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Major Performance Issues with Fiberglass Batts at the Roof Deck

posted Oct 14, 2014, 4:28 PM by Stephen Mogowski   [ updated Nov 3, 2014, 7:25 AM ]
Fiberglass batts remain in existence (and prominence) today not because of their superior performance but rather their cost and ease of installation.  After all, time is money.  With fiberglass batts you save on material costs but also don't have to wait for off-gassing of foam or drying of wet-spray cellulose.

But what we often forget is that after we drywall, our clients live with our choices.  And fiberglass batts strapped to the roof deck are a bad idea, 100% of the time.

We've seen it multiple ways.  Batts 'secured' to the deck with insulation hangers between top chords or more commonly, tension cables secured at the ridge and eaves with eye hooks to prevent fall out.

Here is a fact: If you use air permeable insulation and it is not in direct contact with the air barrier (in this case, the plywood roof deck), you will experience convective loops within your assembly as well as unwanted radiant heat transfer.

Out here in Arizona, when you have 120F ambient temperatures the roof deck reaches outrageous temperatures... even with clay tile roofs outperforming asphalt single roofs.  So the importance of a thermal assembly making contact with your air barrier is even more important!

Unfortunately, not every home is built right the first time.  The IR picture is taken on a typical summer day in Phoenix.  Notice the ridge is illuminated at approximately 118F.  After analysis using BPI methodology for deducing R-Value of misaligned batts, it was determined that the attic insulation for this example was performing at R-7... despite the fact that it was 10" R-38 unfaced batts that were installed.

Building on the previous example, the expected attic temperature was 80F (remember, sealed/conditioned attic space) but in the afternoon it was recorded at 93F (+13F).  The result was that the mechanical equipment and duct work were exposed to unexpectedly higher temperatures.  Because the duct work was initially designed to be in conditioned space, the R4.2 flex was now susceptible to heat gain and the homeowner was unable to cool their home effectively.  Even after upsizing the unit a half ton, there were still comfort complaints.  Especially on supplies that were at the end of the run.

So ultimately, the builder opted to replace the insulation with a pillowed cellulose 
application.  The cellulose made contact with the roof deck and effectively reduced the convective loops and radiant heat transfer to a point where the homeowner was able to be comfortable and have vastly reduced energy bills.

In the above example, the mechanical engineer designed a system utilizing ACCA Manual J, S and D.  The energy rater contracted to inspect the insulation was unable to verify whether the batts were in complete contact with the deck.  It looked okay-- so it was approved.  Had the application been found to be unacceptable, all of the headaches would have been avoided.

The moral of the story?  Pay attention to your material choices and their applications in the design phase.  Spend a little more to prevent big problems down the road.  And if in doubt, consult with an energy rater before making a costly mistake.

Thanks for reading.
Stephen Mogowski
Desert Skies Code Compliance


This article wasn't meant to discredit fiberglass batts in all applications.  But it is meant to clearly state that they have no business in a sealed attic.  Some governing bodies will no longer allow the application due to issues with condensation control.  See below for 2012 IRC language.

R806.5 Unvented attic and unvented enclosed rafter assemblies. 
Unvented attic assemblies (spaces between the ceiling joists of the top story and the roof rafters) and unvented enclosed rafter assemblies (spaces between ceilings that are applied directly to the underside of roof framing members/rafters and the structural roof sheathing at the top of the roof framing members/rafters) shall be permitted if all the following conditions are met: 

1. The unvented attic space is completely contained within the building thermal envelope.
2. No interior Class I vapor retarders are installed on the ceiling side (attic floor) of the unvented attic assembly or on the ceiling side of the unvented enclosed rafter assembly.
3. Where wood shingles or shakes are used, a minimum 1/4-inch (6 mm) vented air space separates the shingles or shakes and the roofing underlayment above the structural sheathing.
4. In Climate Zones 5, 6, 7 and 8, any air-impermeable insulation shall be a Class II vapor retarder, or shall have a Class III vapor retarder coating or covering in direct contact with the underside of the insulation.
5. Either Items 5.1, 5.2 or 5.3 shall be met, depending on the air permeability of the insulation directly under the structural roof sheathing.
5.1. Air-impermeable insulation only. Insulation shall be applied in direct contact with the underside of the structural roof sheathing.
5.2. Air-permeable insulation only. In addition to the air-permeable insulation installed directly below the structural sheathing, rigid board or sheet insulation shall be installed directly above the structural roof sheathing as specified in Table R806.5 for condensation control.
5.3. Air-impermeable and air-permeable insulation. The air-impermeable insulation shall be applied in direct contact with the underside of the structural roof sheathing as specified in Table R806.5 for condensation control. The air-permeable insulation shall be installed directly under the air-impermeable insulation.
5.4. Where preformed insulation board is used as the air-impermeable insulation layer, it shall be sealed at the perimeter of each individual sheet interior surface to form a continuous layer. 


2B and 3B tile roof only0 (none required)
1, 2A, 2B, 3A, 3B, 3CR-5
4A, 4BR-15

a. Contributes to but does not supersede the requirements in Section N1103.2.1.