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AC equipment in an unconditioned attic

I have some issues and a possible solution that I am looking for advice.

Live in NJ. A/C system was installed in the attic. This was back in 1999. The house is a split level. A/C trunk line runs in the upper attic, down in an interior closet and across the lower level attic. The lower level was all remodeled in the past year.

In the winter, you get cold air out of all the lower registers. My thought was this was a convection current with the hot air going into the hallway ceiling return and then cooling and dropping out of the lower registers. I had installed Airtec Ceiling registers with an integrated damper that seals shut. This solved the cold air problem but I was noticing occasional moisture on the floor below one of the registers. I went into the attic to install a new upgraded filter assembly from Aprilaire. I just did the furnace and figured I would just do this now to get the box out of the way. When I opened the filter box I got hit in the face with hot humid air. I also noticed some mold in the filter box and possibly some mold growing on the coils. Hard to tell what it is.

As I have read countless times on this site, the system should be in conditioned space. There is no possible way to move the equipment out of the attic, since there isn't any possible space inside the existing conditioned space. To top it off , the attic doesn't have great headroom. I can try and post some pics. I have both soffit, gable and ridge vents in the upper attic.

Thus, my idea was to try and build conditioned space within the attic (Middle third). If this was doable due to the headroom. My plan was to keep the roof vented. The rafters measure 5 1/2" (just need to re-verify). I know I need an air channel but unsure how high that needs to be. I would then use the rigid foam as the bottom of the baffle. I believe the foam to use would be XPS. Is the Owens Corning pink stuff XPS? Anything better? I also believe is R-5 per inch. If you only need a 1/2" air channel then I could get 5" or R-25 within the rafters. If the air channel needs to be more like 1" then I could get like R22.5. I can pack more down with foam or dense pack then with cellulose. I believe I only need R20, in my zone, to prevent condensation from forming on the interior of the roof sheathing. Not sure weight wise for the roof, which is the better option. Need a min R38 to max R-60. I would then have knee walls for the side walls. How much insulation on those walls? Same R-38 - R60 or is it like an exterior wall, even though its in the attic. trying to see if this can be done. I know I then need to cover the interior with drywall. I was going to use cellulose fr the remainder of the attic, which is why I thought dense packing between the sheetrock and the foam within the rafters. Also adds a fire retardant layer as well since the cellulose is treated.

I then still have the lower attic and the trunk line there to deal with. The headroom here is way smaller and no was to build anything in. The trunk line is pretty much attached high around the ridge vent. I would have it dropped to the joists. I have blow in cellulose here and then can have the insulation contractor completely cover the trunk line to increase the R-value and thus prevent the air from cooling within the ducts. Other option is to put the fan on the A/C airhandler, very slow variable speed, but that would cost $$ but would keep the air moving and not coolling.

Is this feasible?

Asked by todd s
Posted Thu, 12/27/2012 - 16:49
Edited Fri, 12/28/2012 - 06:12

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2 Answers

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1.
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Todd,
There is no way we can determine whether your plan is feasible without a site visit. There are too many variables.

The standard solution to your dilemma is to create an unvented conditioned attic with closed-cell spray polyurethane foam. For more information, see Creating a Conditioned Attic.

It sounds like you have many problems to address, and not much room in your attic. Frankly, it's a mess, and there is no easy solution.

By the way, ventilation channels in insulated sloped roof assemblies should be at least 1.5 inch deep, although some builders get away with 1 inch.

Answered by Martin Holladay, GBA Advisor
Posted Fri, 12/28/2012 - 04:53

2.
Helpful? 0

Hey Martin,

Thanks for the quick reply. I know this isn't an easy one. The issue with spray foam is two fold for me. One is that we had remodeled last year and for me to foam, THey would have to rip all of the soffits out to seal everything up. THat affects the aluminum channel etc holding the vinyl soffits up. Also, I have read countless stories about the smell etc from spray foaming existing spaces. Not sure if there are issues or unfounded. If the house was gutted maybe. Not to mention with the roof slop, I am not sure how easy that would be anyway.

As far as the issue at hand. I am looking for overall advise as to whether what I am thinking "could" be possible. I know there are allot of variables. Would it help if I post some pics of the space as well as the height from floor joists to the 1x making up the ridge? I did read another post about thermax and see that this rigid foam does not need to be covered for fire resistance. It was also R-6.

7" would be R-42 and 8" would then be R-48. Plus an additional 1.5" for the vent channel. That would be like 8.5-9.5" min from the sheathing or 3-4" deeper then the rafters. That should be doable from a height perspective. I am also going to move the attic steps to the center of the hallways so you come up right next to the equipment, which is at the tallest headroom point. Plan was to make the vent channels over what would be the conditioned space and thus end at the side knee walls. I then could just transition to plywood, if I wanted/needed, to carry them down all the way to the soffits. Figured use of rigid foam is not needed there. Might not need to do that anyway.

Answered by todd s
Posted Fri, 12/28/2012 - 08:59

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