Air-Sealing the Floor of a Conditioned Attic
We are going to spray closed cell foam to insulate our gambrel roof, even though I am not a fan of using the product, it appears to be the best option for our situation. We are going to make the attic an unvented conditioned attic. There are supply and return ducts in this space for the second floor bedrooms so we will remove the existing attic floor insulation and spray the underside of the roof deck. We are also going to use insulated roof zip sheeting as the roof deck. We will remove the OSB that is the current deck sheeting. I have requested to have the attic air sealed after the current insulation is removed. My contractor is saying that he and the insulation installer don’t see a need to air seal since the space is conditioned. Are they correct? Everything I am reading seems to suggest air sealing the attic floor is important. We are going to have the ducts areo sealed and the HVAC contractor doing the work is also suggesting to install a 4″ return air supply line to help control the air in the attic. There was talk about even adding a supply line in at that area as well. What is the correct way to move forward? We have had a number of suggestions from contractors with no two the same. This needs to be done correctly the first time so any input would be greatly appreciated.
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The first big question is why are you replacing the OSB with ZIP??? Is it damaged or rotten and have you fixed the big problem that damaged the OSB?
I agree with the contractor that air sealing the attic floor in a “conditioned attic” is a waste of money. The fact that you think it would be beneficial make me think you are not planning on conditioning the attic to more or less the same temp and humidity as the rest of your house.
The fact is conditioned attic is only very marginally better and maybe worse than the current situation. Because your conditioned attic attic will have 50% more surface area losing heat that the current attic floor so you are down 50% if you can some how afford to keep the same R value. Getting a high R value applied to the roof line gets very expensive very fast as the only real option is spray foam and it is the most expensive way to buy an R of insulation per square foot so you are tripling the price of the product while forcing yourself to buy 50% more product. If your ducts are not leaking much into the attic and you have a fair amount of insulation on the attic floor say R30 or so you would likely use more energy after the conversion.
Have you consisted undoing the very poor choice of putting the HVAC system in the attic?
If the ducts must stay consider sealing all the gaps with foil tape and covering the ducts with layer of spray foam buried in R60 of cheap fluffy insulation.
Walta
Just for clarity for other readers, the 50% added surface area number is a rough guess and is really dependent on both the pitch of the roof and the type (I.e. hip vs gable).
Are you aware that Zip-R is not warranted for rooftop installation? My guess is that it will work fine, but it's not something typically done.
I agree with Walta that there is no need to air-seal your attic floor. I also agree that it would be better to move the mechanical system, but I understand if that's not a reasonable option.
Renovation projects are a puzzle, without a single correct answer, so it's no surprise you are getting different info from everyone you talk to.
If you are removing the roof sheathing, why do you think using spray foam and Zip-R is your best option?
There is little reason or need to seal the floor of a conditioned attic, since by being “conditioned” it is, by definition, part of the interior of the building envelope. This is a fancy way to say the attic air space is part of the home’s air space, just as any other room in the home would be. There is no need to air seal between rooms in the same home.
If you had a vented attic, that’s when you’d want to air seal the attic floor. In the case of a vented attic, the upper level’s ceiling, which is usually also the floor of the attic, is part of the building envelope, meaning it acts as an air barrier between the interior of the home and the outdoors. In the case of your conditioned attic, that air barrier has moved to the roof assembly.
I’m also curious as to why you want to use exterior rigid foam AND spray foam. Usually you use one or the other, or exterior rigid foam and cheaper batt insulation on the interior side of the roof sheathing.
Bill
I want to thank everyone for their input.
This project is basically a complete exterior remodel If you look at the pictures I have attached you will see some of the reasons why we need to do something to improve, hopefully, the problems we have reoccurring year after year.
When we bought this house we had electric base board heat and a number of window air conditioners. Due to the fact the previous homeowners had average monthly utility bills of $400 to $500 we put in a geothermal system. Since there were no ducts in the house and shy of tearing the house apart we ran the ducts through the attic to cover the second floors heating/cooling needs.
As you can see in the cross section picture the rafters are 2x6 on the flatter pitch and 2x4 on the steeper pitch This is why we are going to use closed cell spray foam to get some R value out of the space that we have to work with. That was also why we would be using the ZIP roof system to give us more R value since the 2x4 cavity will at best give R-25. I was also concerned about the rafter bridging issue. We thought we might put foam board on top of the existing deck but my contractor said that the crew couldn't frame it and that it would look right because of the large facia you would end up with
Mr. Maines mentioned that the ZIP may not be warranted for use as a roof deck. Could someone please explain why that is with that product?
Since all of this work is being done from the exterior side we will be taking off all of the sheeting to accomplish our goals.
I would like to see this house efficient and healthy to live in.
We are also replacing the windows which I have repaired a number of times and which are no longer made. These windows have rot around the framing since they never flashed anything in the 70's or at least they didn't do it on this house.
We know we probably won't recoup everything we are putting into the project but we would like to live and enjoy this place for another 10 to 20 years.
Thanks again for your input.
OK, so what you're doing is converting a vented attic to a conditioned attic. Conditioned attic doesn't need air sealing between it and other conditioned space.
If you want more insulation, you can tack ripped pieces of 2x4 to the underside of the rafters to make them thicker. You could also attach insulation board to the underside. You could even do both.