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Community and Q&A

Spray foam and batt insulation combination

2wBzrWjBUK | Posted in Energy Efficiency and Durability on

We are renovating our 2nd floor bedrooms due to moisture problems and are seeking the most efficient and cost effective means to insulate. We live in the lower thumb area of Michigan and the the rooms have 2×6 cathedral ceilings. The previous method used was 6″ R-19 kraft faced batts with durovent baffles(improperly installed/long story), with soffet and ridge vents. I do not wish to add to the 2×6’s and shrink the rooms so I need to maximize the use of the 2×6’s. I would like to use 2-3″ of closed-cell spf and fill the rest with either 3.5″ r-15 batts or 2.5″ rigid foam board. I know I will only max out at approximately r-31 but I’ve made my peace with that. I was told not to use kraft faced batts with the foam. Would 2″ of spf against the sheathing plus the 3.5″ batt do the job or would I be better served to put the 2.5″ rigid foam against the sheathing and fill the rest with spf? Or would another method work better. The long story above drained our savings so we are on a tight budget but we don’t want to have to deal with this issue again so we want it done right. Additionally, I read that applying spf in cold weather could be prolematic for the spf. Any truth to this? The insulated area will not be accessible once the work is complete so I won’t know until it’s too late. Trying to sidestep that landmine this time around. Thank you for your thoughts and input.

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  1. GBA Editor
    Martin Holladay | | #1

    Here's an article that explains how to insulate a sloping ceiling: How to Build an Insulated Cathedral Ceiling.

    If all you have is 2x6 rafters -- that is, 5 1/2 inches -- then I think your only choice is closed-cell spray polyurethane foam. I would install at least 5 inches of the stuff.

  2. 2wBzrWjBUK | | #2

    Thank you Martin for the link. It was very informative. Cost is an issue with this renovation and 5" of spf would almost deplete our entire budget for this project. 5" would be well over $4000 for the space I am doing so I need to find an alternative method of insulating. In addition, I've been told that anything over 4" of spf only increases the r-value by a small percentage and that another means of gaining r-value is needed. That is why I want to use a combo of spf and fiberglass batts to get the most r-value I can in the space I have available. Thanks for your input.

  3. GBA Editor
    Martin Holladay | | #3

    The information you have is wrong. If you double the thickness of a given amount of spray foam, you cut the heat loss in half. That last inch matters -- especially when the code calls for a minimum of R-38 or R-49 in your ceiling.

    You could stack up several layers of polyisocyanurate insulation if you want, carefully sealing the perimeter of each piece with caulk or canned foam. This is labor intensive but cheaper than spray foam.

  4. 2wBzrWjBUK | | #4


    Would you recommend combining the polyiso and spray foam together to get the best of both? I recently saw a post where a guy placed batts between the rafters and laid polyiso sheets on the rafters and firred them out for the drywall installation. I was thinking about using foam board either underneath or on top of the spray foam. If I get 3" of spf installed then I would have 2.5" left where I could install the foam board. Could I use the pink xps or the polyiso or are they the same thing? Also is air space between the spf and the drywall ok. I read that once the spf is installed, any space below it is considered conditioned space. Is that only accurate if you have enough spf installed? Lastly, have you heard of engineered thermal performance (ETP) ratings in regards to the r-value figure? Thanks so much for all of your time and information.

  5. GBA Editor
    Martin Holladay | | #5

    Yes, you can combine rigid foam and spray foam if you want to save some money. I would be inclined to install the layers of rigid foam first -- cut undersized, and with canned foam sealing the perimeter of each rectangle -- and cap everything with spray foam from a contractor's truck. Plan the thickness so that you try to fill the rafter bays, or almost fill them.

    You could also install a layer of rigid foam under the rafters as well, to limit thermal bridging through the rafters, if you don't mind losing another inch or two or ceiling height.

  6. 2wBzrWjBUK | | #6

    I was thinking about attaching them to the sheathing but would I need to seal the edges? Wouldn't the spf fill in any voids?

  7. GBA Editor
    Martin Holladay | | #7

    Q. "Wouldn't the SPF fill in any voids?"

    A. That depends on the skill of the SPF installer -- and whether you left a big enough gap all the way around. It's certainly safer to be cognizant of the air sealing issue when you install the rigid foam.

  8. 2wBzrWjBUK | | #8


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