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

President

Desert Skies Code Compliance

P.S.

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.