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Insulation Discussion Forum • View topic - Need Advice on Attic Insulation Plan

Need Advice on Attic Insulation Plan

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Need Advice on Attic Insulation Plan

Postby techinstructor » Fri Jul 29, 2011 11:44 am

Hi,
I'm getting ready to insulate my attic in a new house that is still under construction. I'm struggling with trying to find the best way to gain both thermal and radiant insulation and provide adequate air flow for moisture control and ventilation. Here is a cross-section of the attic space, which is actually designed as a "bonus room."

Image

Here's some background information that may be helpful. The house is in central North Carolina, so we have hot, humid summers, and moderately cold winters. We plan to heat with wood - wood stoves in the basement and on the main floor. Chimney is inside the house on the inside of the gable end wall. We will also have electric baseboard heaters in some locations. We do not plan on installing air conditioning as we are trying to build a home that can be livable on less electricity.

The roof is standing seam metal. It was supposed to have a ridge vent installed, but we later learned that there is no ridge vent, only a cap with a small channel for air to escape every 12" where the panels overlap. There is a continuous 2" soffit vent installed in the eaves. We're considering adding a real ridge vent and wider soffit vent but that will have to be done at a later date.

The entire attic space is sheathed with T&G plywood. We plan to insulate the main floor ceiling/floor for the attic, with Kraft-faced fiberglass batt insulation, either R-30 or R-38 - the ceiling joists are 2x12. The bonus room ceiling will have R-30 Kraft-faced fiberglass between the 2x10 collar ties. The 2x4 knee wall will have R-13 Kraft-faced fiberglass batts.

My questions center on the sloped roof. We would like for the storage areas (the triangular spaces on each side of the attic) to have some protection from excessive heat and cold. One long side of the roof has a direct southern face so we also need a radiant barrier.

We had considered doing something like this:
Image

My concern with this plan is that the radiant barrier would only redirect the heat into the insulation where it would stay. I've read that the radiant barriers need air flow to work properly. What is really needed, is a baffle that is lined with foil, that could be installed in between the rafters. Then we could put the insulation on the other side of the baffle. I suspect that we'll have to fabricate something like this ourselves.

I'm open to suggestions for how to best accomplish this task.

Thank you.
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Re: Need Advice on Attic Insulation Plan

Postby techinstructor » Sun Aug 07, 2011 7:29 pm

We've modified the plan since I did this post. Here is the latest iteration along with some additional information. I would still appreciate any input or suggestions.

The attic/bonus room is considered UNFINISHED storage area, because we can't afford to finish it out at this time. We want to do that after we get occupancy, so we have to insulate the main floor ceiling as if the attic is to be left unfinished. We also want to be able to control the temperate in various areas so we think insulating and isolated the floors, then adding vents where desired would give us more control. The insulation around the attic storage areas is so that we can store items there without worrying about damage from extreme heat or cold. It should also help with the temperature in the main room upstairs, which is one long room - no partition walls.

We have decided to fabricate our own baffles using 1" foil-faced Styrofoam. This will not only give us an addition R-4 or R-5 but will also provide a radiant barrier. Now I'm trying to decide what size batt I can use to get the most value for the cost. I know that you can compress batt to some degree but you lose R-Value. Here's a new graphic showing the revised design.
Image

Which would be better for the following:

2 x 8 (7 1/4") -- R-19 (6 1/2") or R-25 (8")

2 x 12 (11 /4") -- R-30 (10 1/4") or R-38 (13")

BTW...I've measured this lumber and it's actually about 7 1/8" and 11 1/8" respectively. It just keeps shrinking as the price goes up!
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Re: Need Advice on Attic Insulation Plan

Postby SIM » Tue Aug 09, 2011 11:00 am

I can't comment much on radiant barriers as I focus more on northern climates with harsh winters. Most/all radiant barrier products do not qualify for incentives in the Midwest because they are not effective.

I can comment that fiberglass batting is inferior to cellulose for several reasons when installed in an attic ceiling. First, it gives zero insulation above the ceiling joists. Second, the lack of density and no closed air pockets allow for a lot of conduction loops within your insulation, causing the R value to become much less as the temperature drops (when you need it most).

Most people find that hard to believe, but it is an easy thing to test and has been verified by independent research. Because fiberglass has no air spaces within the strands the air movement through it is much greater than cellulose that has a lot of the air trapped within the strands (not just between). This is primarily because it is a dried plant (cellular) material. See graph and reference below.
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