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3 Reasons to Remove Attic Floor Insulation in a Spray Foam Attic


I get asked a lot of questions about spray foam. Do I need an ignition barrier? Should I use open cell or closed cell spray foam? Will open cell spray foam really rot my roof? But the question I get more than any other on this topic is about whether or not the insulation on the attic floor should be removed when insulating the roof deck in an existing home. As you can tell from the title of this article, my answer is to remove it. Here are my three reasons, in increasing order of importance.

1. To prevent moisture problems

Think about temperature. Think about dew point. If you leave the insulation in the attic floor after insulating the roofline, the attic will be cooler in winter than if you remove the insulation.

But a common reason to put spray foam on the roofline is to avoid having to air-seal the attic floor. Thus, the air in the attic is connected with the air in the house. That means it’s more humid than outdoor air and more humid than vented attic air. Cold air is dry air, you know.

But now the attic isn’t vented to outdoors. The attic is much warmer than outdoors in winter but significantly cooler than the living space if you leave the old insulation in the attic floor. That makes the surface of the spray foam cooler, possibly even below the dew point.

Removing the attic floor insulation will solve this problem. It is not, however, the first solution. As Dr. Joe Lstiburek says, we shouldn’t be calling these things sealed attics or unvented attics. We need to think of them as conditioned attics. Once you deal with the air up there, this problem goes away, with or without the insulation in the attic floor.

2. To get a better air seal

Spray foam insulation is an incomplete name. It really should be called spray foam insulation and air barrier. A properly done spray foam job not only insulates but also greatly reduces the air leakage of a home.

But that only works if the installer can get it into the places where most of the air leakage happens. In an attic, the eaves are one of the most critical places to get good coverage with the foam.

If you leave the existing insulation in the attic, it interferes with the foam installation. Yeah, if you do a really good job raking the insulation back, you can still do a good job with the foam. But it’s harder and the chances of success are lower. I know. I’ve done blower door tests on homes where they left the old insulation and the leaks were at the those transitions where the old insulation got in the way. The photo below is one of the homes I tested where the installer didn’t get it sealed up.


3. To reduce odors and improve indoor air quality

If those first two don’t have you convinced, consider this. That old insulation is full of dust, debris, leaves used by that mouse that had a nest up there, rat poop, bat guano, the remains of a dead squirrel, remnants of rat poison that someone put up there ten years ago to kill that squirrel, some teenager’s forgotten drug stash, and possibly even a severed human ear. Those are just a few of the things I’ve found in attics.

Just kidding! I haven’t really found all those things in attics…yet. But those things and more could be hiding in the old insulation in your attic.

The effect of all that nasty stuff on indoor air quality is quite real, however.   The indoor pollutants most likely to be added to your home’s air are fine particles and volatile organic compounds (VOCs).   Fine particles are one of the pollutants most hazardous to your health because they can get deep into the lungs and then into the bloodstream.  Many IAQ researchers are most worried about PM2.5.  Fortunately, good, affordable IAQ monitors for homes* exist now.  I use the Awair Element* and have two of them in my home.

When the attic was vented to the outdoors, all that nasty stuff wasn’t so connected with the living space in your home. Yeah, you probably still breathed some of it, but not as much as you will now with spray foam on the roof deck. By encapsulating the attic and leaving the old insulation up there, that filth is in your conditioned space. You may have odor problems. Your indoor air quality may get worse.

Other considerations

Another potential reason to remove the old insulation is that it might be required. Some energy efficiency programs help you pay for the cost of spray foam in the attic but require removal of the old insulation. Georgia Power’s home energy improvement program for existing homes does this.

Will you have problems if you don’t remove the old insulation? Maybe. Maybe not. I know of homes where they left the old insulation in place and everything seems to be OK. Usually that happens in homes that are still pretty leaky even after sealing the attic.

I’ve been called in to look at other homes, however, where the old insulation led to odor problems. In my opinion, removing the old insulation should be as much a part of putting spray foam insulation on your roof deck as making sure you have whole-house ventilation and conditioning the air in the attic.


