. It is really fast, even for custom widths, and I can get very good installations with just a tiny bit of batt oversizing. A bread knife really is the tool of choice. Way better than most fiberglass. If someone know of fiberglass that installs like this, please share.

Second, the bad:
The odor problem is meaningful for me. At this point, I have a lot of exposed (not yet drywalled) installed roxul and a lot of exposed uninstalled roxul (the bags tend to open at the top with handling). To me, the odor in the house is very noticeable despite significant venting on previous days. There is some new drywall (perhaps 200 sf but no mud or texture yet) but not much other building material to be causing odors. It's not the uric formaldehyde odor I smelled earlier but it is irritating to me.

At this point, I'm planning to deal with this issue by adding a well-installed air and vapor barrier (suggestions welcome but polyE worked on these walls for 35 years so why not) and a service cavity and investigating alternatives to Roxul for future projects.

Thus far, I don't have a reply from the manufacturer.

Answered by Keith H
Posted Sun, 12/29/2013 - 17:12

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Keith : as Martin pointed out, you will have no choice to deal with the air perm of the insulation you chose... so plan and install air barriers as tight as possible on top of the insulation.

Some here also often suggests the use of drywall/paint as air barrier..could be a cheap double up option ...

Answered by Jin Kazama
Posted Sun, 12/29/2013 - 23:16

Helpful? 1

Final update:

The odor did dissipate. It's no longer detectable or irritating. I'll continue to use this product for more projects and wouldn't hesitate to recommend it at this point based on the ease of high quality installation.

Answered by Keith H
Posted Mon, 01/20/2014 - 19:12

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