Open cell foam and cellulose... Can they work together?

Greetings everyone!

   I have a client who would like to have two inches of open cell foam in the attic flat, and then 10 inches of cellulose. Someone mentioned to you that there were issues in doing just that.... I myself cannot imagine what that could be (of course preparing to give the foam enough time to cure)

Any comments?



Tags: cell, foam, open

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Vapor issue is the only one I can think of though if you use a vapor retarder paint you should be fine & is accepted by many codes (do not use plastic & if it is there remove it)

I can't think of any other issues with using it & seen it used elsewhere

I got to ask 2"? That is typically a Closed Cell number, while 3 to 4 is more typical of Open Cell

The only things to be careful of is making sure they spray it in properly & don't leave any gaps or pinholes - I know to prevent that with closed cell they do what is called picture framing by spraying the studs first & then spraying the center, I think they can do it that way also but not positive

Should be 2 inches of closed cell and then cellulose. The closed cell will act as the vapor barrier. Cellulose will act as the flame retardant. Both can work as a system.

Most importantly the installer's credentials and insurance must be carefully verified when working with spray foam. I mean the installer, not the salesman. Again, INSTALLER. The finished product fall's on the installer's ability.

I'm not a fan of spray foam or SPFA but, you must do your home work or this could prove to be the most expensive insulation project you ever gambled on. See.

NEVER use closed cell in an attic space! Closed cell is just that, water will not pass threw. So you get a leak in your roof, the water will go some where. It may build up, evaporating back into the attic space, it may find it's way to an outside wall and drain down from there. You could have major damage before you even knew you had a leak.

Closed cell should be used as a barrier to moisture getting to structure, such as in a crawl space. 

Why not just seal the actual leaks in the attic floor, then blow with cellulose and save a bunch of money?  This is similar to flash-and-batt in exterior walls and I have yet to understand why this is done.  My theory is that the closed cell people were looking for something to do with their 2" restriction and came up with this.   It does not add any thermal break, and the open cell is the same R-value as the cellulose.  Even if it were closed cell, the added R-value per inch couild easily be made up with added thickness of cellulose.

I am in the mid-Atlantic and don't find there is any significant vapor transmission through normal construction materials - just seal the holes like we have known to do for 35 years.

I am right there with you Ed but with that many use the open cell to do the air sealing for them & then top it off with cellulose - the system does work great (though it should be 3-4" based on specs / test results & not 2" for air sealing purposes)

As for closed cell, first some of you guys need to find better roofers & having done roofing for enough years I can't tell you how many people never realize they had a leak & yet we had to replace sheathing left & right with a regular vented roof. Personally I think Open cell is the best product as described above but if someone wants to use closed cell I wouldn't pull out the catastrophic leak card. As for building up & evaporating back in the attic, that is what would happen no matter what insulation product you choose if water does get in.

Ahh it needs to act as a vapor barrier so use... no sorry that doesn't fly either as diffusion is greatly exaggerated - the issue is air leakage & people not venting appliances / bathroom outside - you control the moisture inside & any diffusion will be handled just fine by a vented assembly. As for hot roofs, then yes it does need to be closed cell or an open cell with a retarder applied as it is now part of the conditioned space & it is the air barrier instead of the drywall


I am still wondering why we need airsealing in the center of a piece of plywood that is already airtight.  The money would be better spent on a layer of rigid foam on the outside to cut down on thermal bridging and to add R-value.

Ahh but we are talking the attic floor & not the roof sheathing, no?

As for hot roofs aka encapsulated attics, the issue is dew point considerations & as Richard points out, Lynene spread so much dis-information it made it nearly impossible for them to start selling CC & instead opted to call it Med. Density Foam until most of the market forgot. As for thermal bridging, yes I like the outside the sheathing method, but if CC is installed properly which includes spraying the rafters, etc... it is minimized if not nearly eliminated

This link show's a risk you face when using closed cell without a balanced HVAC system. Your risk using open cell I described previously. This only shows insulation alone will not eliminate your problems if you do not plan properly.

Sorry Richard, but that has nothing to do with the HVAC, that had to do with to thin of a layer of foam not meeting the dew point depth required. To top that off I would also state that they probably were not maintaining moisture levels in their home - that also happens to be an enclosed / hot roof assembly while the bulk of this conversation & the OP's post is about insulating the ceiling plane / attic floor

Sean thanks for pointing that out. You may have missed my point. If a home has high humidity and you add SPF, it needs to be dehumidified to within published standards. A well balanced HVAC system helps maintain a balanced home. In some homes the HVAC system adds humidity via a built in humidifier. Once SPF is added to the home the HVAC system needs to be revisited and tuned in. We all know they are not always balanced and in some cases add more moisture than required. The end result is shown in that picture and is a large contributor to open cell spf failures without vapor barriers. I did not mean to imply the results explained in that article were a contribution to this discussion, only the photo. Sorry about that. I hope this clarifies my position.

Guy's this leak stuff is a made up story by a major spfi chemical manufacturer. At that time when the story was flying all over town, this same manufacturer did not have closed cell in it's inventory. People believed it and obviously still argue the details. wrote a story recently about open cell. The roof sheathing was rotting and covered in mold. I know of a home here where open cell was used and at the time of installation the same manufacture who started the rumor your speaking about also said a vapor retarder was not needed and paint was sufficient. This is published in their 2002 data sheet. This home was a very large home where cost was not an issue for anything. Five years later the roof was cut off and all that open cell was removed. Now this same manufacturer makes it clear a vapor barrier must be installed with their open cell foam. 

Regardless of the insulation used, once the vapor barrier is installed and done correctly, should the roof leak your not going to know until the water finds the least path of resistance and shows it face. 

Again, when all products used are installed correctly.

The same would apply to the roofing materials used. The roof and insulation should act as a system. Nothing is bullet proof and as you all know this industry uses our homes as case studies to write the standards for the future from our own failures. 

Here's a helpful link, feel free to navigate around.

Wishing you all tremendous success!


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