All fiberglass insulation must be encapsulated!

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.

Hall (210)Hall (211)

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.

Colburn-full (298)Jorg 229

We should never see exposed batt type insulation around skylight chases.

Copy of Snell 325

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.

Colburn-full (310)

We should never see exposed batt type insulation on ductwork.

Gray 253

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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  1. Robert Haverlock says:

    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.

  2. 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.

    • Charles Buell says:

      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.

  3. Please don’t confuse insulation with air barriers. No insulation is completely effective if air moves through or around it.

  4. 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.

    • Charles Buell says:

      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.

  5. Thomas Sawyers says:

    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.

    • Thomas Sawyers says:

      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.

    • Charles Buell says:

      No it would not be OK because it would be adding vapor barriers in the insulation away from the interior surfaces.

  6. martin wellens says:

    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.

    • Charles Buell says:

      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.

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