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Discussion Starter #1
So I am thinking about installing central air, and want to put the air handler in the attic. I want to make the attic conditioned space. I have a very simple roof.

I want to vent the interior roof deck, and then insulate under it. My roof rafters are only 2x4's.

I was thinking I would build custom rafter bay baffles out of rigid foamboard insulation. I would cut 1"x2" strips of the rigid insulation, run them from the soffit vents to the ridge line in the corners of each rafter bay. Then cover the strips with a .5" thick piece of hardboard insulation. Basically having a closed 2" high vent running the entire width and length of each rafter bay from soffit to ridge line/vent. This way the the roof decking is getting plenty of air under it. I would do my best to air seal all joints of this custom built rafter bay channels.

Then... I was going to cut 4" thick strips of the rigid insulation and tack them to the roof rafters. Basically extending the roof rafters into the attic, making each rafter bay another 4" deeper. So the total rafter bay depth would know be 8", minus the 1.5" for the vent channel. This leaves me 6.5" for fiberglass bats to fill the the rest of the rafter bay.

Which apparently gets me nowhere near the required R-value for the roof in NJ(zone 6) of R-33.3

So seems like I am totally off target here. Seems like no real good way to do this?

Any thoughts/suggestions as to how I can get my attic/roof deck insulated to what it needs to be so that I turn the attic in to conditioned space for central AC air handler and duct work? I really don't want to put it in the basement due to ceiling height issues in the basement.

Thanks! The new guy! :)
 

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First, I would determine the actual code for a non-living space as you propose.

Also, be sure your trusses are rated for that extra load. What dimension are the floor joists?

What you are suggesting is really an insulated vaulted ceiling or cathedral ceiling as opposed to a conditioned unvented attic. Here is a related article. http://www.greenbuildingadvisor.com/blogs/dept/musings/how-build-insulated-cathedral-ceiling

The ½" of rigid isn't quite thick enough for the amount of additional insulation you want to add to the inside. There is guidance that tells us what thickness is needed to ensure that the inside surface of the rigid foam never drops below the dew point, a winter condition but it sets the minimum. The guidance is about walls so not sure where to find anything on vaulted ceilings. But the objective still applies, no condensation.
http://www.greenbuildingadvisor.com/blogs/dept/musings/calculating-minimum-thickness-rigid-foam-sheathing?utm_source=email&utm_medium=eletter&utm_content=20111005-r-value-or-thermal-mass&utm_campaign=green-building-advisor

Starting with 2x4 (3.5") material it is difficult to achieve the code minimum insulation levels. Verify what the code official wants to see it may be less since it is not living space.

As a note, have you looked at a mini-split system?

Bud
 

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There's hundreds of thousand of homes far further south then you with the air exchanger in the attic that work fine.
Seal the ducts and insulate them and save yourself a ton of money and time.
Make sure to have them install an auto shut off so if the emergency drain pan starts to fill with water it shuts the system down before it floods the ceiling below.
 

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Discussion Starter #4
Bud,

Thanks for info. Your links are spot on.

I have been considering the split ductless systems, but that still means running the coolant lines through the super hot attic. I know the coolant lines typically have some like pipe insulation around them, but is that enough? For some reason I am extremely concerned with losing paid for "conditioning" to the uninsulated attic.

I could build a small enclosure to run the coolant lines through and heavily insulate the enclosure.

But.. I am not sure how nice the split duct systems really work, how the look, do you feel like fan is blowing on you, etc... But I think this is a topic for a thread in a different group! :)
 

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Discussion Starter #5
Hi Joe,

Yeah, I realize it is done all the time, but everything I read by "the experts" says it is just a huge waste of money, extremely inefficient, can lead you to oversize the system, etc...

Maybe my plan of trying to do what I am talking about is so expensive that it would take years and years to break even vs the increased electric bill of running everything in the attic as is and just trying to keep the ducts sealed and insulated well?

I could see that...
 

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Joe is correct and if you go that route you have a big advantage, you get to do everything right from the start. Air sealing an existing system can be a pain with less than 100% results. There is no reason why a new system with careful oversight wouldn't be perfectly sealed. Then, insulation can benefit from the same new factor as the air sealing. Your bottom line could be very little added expense.

Add to that, if you aggressively air seal and insulate that attic the size and operating costs will drop further.

There is one big advantage of having the cooling system above the desired space, easier to blow the cold air down than up from the basement.

As for the mini-splits, they seem to be popular and even see many way up here in Maine. I will be pricing one for a family member this week. As for the coolant lines running through the attic, just add more insulation to them.

The traditional open and ventilated attic has proven it works well. If you were to convert to an insulated cathedral roof you should also condition that space and there would be an energy cost to doing that, especially if you don't reach a high level of insulation. Technically, you would want to remove the current insulation as leaving it could create problems.

Bud
 

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Discussion Starter #7
So with the input from Bud/Joe and all the research I am doing it is starting to seem like the best solution will be to just do a traditional central AC in the attic and seal and insulate the heck out of the ducts, and leave the attic as a well ventilated attic.
 

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I'm a retired energy auditor and have always advised home owners to invest their time and money where it will do the most good. What you will save in time and money with the direct installation as opposed to converting that attic to an insulated space could be invested elsewhere to save far more than what you would have achieved up there.

It is hard to say where electric prices are going, but you could take those savings and invest in a pilot solar project. I think we will eventually see solar panels on every home in America.

Best,
Bud
 
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