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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