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Awair Element indoor air quality monitor

The Awair Element indoor air quality monitor is one of the best tools for keeping an eye on how good—or bad—your IAQ really is and whether or not your filters are keeping the PM2.5 levels low enough.  I have two of these monitors in my house.


Related Articles

Will Open-Cell Spray Foam Insulation Really Rot Your Roof?

California Mistakes Put Spray Foam Insulation on the “Bad List”

Does Your Spray Foam Insulation Need a Thermal or Ignition Barrier?


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This Post Has 29 Comments

  1. I’ve been in one of these
    I’ve been in one of these homes several years ago before I even knew such a thing existed. I climb up in the attic and like freak out how cool it is up there.

    Then I start looking around and I don’t see any insulation on the floor of the attic and I say ‘WTH?’

    But then I think, I really like how cool it is up here. Then I glance around and notice the roof is insulated with foam? Ok, this is a freak of nature whoever put this in had to be someone who has spent too much time in hot attics. (LOL)

    Upon further inspection there was an open duct connector coming off the plenum. At first I was thinking someone forgot to hook a duct up… but after assessing the situation I decided this was by design for clearly the reasons you suggest.

    Now if only all the attics I ever had to play in were this comfortable. The other problem with this is if a section of the roof deck has to be replaced due to fallen tree or other it would increase this expense and many times the roof deck is just replaced and they leave a big gapping hole where foam insulation should be.

    This attic I visited had exactly that problem.

    1. Yes, Ray, if part of the roof
      Yes, Ray, if part of the roof deck is replaced, the insulation needs to be replaced as well. I imagine if that house you were in had an uninsulated part of the roof deck, you would have noticed a big difference in temperature there.

  2. Seems Joe Lstiburek’s 2015
    Seems Joe Lstiburek’s 2015 article referenced in item 1 recommends small supply and return ducts for the conditioned attic to prevent roof deck moisture problems. Then add a fire detector. Is monitoring moisture in the conditioned attic a consideration without the supply/returns? Especially down south in hot and humid climates where it does not get excessively cold for extended periods? The common practice down here is to seal attics with open cell and use no supplemental attic conditioning.

    1. RJ, adding a supply and a
      RJ, adding a supply and a return is one way to condition the attic. Using a dehumidifier is another. Ventilating with either an energy recovery ventilator (ERV) or small exhaust fan can do the trick, too. If someone doesn’t have any way of conditioning the air in the sealed attic, they absolutely should monitor the humidity, and they should put the sensor near the ridge. More on this in another article.

  3. Thank you for writing this
    Thank you for writing this article! For almost 10 years now we have been recommending that if a client spray foams the underside of the roof deck (we prefer closed cell) then they should drop both a supply and return vent into that attic cavity – it essentially becomes another “room” in the house, though of course we are not as concerned about proper sizing of the vents as we are that we are helping to fight humidity in the attic. We see far too many builders skipping that step, or offering a supply and NO return, to which we say this can also lead to issues.

    1. Charles – I have heard that
      Charles – I have heard that adding a supply and return into sealed attics creates a conflict with the building code where the interior of the attic would need to be completely finished or brought up to a fire resistant level for which most homeowners do not want to pay. I can see this being probable. Have you experienced this conflict?

      1. Joe Lstiburek suggested that
        Joe Lstiburek suggested that the addition of a smoke detector along with an shut-off switch may be sufficient to gain approval from code officials.

    2. How does not having a return
      How does not having a return lead to issues? You need to insure there’s adequate transfer through the ceiling plane (either deliberate or through existing leakage paths. Zonal pressure imbalances are very easy to test for, and correct.

      In fact, the ceiling plane should NOT be deliberately sealed beneath an encapsulated attic, except for ceilings over high moisture areas.

      Unless the homeowner just wants to pay extra (in first cost and ongoing energy costs) for a more comfortable attic, it doesn’t make sense to condition an encapsulated attic any more than necessary to avoid condensation. Joe’s advice to directly condition an unfinished attic somewhat defensive, as it should be as a general guideline. But if you build it right, no direct conditioning should be necessary.

      In new construction, there are other ways to get HVAC inside the envelope (especially if the house is designed for that), in which case there would be no economic rationale for spending all that money to insulate the roof deck.

