Closed-cell foam vs. fiberglass
dav_h
| Posted in Green Products and Materials on
I have a shop bldg and am planning on insulating it for comfort. I was thinking of using closed cell foam on walls and should be straight forward. My question comes to the ceiling insulation. I have 9 1/2 inch manufactured H beams at 2 foot centers. I have a metal roof on top of bldg. I have soffits in front of bldg for air flow.
How can I insulate the ceiling with spray foam and not loose the ability of the cavity to breathe from soffits??
thx
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Replies
Dave,
I don't know if you are insulating a flat ceiling under an attic or a sloped roof. If you are insulating a sloped roof, the roof assembly can be vented or unvented.
If you choose to use spray foam, you don't have to make a vented roof assembly. It's perfectly possible to make the roof unvented.
However, if you prefer to have vent channels, you can do that too -- even if you are installing spray foam. All of these details are up to you.
The situation is complicated if you have steel framing. I'm not sure from your description whether your building has steel framing or wood framing. One other consideration: you don't want to install spray foam insulation directly against the underside of steel roofing.
For more information, see this article: How to Build an Insulated Cathedral Ceiling.
Martin, thanks for the help. The shop has a sloped roof with 1/2" chip board under the metal roof. When you mention vent channels would these run the entire length between trusses, if so how many channels would be required for a 35' X 20' area?
If I insulate to a depth of 1" , I will still have a void of 8 1/2" from the chipboard to the sheetrock.. If I go with the unvented solution, will I just insulate the entire cavity from freeze block to freeze block?
Dave,
Q. "When you mention vent channels, would these run the entire length between trusses?"
A. Yes. Every rafter bay needs a vent channel if you are building a vented roof assembly. These vent channels extend from the soffit to the ridge vent.
Q. "If so how many channels would be required for a 35' X 20' area?"
A. One channel in each rafter bay. The vent channel must extend for the full width of the rafter bay, all the way from the soffit to the ridge vent.
Q. "If I insulate to a depth of 1", I will still have a void of 8 1/2" from the chipboard to the sheetrock."
A. One inch of insulation isn't enough. In the U.S., building codes require a minimum of R-30 insulation in Florida and R-49 insulation in Minnesota. If you install closed-cell spray foam insulation, you will need at least 5 inches of foam in Florida, or 8 inches of foam in Minnesota.
Q. "Will I just insulate the entire cavity from freeze block to freeze block?"
A. The depth of your insulation must at least meet the minimum R-value requirements of your local building code.
Insulating a shop bldg for comfort may not trigger code requirement for full bore insulation R-value / thickness.
When starting from an uninsulated base, the first few "R"s have dramatic effect, the rest, far less so.
What is the objection to applying foam directly to metal roofing?
Curt,
Applying foam directly to the underside of the roofing makes a sticky mess that complicates the removal of roofing panels (for roofing repairs) or roofing replacement.