Affordable Low Energy

[mrd999]

Certainly, we want an air barrier between the conditioned interior and exterior air. If our insulation may be infiltrated with air, such as with fiberglass or cellulose to a lesser extent, then the location of the air barrier affects the potential for condensation.

Warm, moist air in the attic may condense due to cold air conditioning. Warm, moist interior air may condense due to the cold attic. The air barrier above or below the insulation layer will prevent one or the other.

Air barriers both above & below such insulation would deter either condition from occurring. However, depending on climate, you may only need to consider one condition.

On another note, it would probably be easier to assure a continuous air barrier above the ceiling joists, as there aren't intersecting interior partition walls to contend with. There are surely less expensive air barrier materials than plywood available, but as it may also be acting as a floor for attic storage, it could be a sensible choice.

[John B]

Originally Posted by JohnCR

JohnB, Why would a plywood sheathed attic be better than an airtight drywall method ceiling?

JohnR

John R,
I think it could work well in conjunction with a wall sheathing system (like ZIP) because the attic floor deck and the wall sheathing would meet each other in a simple and continuous fashion.
The "layers" would align.
Sheathing to Sheathing.
Structure to Structure.
The ceiling structure(ceiling joists) would be "inside" the conditioned envelope.
Wires and fixtures would not interfere with the Air Control Layer.
The Airtightness could be Blower door tested BEFORE drywall.
The attic deck air tightness could actually be repaired in the future if need be.
Sub contractors could easily understand Where the Air Control layer is.

[John B]

Also the top face of the drywall and the bottom face of the sheathing would each have Air film R-value
Another possibility is the sheathing could have a radiant barrier.

[John B]

I am also considering the possibility of an "interior thermal mass" advantage.
The volume of the ceiling joists would now be Inside of the Thermal control layer instead of part of the thermal control layer.

[Allan Edwards]

John

Where does HVAC equipment live?

[mrd999]

So, what is your treatment of the sheathing edges? ZIP tape?

[John B]

Originally Posted by mrd999

So, what is your treatment of the sheathing edges? ZIP tape?

Possibly Zip tape or the European AirStop Tape

[John B]

Originally Posted by Allan Edwards

John

Where does HVAC equipment live?

Allan, see post #9

[JohnCR]

JohnB, Sounds like you would not have insulation in the ceiling joist, we use 15 inches or more here. Is this correct? If not, where is it? Truss roof guys SOL as it only seems practical with a framed roof and minimum of supports.

John

[John B]

Originally Posted by JohnCR

JohnB, Sounds like you would not have insulation in the ceiling joist, we use 15 inches or more here. Is this correct? If not, where is it? Truss roof guys SOL as it only seems practical with a framed roof and minimum of supports.

John

John R,
There would be a generous amount of Not-So-Expensive insulation above the attic deck.

This is not for troof truss construction.

I think I read somewhere that John Straube practically filled his attic with loose fill...
Not because he thought he needed that much... but because it was cheap.

[Allan Edwards]

John

So obviously you are talking about a vented attic in this scenario. And since I guess the rafters sit on top of the plywood on a plate rather than next to the joists, so as to really create a total enclosure. Something to think about, although if you have a lot of HVAC equipment and ducting it might become a bit crazy with chases.

[John B]

Allan,
Have you looked at the Daikin Multi Split Air conditioners?
They have a ducted version that is very,very compact and quiet.
It works well with multiple zones.
High end clients love it ...better bling than a geothermal.

I am thinking about a vented attic....but not the typical vented attic.
I am thinking controlled ventilation for the attic...depending on season and/or outside conditions.

[Allan Edwards]

John

You've got me thinking. Icynene is starting to catch on here. I like it sprayed into the walls because it is able to get into every crack and crevice, has a very high R value, and fiberglass in walls gets compromised so much.

However, when you take it up to the rafters, and seal an attic, and don't insulate between the living area and the attic, I start to wonder about this. I will say putting duct work in our hot attics, even with radiant barrier, is something that has always bothered me.

So, why not foam the walls, use even a something like blown fiberglass in the attic area. Its coverage is good, relatively inexpensive, can do high R value. Forget the sealed attic, but try to relocate duct work into conditioned area.

[John B]

Allan,
Icynene does NOT have a very high R-value!

[John B]

Allan,
Reworking the ac into the condtioned space sounds good ... but do not ignore air sealing.
There are other ways to air seal other than what I am proposing.