  4. Agreed on #3. The blown
    Agreed on #3. The blown cellulose that was in my attic was terribly dirty when pulled out. It wasn’t cheap to vacuum it out but it was an important piece to do.

  5. We compromise, we remove some
    We compromise, we remove some but not all. We are in a dryer climate, Denver CO. Fairly often folks want more conditioned space,or simply more storage space, so we leave the insulation between new floor over old ceiling, for sound control. Sound principal?

  6. Allison, you said a
    Allison, you said a conditioned attic is cooler with floor insulation than without. Since the AC supply vents are all in the house and the roof receives nearly all the solar heat load, how is this the case.

    You also said humidity inside the house is higher than outdoors. Where i live outdoor dew point is 74-78F… I can assure you it is far lower inside. I am thoroughly confused by what you say today. At the very least it deserves to be explained further.

    OMG I just realized you are talking about Yankee winter problems on a day when Texas is setting records for AC use. Over 70,000 MW. Never mind.

    1. Sorry, Mark. I’ll go back and
      Sorry, Mark. I’ll go back and see if I need to clarify the text of the article.

  7. In the Northwest where foam
    In the Northwest where foam has been sprayed directly to the underside of roof decks for over a decade the building officials interpret the code to require removal of existing insulation. The code allows for insulation directly to the underside of the structural roof sheathing providing “the unvented attic is contained completely within the building thermal envelope.” This language is in both the residential and commercial codes. If the nasty old insulation was left on the floor of the attic the thermal envelope would be defined by that first layer of insulation so the roof deck insulation would be subject to much different temperature and humidity as Allison stated in Reason 1 above.

    Please, please, please remember to never ever, ever pull the trigger on a home or building air-tightening project until there’s a ventilation plan in place. This applies to any method of insulation and air-sealing, not just spray foam.

    Ventilate X 3.
    1. Ventilate during the project to suck out the old insulation fibers in the case described above as well as the vapors and odors from the new insulation.
    2. Ventilate shortly thereafter. Continue the aggressive ventilation until all odors are long gone and until any coatings are completely dry and cured.
    3. Ventilate forever thereafter. A permanent code-compliant mechanical ventilation system must be installed in every air-tight house and building.
    Building Science Corp has very good guidelines for filtered, balanced and distributed systems which will enhance IAQ and not negate all of your energy savings from the advanced insulation and air-sealing application.

  8. Great article!
    Great article!
    I’d only add two points. It’s also much easier to see where your walking.
    Once the old insulation is removed the conditioned space is a much nicer storage area.

  9. Right on point Allison. Due
    Right on point Allison. Due points during the winter will be on the foam, summertime the due point will be driven to the insulation in the floor. We teach that bringing in vented spaces into the “condition space” is called climate controlled zones. I don’t live in the zone, just want to control climate so appliances operate more efficiently.

  10. Great article, great comments
    Great article, great comments. Thanks, everyone.

  11. Well, I hear all of these
    Well, I hear all of these engineers type answers to questions. Most have not sprayed much foam. I got started 13 years ago and about half of our business is retrofitting attics in a hot humid climate. We offer removing the insulation, 99.9% of the people do not want to pay the extra cost. Most people will receive a 30% reduction, sometimes much more by retrofitting the attic. I will concede, if you suck out the old insulation, your savings will increase., by possibly as much as 30%. Why? Well the old insulation was used to protect the living area from the hot attic. It did not work well, but it does slow it down. When the attic is sprayed with foam, now the reverse is happening. The old insulation is now keeping the cool air from filtering into the attic, providing a better semi-conditioned space. We have always said air leaks which use to be often 25% were our friend. Now I know this works better if you suck out the old insulation, but why not just drop 2-3 4″ vents into the attic; you have the extra capacity once you foam, then you are creating a forced leak that changes the environment like sucking the old insulation out. Much less expensive and works well. For all of those that say if you leave it you humidity will go up, I went to a house in Marquet, TX that was having terrible humidity problems both inside the house and in the attic. I foamed the attic, and his humidity in the attic went from 88% to 45% in 24 hours in the middle of August. He still has his old insulation and a bill that went from 1200, to about 500 per month. The proof is in the pudding, not in the theory. Many hundreds of jobs don’t lie. I cant speak for the North, but this is how it is in Texas! Greg Pruitt

    1. I,m with Mr. Puritt,
      I,m with Mr. Puritt,

      Perfection based on theory is for the rich guy. The old insulation is, by definition, in the conditioned space. Passive ventilation can easily be added as he suggests.

