I suspect this will rub up against common wisdom and even standard building practices, but it is an idea that’s time has come.
Un-encapsulated insulation is at its best a good filter, as is demonstrated over and over with the dust that shows up from air moving through it. It is not surprisingly–used in filters–like furnace filters. That same insulation “quantity” becomes much more effective when it is enclosed on both sides. In this first picture we can see black sooting of the insulation where air is leaking around an ICAT type can-light buried in 16 inches of white fluffy fiberglass insulation.
Fiberglass insulation, without an air barrier on both sides, is very poor at stopping air moving through the insulation and into the attic around can lights, junction boxes, exhaust fan housings etc. Of course these penetrations should be properly air sealed regardless, but when there are breaches, the fiberglass cannot help, as can be seen in the next two pictures.
The R-value of any insulation is only as good as its ability to stop air movement. Fiberglass insulation is very poor at stopping this air movement. Loose fill cellulose fiber, spray cellulose fiber, foam boards and the spray foams are much better at this.
So one might ask, well that is fine for side walls, but what about attics and crawl spaces? The answer is simple–don’t use fiberglass in those installations. Pick an insulation that is better at stopping air movement or figure out a way to encapsulate the insulation.
This would not be impossible for crawl spaces, but quite difficult for attics without special framing changes or truss design changes etc. It is much easier to simply properly air seal the ceiling and then insulate with cellulose fiber or spray foam.
We should never see exposed batt type insulation in the transition walls between different levels of a home visible in the attic.
We should never see exposed batt type insulation around skylight chases.
We should never see exposed batt type insulation in the walls of knee-wall attics or the walls between crawl spaces and conditioned spaces, such as the next picture.
We should never see exposed batt type insulation on ductwork.
I am pretty sure it is time for the building codes to recognize this serious defect regarding the use of fiberglass with no encapsulation, and require that no fiberglass insulation ever be visible–that a proper continuous air barrier be installed on both sides.
Another requirement, related to good air sealing, would be that all drywall be continuous bead glued to at least top plates and bottom plates. We can discuss that another time.
PS, if you are thinking of fixing your own ineffective fiberglass installation, all I can say is don’t. It is not simple to do and things could co horribly wrong. For example you generally would not want to just add plastic sheets to the underside of your floor joists to “encapsulate” your insulation. This would be a very bad idea in most climates if the plastic is a vapor barrier. Consult with a qualified Building Performance Professional before making any changes to the way your house is insulated.
By Charles Buell, Real Estate Inspections in Seattle
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Hello Charles, This answers my questions about fiberglass insulation. I don’t like it! and I think its the least effective insulation on the market, no matter what air sealing is accomplished. I’ve seen this more often then not. Bats sag, air drives through fiberglass no matter how tight. I will give some credit to Knauf for a better insulation, but still don’t like it…
I use Roxul and blown-in-Cellulose as well as FoamGlass, (which is rigid) which I think and know packs in much tighter and is more air-resistant then any fiberglass product.
Your comments make no sense. I previously spent 29 years working as a heat transfer engineer and I totally disagree with your logic. All to often we find notions like this being posted without technical justification. They then become “common knowledge” that leads folks to spend large sums of money to “fix” a nonexistant problem.
Bill, I thought about whether to let your comment stand or not but decided what the heck. My experience with this goes back further than yours and there just may be something you are either miss-understanding about what I said or something else. While you say you disagree with my logic it might help if you explained what you mean by that. If you actually think that fiberglass insulation is just as effective not enclosed as it is enclosed you are surely mistaken.
Please don’t confuse insulation with air barriers. No insulation is completely effective if air moves through or around it.
I am not confused about it 🙂
First, let me say I agree that fiberglass insulation performs best when encapsulated on all 6 sides in an airtight cavity. You’ve shown some great photos here of bad installations of fiberglass.
But, when you say “Un-encapsulated insulation is at its best a good filter,” you exaggerate. Fiberglass that’s open on one side doesn’t have zero R-value. It still insulates but at less than its rated R-value.
Also, I think Wynn may be right. When you write “The R-value of any insulation is only as good as its ability to stop air movement,” you may not be confused in your mind about the difference between air barriers and insulation, but you’re creating confusion for readers.
Insulation itself doesn’t have to stop air movement. Air permeable insulation is fine as long as it’s part of an airtight assembly.
Allison, thanks for commenting. My pictures of fiberglass insulation is so over and over again that it is hardly just bad—it is typical in my experience. To some degree my stating that fiberglass makes a good filter is not all that much of an exaggeration really. Of course it does not go down to zero in R-value and we could perhaps argue all day as to how much but how much the r-value is lowered, as it is in itself related to how good the air sealing of the five sides is. I am quite sure that if we all stopped seeing fiberglass as insulation none of us would be confused at all. Sure it CAN work in an ideal installation but what installation is ever going to be perfect? Why not just use insulations that are in themselves decent air barriers so that the air sealing of the six sides does not have to be so dang perfect? I would argue also that air permeable insulation is never fine. That is just an attempt to make fiberglass insulation OK. The costs involved in making fiberglass insulation fully functional is a cost that simply is not necessary.
I have cellulose insulation in my attic. I was recently working up there and pulled back the insulation to just below the joists as to not compress it and laid down plywood. I was thinking about enclosing some fiberglass insulation in contractor type trash bags and laying it on the plywood. Is this ok.
