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Question about adding insulation above vaulted ceiling

rogusnic | Posted in GBA Pro Help on

My name is Nick and I live in Middlesex County in central New Jersey, climate zone 4A.  House is a split level built in the mid-1950s.  Last roof installation was June 1995 – scheduled to get new roof within next month.  My living room is approximately 12 x 20 and has a vaulted ceiling.  A few years ago in early Fall I noticed an oval-shaped stain near the center of the vaulted ceiling that looked like a bullseye and about the size of a baseball. 

The drywall was not soft to the touch and the stain only appeared to be re-wetted one year later on the first morning in early Fall when there was a large gradient between day time and night time temperatures.  There are no other stains on the ceiling and it has not changed in size after significant rainfall.  The stain has not grown in size in the last year and has been attributed to condensation. 

There are no soffit vents at the lower end of the vaulted ceiling and the house was constructed with a board above the living room ceiling that runs perpendicular to the 2×6 rafters, running across the middle of the ceiling, halfway between soffit and ridge.  Laying in the attic and looking down the rafter bays that are over the vaulted living room ceiling, there are a few locations where there is a piece of sheet metal above the wood sheathing.  Perhaps the sheet metal is from previous patching of roof and the original wood sheathing may have been damaged from years of poor ventilation above the living room ceiling.  Any damaged or compromised roof decking will be replaced with the upcoming roof installation. 

Originally, a roofing company suggested that one solution to the condensation problem would be to tear off the tongue and groove decking above the vaulted ceiling, remove original fiberglass batt insulation from the 1950s and install air baffles with new insulation.  This would require making holes for airflow in the board which runs perpendicular to the rafters.  A certified home energy specialist/energy auditor/contractor suggested a better solution would be to attack the problem from the inside after the new roof is on the house.  In addition to air sealing the main attic, he proposed two possible solutions for the living room:

Option #1: Remove existing living room vaulted ceiling and old fiberglass batt insulation and completely fill space between 2×6 rafters with ~5.5” of closed cell spray foam


Option #2: Remove existing living room vaulted ceiling and old fiberglass batt insulation and apply 2” of closed cell sprayfoam against sheathing.  Fill balance of void with unfaced R15 fiberglass batting.  From reading multiple articles on your site, I learned this is called the flash-and-batt approach.  This option was suggested to keep costs down. 

I was hoping to get your advice on the following:

1.      Is option #2 acceptable to eliminate the condensation issue above my living room ceiling?

2.      Do you recommend to use fiberglass batting against the closed cell spray foam in flash-and-batt application for vaulted ceiling, or are there advantages to using a different material such as mineral wool with the ccSF?

Thank your for your time.         

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Nick.

    Both of those approaches can work if you reach the appropriate R-values for the overall roof and the right ratio of spray foam to fiberglass insulation. I recommend that you read this article, which should answer all of your questions: How to Build an Insulated Cathedral Ceiling

  2. Expert Member
    BILL WICHERS | | #2

    I had a similar issue in a large room in my own house. We had a tongue and groove ceiling, and had lots of problems. The insulation was done incorrectly, was insufficient (only about R10!!), and the venting was all messed up. This was also a very shallow pitch roof so it couldn't be vented properly anyway.

    I put in R38 of closed cell spray foam. Problem solved. Detailing around the skylights was very important to avoid condensation around the perimeter of the skylight frames at the top of the skylight wells.

    Either of your options will work. BE SURE if you use option #2 that the contractor installs the spray foam a MINIMUM of 2" EVERYWHERE. They will often say "well, the average is 2" so you're good", but you can have condensation in spots that are too thin and that's what you need to avoid. Make sure you air seal any areas that the spray foam isn't fully encapsulating (gaps between sistered rafters, etc.), since those are also sneaky places water vapor can get into and condense.

    If you want to keep a tongue and groove ceiling (I'm not entirely clear if that's what you have or not), best is to put up drywall, tape/mud and prime the drywall, then install the T and G over the drywall. The drywall becomes your air barrier.


    1. rogusnic | | #3

      Thanks for the advice Bill. The ceiling in the living room is drywall. The roof sheathing above the ceiling is tongue and groove.

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