      Dew points will not be seen on the underside of a properly insulated deck.

      1. Morgan, it’s not about dew
        Morgan, it’s not about dew point. It’s about IAQ, and that’s for everyone, not just “the rich guy.”

    2. Greg, the moisture issue is
      Greg, the moisture issue is the least important of the three I mentioned here. Indoor air quality is the most important. If you leave dirty, smelly insulation in the attic, you’re leaving the occupants an IAQ problem. Yes, I know it’s expensive. Yes, I know homeowners don’t want to pay it. I’ve spent my time in the contracting world. I understand the realities.

  12. “possibly even a severed
    “possibly even a severed human ear.” Fan of Blue Velvet are you now?

    I have a question about converting attics to conditioned space. My attic is a nightmarish obstacle course of pre-fab trusses used to create semi-vaulted ceilings in the three front upstairs rooms. I’ve done a fair amount of work up there installing cables over the years, and I can say that access is very, very cramped and difficult. Does an attic like like mine present too much of a problem for spray foam installation? To give you an idea there is really no place in the attic where a person can stand up and movement is always hampered by the forest of truss members. Plus the trusses were assembled using those sharp-edged steel cleats so it’s danerous too. I really want to do a major energy upgrade on this house someday so I need to get an idea what what is possible. And yes, the existing insulation is filthy and absolutely would be removed. I’m in Austin, Texas so the humidity is very high here most all year.

    1. It can be sprayed but it
      It can be sprayed but it really requires an experienced two man team in the attic and someone in the truck. I might think twice about encapsulating ac wiring other than at the soffits. Being able to seal and then inspect the work at the soffits is an important consideration in a tight space.

    2. @Jeff, clearly the best way
      @Jeff, clearly the best way to find out is to invite an insulation contractor to inspect your attic. But in my experience, what you describe is not uncommon. These guys (and gals) are used to difficult access and have ways to deal with it.

      If you decide to encapsulate, be sure to do your diligence on the contractor you select. Just like any other contracting work, quality control is not something you can take for granted. However, in the case of spray foam, a botched job can lead to repair costs far beyond the cost of the foam. For example, if the foam isn’t mixed or applied correctly and fails to fully cure, it can make your family sick. Also keep in mind that some people are much more sensitive than others to the chemicals involved.

    3. You got it, Jeff! You win the
      You got it, Jeff! You win the award for being the first person to spot the reference.

      RJ and David Butler gave good advice about spraying foam in your attic. Don’t just settle on the first contractor who shows up. Verify that they do good work. Call their references. Require blower door testing. If you’re going to pay a lot of money to do it, make sure you get the most qualified contractor you can find.

      1. Thanks Allison and everyone
        Thanks Allison and everyone else too. Solid advice.

  13. A certificate of insurance
    A certificate of insurance would be a good first step in interviewing foam applicators.

  14. I have a new home. Blown
    I have a new home. Blown fiberglass walls and form on bottom side of roof sheeting. My ceilings are not insulated. I have ship lap and reclaimed wood on most of my ceilings that have small open gaps throughout. I am considering placing 3.5″ bats insulation on top of ceilings to help seal these openings. Do you recommend this? THanks

    1. Donald, fiberglass is not an
      Donald, fiberglass is not an air seal. There are ways to seal the ceiling, but first be sure to define the goal. If there aren’t any problems you might want to leave it alone.

  15. Awesome! A number of clients
    Awesome! A number of clients have noted how their attic smelled better after removal, and how they didn’t notice the smell before or thought it was normal.

    We recommend removing insulation very strongly. We are ok if clients don’t do it, but we inform them that any failures in the project are 100% on their shoulders at that point and ask them to sign saying they acknowledge that fact. Haven’t left insulation in an attic yet. =)

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