GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Dormer Project Insulation Questions, R49, Spray Foam, Batt, Both?

Chrishere01 | Posted in General Questions on

Hello everyone, 

I have been surfing the Q&A for a while and looking for the answer for a few questions. I apologize in advance if these may seem obvious.

I am currently in the process of dormer my 1950s cape style house in the northeast. 

Currently the code requires R-49 for Ceiling R-values. To get that I was thinking of doing a mix of spray foam and batt insulation or even blown insulation depending on the the availability. 

I am working with a very limited space and I am thinking to spray foam in the first section indicated by the green which is about 4 ft from the back wall The remaining insulation would be batt or blown in indicated by the blue. This could help keep costs down as well

(see attachment 1)

I have read a mix of different things online related to ventilation with spray foam and not to vent with spray foam, where R-value should be related to the thickness and not to air tightness.  Not sure if this first scenario is advisable. 

The second question I have if the first scenario isn’t possible, with closed cell insulation needed for the r-49 value I am assuming that would need a backing so the spray foam has something to stick to. Considering 8 inches is needed,  the issue comes from the collar ties 2×8,  Would I need to pad out 1 1/2 to 2 inches for the backing to meet the R49, leaving some clearance for drywall?  Also can the backing be something as simple as a vapor barrier?

The last question comes from something I saw come from “A solution for high performance attic” (lol).

(See attachment 2)

It appears as though the roof rafter are insulated and the ceiling is insulated as well. Looks as though the air is being ventilated through this  insulated space. I was wondering if the ceiling can be insulated with R-30, and then the roof rafters insulated with R19 (or greater), to give the  R-49 total insulation? My initial instincts are no due to the ventilation, but maybe I am wrong. 

Let me know if you have any other suggestions too!
Thanks everyone in advance for reading!


GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. plumb_bob | | #1

    Hi Chris

    You need to figure out if you are trying to achieve an unconditioned, ventilated attic space or not. If yes, then the ventilation details are critical. Your limiting factor will be the height available for insulation at the eaves. You want to achieve maximum R-value at this location while still providing an adequate ventilation channel. The spray foam to 4' back is an option, but make sure you have a vent channel that is rigid enough to handle the spray foam. The detail you show has the insulation completely filling the attic to the peak of the roof, this needs to be reduced so that the ventilation channel is continuous to the peak and is vented through a ridge vent or similar.

    The second image makes no sense as the insulation that is on the underside of the roof deck is outside of the insulation plane of the rest of the house, so has no benefit. Also, fluffy insulation should never be placed tight to a roof deck.

    If this project is being inspected, talk to your inspector about how they would assess this project. Most codes have a relaxation where working with existing structures, so trying to achieve R49 may not be necessary.

  2. Chrishere01 | | #2

    Thanks Bob,

    You are indeed correct about existing structures. The town allows for r30 where r49 is not achievable, but that is limited 500 sqft or 20% of the ceiling area, whichever is lower. Given this leniency, I was going to run baffles to 4 ft or so to the ridge vent allow for ventilation adequate ventilation to from the soffits.

    I have my own reservations on the ridge vent for the low pitch roof may discuss with the contractor with installing a fan or louvre instead of a the ridge vent. But the combination of 4ft of spray foam and batt seems to be most cost effective option. savings of around 60%.

    I could spray direct to the decking, but would like to insulated along the collar ties and back it with a vapor barrier or even some board.

    The second image made no sense to me as well so I appreciate the sanity check.

  3. plumb_bob | | #3

    If you do spray direct to the decking, you are creating a conditioned, non ventilated attic space at that location. This is ok, but you have to figure out how it ties into the other attic spaces of the house, and how it may disrupt the ventilation of the other spaces.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |