Roofing Roof Insulation, warm moist air, roof insulation

Roofing Roof Insulation, warm moist air, roof insulation

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Expert: Ron Haynes — 11/20/2006

Question

This has been extremelly helpful, and I have two more questions to complete my install. I am considering adding in the reflectix bubble type insulation, with my fiber glass insulation I am installing. The question I have is should I install the fiberglass (pink) first, then the reflectix or the reflectix on top of the pink insulation. Second, with this combination, would creating the air flow give me better insulation, or not having it really not make much difference at this point. Noting the earlier comments about ice damining to which I have no concerns, but I am worried about conserving heating and cooling.

Notes:

The celing is unfinished and no inslulation has been installed. The roof is on.

Thanks again.

The text above is a follow-up to.

——Question——

I have an existing barn, that I am converting to a workshop. It has a metal roof with 2×6 construction. The roof seams were never sealed, and no soffits were installed. I have changed the inside to be a cathedral ceiling. My question is, how do I insulate the roof and provide air flow?

Thanks.

——Answer——

You would have to create an insulation cavity with an air space for free air flow from eave to peak, and with vents at the eave/soffit and ridge area, if you want to ventilate this space. The air flow path needs to be above the insulation plane. However, if the cavity is filled with insulation, with a metal roof you really have no need for ventilation except as related to energy conservation. That is to say, if no warm moist air can enter the space there is no need to vent it. If in cold climate, there could be some ice dam effects as interior heat melts the snow by heat transfer from ceiling to roof and it re-freezes at any overhang there might be. If a steep gambrel-type barn roof this is not an issue but with shallower slopes it may be.

To insulate, you need to install insulation above the ceiling between the rafters. This should be done before the cathedral ceiling is installed. There is not a good way to do this after ceiling installation. To create a ventilation air flow space, you would install baffles between the rafters and next to the underside of the roof before installing the insulation.

On a side note, you mention the seams in the metal roof were never sealed. This leads me to believe some level of moisture may enter through the seams. This probably was not a big concern with a barn but will be to a finished and conditioned (heated/cooled) interior space. Ideally, the seams of the metal roof really need to be sealed to prevent water intrusion and to serve as an air barrier.

My suggestion, install fiberglas insulation blankets above the ceiling (which at this point means remove the ceiling or the roof panels). Use a kraft-faced or vinyl-backed insulation with the facing toward the ceiling. This will prevent any interior moisture from migrating into the insulation. Any moisture that might migrate through the roof panels can dissipate through the solar heating of the roof.

Ron

Follow-up response: Place the insulation in the rafter cavity so that the insulation facing will be against the top side of the interior ceiling gypsum board.

Answer

Michael,

The reflective property of the Reflextix is intended to reflect heat in desired direction. So, if the main concern will be to keep the heat out of the interior space, the Reflextix should be atop the pink insulation in the cavity. If you are in a cold climate and want to reflect interior heat back to the interior, then the reflective facing should be turned toward the ceiling on the underside of the pink insulation.

You may get some real benefit if used atop the pink insulation (keep heat out of the space) but I think the benefit will be minimal otherwise.


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