Attic w/ HVAC Conditioning ?s

bridgettesOctober 12, 2011

Hi,

I was hoping I didn't need to post a ? but I am stumped. I plan on installing soffits and soffit vent baffles and solar powered attic fan in gable vent (cost prohibitive to do ridge went in metal roof) then putting Roxul Wool 3.5" deep btw 23" OC rafters and 2" R-Tuff polyisocyanurate board to bottom surface of rafters. The problem I have is how to insulate the area where the attic (an addition) crosses over the existing roof. Please see pics of the whole attic on my picasa album.

https://picasaweb.google.com/1040152...eat=directlink

Thanks,

Doran

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worthy

I know you didn't ask this question. But there is no logic to insulating the floor and ceiling of a vented attic. All the insulation goes on the floor, between the joists and piled above. Furthermore, an attic fan, solar powered or not, serves no purpose and is counterproductive, according to Dr. Lstiburek.

Your pic link doesn't work.

    Bookmark   October 14, 2011 at 12:49PM
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doranaatman

Sorry this link should work.
Attic Conditioning Project
Thanks for the info. There basically is no insulation in the floor so instead of ripping up the plywood I hope to make the the attic a conditioned space esp since the HVAC is up there.

    Bookmark   October 14, 2011 at 2:49PM
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Billl

If you are venting, it is not a conditioned space. You need to insulate the floor. It doesn't do any good to insulate the rafters if you are going to pump cold air between that insulation and the living space.

The old roof line seems like it would lend itself well to a rigid foam insulation.

    Bookmark   October 14, 2011 at 3:26PM
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doranaatman

There isn't cold air btw the rafter and living space. I am installing soffits, ventilation baffles, and a solar sttic fan at the peak of the attic ceiling. This is to keep all moisture from collecting against the inside of the roof. ALL insulation is going below that. Roxul wood in the lower 3.5" of the 2x6 rafter and polyiso board beneath. Does that make sense?

    Bookmark   October 14, 2011 at 5:03PM
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brickeyee

Not much R value..

Check the DOE web site for recommended ceiling R values.

    Bookmark   October 15, 2011 at 1:27PM
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doranaatman

Yea, I know. Is it going to be R13 for 2" R-tuff polyiso and 15 for the Roxul wool. So R28 instead of R38 but did you see the pic of the HVAC unit shoved right up against the rafters. Is it worth is to add another later of polyiso when I can't in that area?

    Bookmark   October 15, 2011 at 6:29PM
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Billl

This is just a really bad plan. You are going to do a lot of work for very little benefit. You need to either vent the space or seal it up tight. If you vent it, vent the whole space and insulate the floor and knee wall. If you seal it up at the joists, use spray foam and eliminate all air movement. Trying to combine the 2 approaches just doesn't work. You either want lots of air movement or no air movement. Designing for something in the middle is just asking for trouble.

    Bookmark   October 17, 2011 at 8:28AM
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brickeyee

You might consider a lightly framed enclosure with sheet insulation around the HVAC equipment and ceiling insulation for the rest of the attic.

Getting enough insulation in rafter depths is often an expensive proposition, and sometime you simply cannot get enough.

    Bookmark   October 17, 2011 at 12:18PM
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doranaatman

Thanks for the responses. I can't grasp the all or none concept from Billl though. Basically how is creating a mini attic (still equalizing intake soffit sq inches to gable vent exhaust square inches) devaluing the effectiveness of the insulation? I want to avoid the spray foam b/c the idea of not being able to find a leak on a 76 year old metal roof is almost scary.

    Bookmark   October 17, 2011 at 5:43PM
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brickeyee

Insulation only works if the air is NOT moving.

Allow air to flow and the value of the insulation goes down very quickly.

    Bookmark   October 17, 2011 at 7:47PM
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doranaatman

If I went with insulating the floor and kneewall what is the best insulation to use in each area?

    Bookmark   October 17, 2011 at 8:38PM
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worthy

What you are proposing is a vented cathedral roof assembly (See Figure 1 in linked document.) In that case, all the insulation is at the roof.

A conditioned attic also has all the insulation at the roof, but without venting.

Or, considering the limited rafter space, isolating the HVAC, as suggested above, may be the simpler approach if you can provide access to the equipment for servicing. Then treat the rest of the attic as a vented attic and insulate and vent accordingly.

I use blown cellulose in attics and am a fan of XPS boards on vertical surfaces.

Here is a link that might be useful: Conditioned Attics

    Bookmark   October 17, 2011 at 11:29PM
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Billl

"I can't grasp the all or none concept from Billl though. Basically how is creating a mini attic (still equalizing intake soffit sq inches to gable vent exhaust square inches) devaluing the effectiveness of the insulation? "

Mini-attics don't work. All the batt types of insulation are mostly air. They only work as insulation if air is not flowing through them. If you are going to use them in a vented situation, you want to keep them away from major air movement. You don't want to create a tiny channel 1-2" directly next to them where all the air currents in the entire attic will need to flow.

So, your effective options are to use a venting system so air can flow through the space but then keep the insulation by the floor so air isn't rushing by it OR insulate and seal off the rafters so there is no air movement. Trying to pick something halfway between the two just doesn't work.

    Bookmark   October 18, 2011 at 8:36AM
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doranaatman

OK, I am REALLY glad I posted. There is so much misinformation on the net.

Here's the NEW plan.

Build an openish 2x4 48" OC enclosure with removable pink foam around HVAC.

Install 4" round soffit (can't do continuous b/c soffit is wierd aluminum paneling) with baffles and leave gable open.

Remove as much old batt insulation from under plywood by cutting strips in ply or removing ply

Install vertical ply blocking off areas to spray cellulose
HOW MUCH SHOULD I HAVE SPRAYED ON TOP OF PLY?

In areas I want for storage and HVAC access lay R-27.5 worth of rigid pink foam, taping or foaming joints and ply on top of that

    Bookmark   October 18, 2011 at 4:32PM
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doranaatman

Ok, we removed most of the plywood and had 18" of loose fill cellulose blown in. Around the HVAC, we had cellulose blown in under the ply. Can we now put rigid foam on the ply and ply on that. Or is sandwiching ply btw cellulose and rigid not a good idea.

Thanks!

    Bookmark   December 7, 2011 at 7:18PM
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motoguy128

I have my HVAC in the attic. I however don;t have traditional soffits and didn't have any roof venting. Since it was already sealed, I elected to keep it sealed and spray foamed the roof deck.

I know, I know there my shingles are going to all curl and fall off. Guess what... its was unvented before which means the shingles received minimal cooling and attic was hot as all he**, and the shingles seem fine.

It's only R20, but because there's no air movement, it performs well. I also have beadboard under the roof deck so that's another R2 or so.

I actually need to remove the existing insulation in the rafters. My furnace ductwork leaks so bad that it's 85F in the attic after it runs. All that heat was going out the roof before. Now, at least 1/2 of it goes back into the house.

Just saying, it's an option. MY attic only gets up to about 85F in the afternoon in summer even when its 100F outside.

    Bookmark   December 8, 2011 at 9:19PM
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brickeyee

"I know, I know there my shingles are going to all curl and fall off. Guess what... its was unvented before which means the shingles received minimal cooling and attic was hot as all he**, and the shingles seem fine. "

A lot of his appears to originate from the shingle manufacturers without any solid data to back it up.

When the color of the shingles can result in large temperature differences, along with northern or southern exposure, the rise in temperature from not venting an attic (or insulating directly under the roof deck) does not sound like the issue it has been made.

    Bookmark   December 10, 2011 at 2:16PM
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