Should we make the attic a conditioned space? Should we hose it down with several inches of spray foam and close off all the vents, maybe even remove the existing insulation sitting above the ceiling? This is a building science topic of debate on many different levels (economics, potential moisture issues, indoor air quality, etc.). Especially when you are in the south, you can quickly begin to debate whether or not there is a likelihood of future moisture problems if the job isn’t done correctly. In my opinion, all climate zones have the potential to experience indoor air quality issues with fully encapsulated the attic. Even when it’s done in the recommended fashion- meaning you have a dedicated return air vent and supply vents for the attic space. So, the indoor air quality debate asks the question, “Do you want to breathe the attic’s air throughout the rest of your house?” The answer depends on whether or not you are OK with dusting and cleaning your attic from time to time. I’m just kidding, or am I?

I find so many companies in the mixed-humid climate of central Virginia selling these kinds jobs, even in situations where the scope of work doesn’t include properly space conditioning the attic. And that scares me. The foam companies should learn to better communicate with knowledgeable HVAC tradesmen to ensure their encapsulation jobs are properly dehumidifying the attic space. If you encapsulate the space, then it must be conditioned. Yes, open-cell foam, when applied several inches thick, may have a perm rating of about 30 or so. But older homes, with moist crawlspaces and basements, in particular, tend to require the evacuation of an abundance of moisture. And when hot air rises, it carries moisture to the attic. Even open cell foam can bog down and create attic moisture issues when proper dehumidification does not exist. Especially when a fire-rated coating, like intumescent, is installed on the underside of the foam, and code requires this fire-rated paint to be. The well-respected building scientist, Joe Lstiburek, has published an abundance of information on the topic of encapsulation. I recommend you find his work if you choose to continue your research beyond this blog. A lot of the information I’m providing here comes from my studies of his work. After all, he is my building science hero. I remember watching VHS tape interviews of Dr. Lstiburek back in Building Science 101 in grad school. Oh, the memories…

Let’s get back to the space-conditioning issue. Yes, if the attic has a dedicated HVAC return and supply, then it is conditioned, and this should theoretically remedy moisture issues. But, do you want to breathe what is in your attic? I don’t. When that return is sucking your moist attic air in and then distributing it throughout your home, it doesn’t seem healthy to me. Unless you intend on dusting your attic and keeping it clean- I see eminent indoor air quality issues. And so many companies cut in an HVAC air supply line into the attic and call it quits. This just doesn’t work. The HVAC system must also suck air from the attic space to properly dehumidify, plain and simple. And when you suck air in to dehumidify the attic- this air is pumped throughout your home for your family to breathe. The cost of this work is high and the return on investment (ROI) is long. Some people are willing to pay for the comfort provided by the conditioned attic space, and everyone can have their own opinion on the air quality thing. I’m not claiming to have all the answers; I’m just providing my two cents.

You see, the logic for encapsulation is a good one. Your HVAC ducts are already leaking air in your attic. And attic temperatures vary wildly, so the HVAC air handler is working ever so hard to do its job when it’s 20 degrees in February or 90 degrees in August. So, if you spray foam the underside of your attic roof deck, the leaky ducts won’t matter, and your HVAC system will have a longer life, and do its job more cost-efficiently every month. The foam encapsulation also slows down the “stack effect,” which is the movement of hot air upward through your home. But, what does this hot air carry? It carries moisture. And moisture-laden air rises more quickly into the attic due to it being lighter and less dense than dry air (hygric buoyancy). Now that you’ve sealed off your attic venting, what is required to take care of the moisture? Space conditioning. And not just the existing leaks in your HVAC ductwork. You need around .3 to .5 air changes per hour (ACH). This equates to around 50CFM for every 1000cu/ft. of attic area.

I work in Blacksburg and Christiansburg, VA, and I come in behind spray foam guys all of the time who have recommended or already encapsulated attic spaces for clients with no dedicated return or supply air. I have yet to see this insulation system installed with the proper level of attic space conditioning (back to that .3-.5 ACH). Just don’t do this, because that approach takes the logic for encapsulation right out of the mix. It is a no-go if you don’t properly size the CFM of air exchange. Moisture problems will find you- and it won’t be pretty. Especially after you paid 13 grand for encapsulating your attic. So, if you’re going to do it, do it right. Make sure the company you use knows about HVAC systems and how to size the new vents and return for your attic.


However, I prefer a different approach altogether. I prefer to seal the ductwork, install cellulose insulation, and air seal the ceiling plane to establish your thermal pressure boundary. How about a 3-4 year ROI instead of a 15-year ROI? Spot foaming is good for penetrations, double top plates, and exterior wall areas. I want my attic detached from the space my family occupies. If you’re doing new construction, then I have an easier time accepting the benefits of encapsulation. There is such a vast difference in cleanliness between new construction and a home built in 1958. The 1958 attic almost certainly has had critters in it, bugs, mice poopies, maybe a dead squirrel if you’re one of the lucky ones. Maybe a little mold, a lot of dust, and pollen that’s made its way in from the attic vents too. And there was almost definitely some kind of insulation already installed on the attic floor. Probably not much of it, but enough to leave you some nasty particles behind, even after it’s extracted before the encapsulation. I see most companies leaving that behind when they encapsulate. If they can’t easily get to the insulation and remove it, then you will breathe it in the future- and that is not good.

Back to new construction- first off, new construction encapsulation would allow you to properly design your HVAC system, and you wouldn’t already have insulation that would need to be removed from the attic floor. I’m building a house right now with an attic so clean you could eat an omelet off of it. Well maybe not, but you get my point. You haven’t spent any money on insulation for your attic floor, you don’t have to pay for the removal of this insulation, and there is no chance you will breathe the remnants of this insulation at a later date. Not such a bad idea, the whole new construction angle. But, darn it; give me an attic with a low pitch and not a huge cubic volume. Show me a 12/12 pitch on a big rancher, and I’ll tell you the economics will never add up. I didn’t encapsulate the attic of the home I’m building. It’s an energy-efficient spec home, and the financials on encapsulation, coupled with the air quality debate was enough for me to go the old school route of sealing the ductwork and installing insulation on the attic floor. Yes, encapsulation would be higher performing energy-wise. I get that. But how long will it take to pay for itself? A long time.

So what is the answer? Hmm… I guess the answer to this question is that there is no right answer. If you want to stay in your home forever, or you are designing your dream house, then consider it. But also consider the air quality issue. You can install a return vent filter on the return, but are you willing to go up that ladder every few weeks to change it? And how quickly will it clog if there is a lot of airborne debris and dust? Again, you MUST condition the space to at least. 3 ACH if you encapsulate, but it is up to you as to whether you are comfortable breathing your attic air inside the living areas of your home. You obviously know my opinion.

There are a lot of building scientists who like full encapsulation. It seems the consensus on building science issues change depending upon the latest research. So, do your research and make an informed decision. In the meantime, I’m going upstairs to eat an omelet in the attic. The best test I know of for judging the cleanliness of a space. Wait a minute; my attic has 2-feet of cellulose on the floor, never mind.


I have a whole different angle on encapsulating conditioned crawlspaces (I really like them in the right situations). That will be next week’s juicy building science nugget. Until then my friends…

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