I forgot to mention that the plywood only covers about a 4×10 area. I was thinking that if it is in the plastic I can move it easily if I need to traverse on the plywood again, and limit the fibers in the air.
It would be better to elevate the platform and keep the insulation under the platform walkway
No it would not be OK because it would be adding vapor barriers in the insulation away from the interior surfaces.
I have trouble accepting that encapsulated fiberglass is superior to regular fiberglass. Consider that I’ve already insulated between ceiling joists and am adding an additional layer. Yes encapsulation would prevent air from penetrating through the barrier, but doesn’t it also make it easy for air to slide between the batts/rolls? I want the consecutive rolls to touch and “lock” together, so air can’t get between them. Encapsulation seems to defeat this goal. Please explain why I am wrong.
You do not want to encapsulate insulation and cover it with another encapsulated batt. Fill the whole cavity and cover all six sides is all and make sure all six sides are air tight.
Would a house wrap type of material work for encapsulating an attic which shares a wall with an interior wall?
likely better than nothing
Charles, what a great post. Lots of people write that fiberglass insulation must be encapsulated, but it’s a rare article that gets into the challenges.
In my 2003 home in greater Seattle, I am thinking about two issues pertaining to encapsulating fiberglass insulation:
First, I am trying to figure out how to improve exposed batt insulation in the transition walls between different levels of a home visible in the attic. I have a surprisingly large amount of this due to the shape of the house, second floor hallway ceiling height versus bedroom ceiling height, etc. Could I apply FOAMULAR 250 XPS rigid foam board to encapsulate the fiberglass in transition walls? All these transition walls are accessible via attic, and there are minimal services in the way. I did my kneewalls this way and was thrilled by the improvement in comfort. Any concerns about vapor barrier?
Second, I would like to encapsulate the fiberglass on the attic floor. Your recommendation is “don’t use fiberglass in attics”, which is good advice (and I agree 100%), but the prior owner built this house and I have to work with that. It is in good condition and relatively well installed (though probably not quite Grade 1), so would be a shame to throw it out. But I would like to improve the insulation for comfort and performance.
I would not recommend that material at that location as it is too much of a vapor barrier. You might get away with it here in our mild climate but would not see it being worth the risk. You could use tyvek type materials or even fiber board for the most part.
Charles, thanks for the excellent advice from your years of experience in the industry. I have a stupid homeowner question, so please indulge me. I installed encapsulated fiberglass insulation (“ComfortTherm”) in my basement walls that I framed. The walls have been sheetrocked and in use for several years. The code in my city which is consistent with ICC, states that 16 inches of fiberglass (and certain other) insulations qualify as a vertical fireblock between floors. However, there is a reference to unfaced insulation. My guess is that this relates mostly to the paper facing which has been in use for many years because, presumably, the paper can burn, negating its usefullness as a fireblock. I do not see the same propensity for fire in plastic encapsulated insulation, which I chose for both insulation and vapor barrier qualities. I should note I live in Utah where humidity is very low and moisture is not generally an issue in my basement anyway.
In addition to complying with code to slow down the flow of air as a vertical fireblock, I believe the encapsulated insulation also helps as a horizontal fireblock as I placed my stud walls against the concrete basement wall instead of leaving a large gap, so the plastic backing, though perforated for vapor penetration, also serves to restrict air from flowing horizontally from one 2×4 chamber to the next (one of the requirements for using fiberglass insulation as a fireblock is that it must fill the entire chamber, which this does from floor to ceiling in all walls).
Am I placing too much confidence in the encapsulated insulation as a fireblock even though it is allowed by code? Does the plastic encapsulation count as “facing” the same as paper facing, and does it therefore negate the qualities of the fiberglass for restricting airflow as a fireblock?
I am sorry, this is a better question for your local jurisdiction. I personally do not think fiberlass makes a very good fireblock material as it does nothing to stop air movement. Most areas around here want solid wood blocking at transitions from vertical to horizontal spaces.
People are driving themselves crazy over insulation these days. There IS a law of diminishing returns. Fiberglass is a fine, cheap, easily available option. Building scientists are looking for perfection. Lots of these new techniques require perfect installs. This never exists in the real world.
Fiberglass simply is NOT fine as you say. Unencapsulated fiberglass is a waste of that cheap insulation. While it has “some” R-value, it does little to stop air movement, unlike some of the more air-stopping type insulations—like cellulose fiber. I do agree some building scientists violate one of the important rules of building of “keeping it simple” whenever possible.
Can I ask a question? I have a shipping container that wants to be insulated so I can heat/cool the thing. I live in Minnesota. If one was to wrap (cheap) 3-1/2″ un-faced fiberglass insulation in a bag and totally seal it, such like in a 4 mil poly, assuming no moisture could go in or out of the bag, wouldn’t that work in the container to insulate and stop the condensation inside the wall? I’m using 2×4 glued to the sidewalls and the ceiling hence the 3-1/2″ thick insulation. I am planning on sheeting the inside with 1/2″ CDX plywood. I’m not sure how to do the floor yet other than laying down a few more 2×4 the thin way and using 1-1/2 rigid foam and covering with 3/4″ plywood. I am mainly concerned about the sidewalls and ceiling insulation in conjunction with the steel container and condensate. If anybody would like to comment that would be great. Thanks.
I really do not have much to help you with, but I know I would not try to do what you want to do with fiberglass.
You are getting condensation because you have warm inside air hitting a cold surface. You would be well served looking into a spray foam install. You have to keep that warm air off of that cold steel…