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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 aboutdew 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 the contractors left the old insulation in place, and the leaks were at the those transitions where the old insulation got in the way. The photo below (see Image #2 at the bottom of the article) is one of the homes I tested homes 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.
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 programfor 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.
Allison Bailesof Decatur, Georgia, is a speaker, writer, building science consultant, and the author of theEnergy Vanguard Blog. Check out his in-depth course,Mastering Building Scienceat Heatspring Learning Institute, and follow him on Twitter at@EnergyVanguard.
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10 Comments
For the attic to be a wintertime dew point problem...
... it has to be really thin insulation at the roof deck, or really fat insulation at the attic floor. In US climate zone 6 as long as 50% or more of the total-R is at the roof deck, the average temp in the attic will be above the dew point of the conditioned air, and little to no moisture accumulation occurs. By climate zone zone the minimum fraction of the total that needs to be at the roof deck is roughly:
Zone 7 - 60%
Zone 6 - 50%
Zone 5 - 40%
Zone 4A & B -30%
Zone 4C - 20%
Zones 1-3 - 10%
所以R20阁楼和R20屋顶甲板isn't likely to become a moisture problem for any location south of zone 7, but putting R20 at the roof deck with R38 at the attic floor (roof-R = 34% of the total) could spell winter moisture trouble in zones 5 and higher, but it can be monitored. In many cases the thermal bypasses from the fully conditioned space undercut the effective R of the fluff on the attic floor, resulting in higher attic temps than the simple-math model might indicate.
If installing the IRC 2015 code minimum R at the roof deck, it's highly unlikely that any pre-existing attic floor insulation would push the attic temps into the moisture accumulation zone.
If the floor insulation is full of bat crap or reeks possum pee, sure get rid of it. But if it's reasonably clean there's no reason not to keep it. Although the insulation on the floor DOES provide a thermal benefit. But code compliance purposes adding the floor-R to the roof deck-R is disallowed- you can only count the roof deck R when there is a thermal bypass as large as an attic.
Pulling back the floor insulation at the edges to get the better air seal with the roof foam is legit, but it can be pushed back once the foam is in place.
Actual Problem Case (Cold Climate)
An actual unvented/conditioned attic problem case I have seen was in Zone 6 (Maine), with ~2" closed cell spray foam at the roofline, and ~R-30+ left in the attic floor. This was a "what were they thinking?!" kind of case--I can't really explain the logic of the installer. As you would expect, the dewpoint in the attic was similar to interior conditions, and in winter, the surface of the underside of the ccSPF (and framing) were cold enough to cause condensation and mold growth. The moisture problem was solved by cutting vents back in at ridge and eaves. Unfortunately, it did not do anything to solve the air leakage problems at the attic floor.
Response to Kohta Ueno
Kohta,
Thanks very much for the comments and photos. This case shows that (a) methods that seem obviously risky to those of us who think about dew points don't necessarily seem obviously risky to many builders and insulation contractors, and (b) scenarios that seem unlikely are discovered all the time in the field.
和yet...
...that ~R12" foam + ~R30 fluff would probably have worked just fine from the warm edge of zone 4 on south.
In Zone 6 removing the R30 and leaving only R12 at the roof deck would have been pretty energy piggy, nowhere near code-min!
Hello,
South Carolina modular 2nd story unfinished, and working on air sealing. Attached are 3 pictures of a somewhat cleared rafter bay. I also managed to take a picture of the soffit opening. I was wondering if I should attempt to air seal both cracks seen in the pictures. Also not sure why the builder placed OSB on horizontally on the drywall, but it had been foamed previously. Somehow the pictures didn’t orient properly, sorry. The 3rd is upside down.
My overall goal is to turn the 2nd floor into a living space and condition/insulate the whole area. 1450 sq ft total with a staircase.
It seems that by removing the insulation from the attic floor in a conditioned attic, you will remove a good bit of the inherent sound attenuation that was in place. Any thoughts on how to safely reduce sound transmission through these surfaces without creating condensation issues?
Jonathan,
I assume you mean sound transmission from mechanical equipment, as the amount of insulation between you and the outdoors doesn't change, it just moves.
I'd suggest setting any HVAV units you are worried about of anti-vibration matts, which also attenuate sound.
It’s that and I’ve heard generally the floor below becomes “louder” when you remove the insulation as you can more easily hear conversations between rooms.
Ah - flanking sound between rooms, I hadn't thought of that - although unless there is some insulation in the walls between them, that path is going to be the source of much more of the sound transmission that the ceiling.
你可以留一些绝缘天花板上long as the proportion to that in the roof was low enough not to cause condensation, or you could leave it all as long as there was a large enough opening between one of the rooms below to keep the attic warm.
Thanks for the insight.
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