Adding Foam Board Insulation & Plywood To Underside Of Roof - Insulation - DIY Chatroom Home Improvement Forum

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Old 05-04-2011, 06:43 AM   #1
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Adding foam board insulation & plywood to underside of roof

I have a gambrel style roof and the attic area is used for storage, so people occasionally walk around up there.

I'm about to install radiant barrier, but before I do -- is there anything I can do to protect the foil from people bumping up against it with boxes or heads/arms/legs.

I am installing from rolls and thought about installing a thin OSB or plywood under the radiant barrier (on the rafters). Would that be OK as far as code goes?

Second, if the plywood is OK, could I take that a step further and add a 1" layer of pink foam board under the plywood?

So, this is the order of materials I'd like to have:
foam board
radiant barrier
The question is whether this will be to code. The area is technically an attic, though it is used as a storage area.

Thoughts, advice?

(PS: I posted this in the roof area and was asked to repost here. Sorry if this is the wrong place.)
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Old 05-04-2011, 07:08 PM   #2
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I'll move you to "Insulation" for a better response.

Any plywoo0d or OSB product is a vapor retarder with 0.75 perm value:

If the supply inlet vents are lower than the plywood install, I would be concerned with ventilation. Without it the attic space can get full area venting or with correct conditions, cross venting, all depending on wind direction, vent sizes (NFVA), location, attic pressure, etc. The plywood may block some venting- collecting condensation to grow mold, may not, hard to see without doing a smoke test. It seems to me the air would be forced against the roof rake, not picking up as much of the inside attic air, acting as a giant baffle. If you have a leaky (air) attic, air seal it first as this is more important than fiberglass insulation against ice dams. Air travels right through f.g., though it is a good filter. If in a cold climate, warm moist attic air could condense on the cold backside of the plywood at outdoor temp. from the rafters thermal bridging and the cold outside air in the cavity. This would be a stretch, depending on the attic and outdoor temperatures, amount of moisture in the air, and plywood coverage, along with airflow, but a possibility.

Radiant barriers are effective if you live in FL or other hot climate without minimum Code value of insulation. Don’t substitute radiant for insulation. If in a heating climate, the values of radiant are minimal or even a hit against you. You want the solar gain in the winter to help with the heating loads below. Only a 2-10% reduction in cooling bill. Need an air space next to radiant barrier.

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