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Jeremy Hillary Boob, PhD 07-31-2009 10:22 AM

rafter insulation question
 
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I'm enclosing a small entryway area and I'm ready to insulate the ceiling, which you can see in the attached picture. I believe that the roof has a small bit of styrofoam insulation (at best, R-11) but I'd like to have more insulation than what is there. As I understand it, I should allow air to circulate between the insulation I add and the roof sheathing. I'd like to maximize the insulation, so my questions is, how important is the air flow? Our climate is fairly mild, with a few summer days in the low 90s and the coldest winter days in the upper 30s, and almost no direct sunlight on the roof (due to tall fir trees and redwoods). In other words, would it really be such a bad idea just to fill the rafter spaces up with fiberglass insulation and not worry about venting? That way, I could get the added benefit of whatever insulation is already built into the roof. Also, would styrofoam insulation be any better/worse than fiberglass in this situation? Thanks.

Scuba_Dave 07-31-2009 01:53 PM

The venting also allows any moisture/condensation to dry out
So its more then just a heat issue

Maintenance 6 07-31-2009 02:11 PM

Eliminating warm moist air from the cold side of the insulation is the primary reason for venting. The roofing material provides a barrier that is impossible for moisture to travel through from above (obviously) and below. If you insulate tight to the bottom of a roof surface, any warm moist air that penetrates through the insulation will reach the underside of the cold roof surface and have nowhere to go. The latent moisture in this warm air will condense and drip back into the insulation. You need to have an inch and a half of air space between the top of the insulation and the bottom of the roof deck. This air space needs to have relief in the form of ridge and soffit vents to promote a chimney effect that keeps frsh air circulating to prevent any condensation issues. The situation you describe with temperature swings well below dew point and little heat on the roof from sunlight makes this a roof system that will condense moisture quite easily.

Jeremy Hillary Boob, PhD 07-31-2009 03:25 PM

Quote:

Originally Posted by Maintenance 6 (Post 309079)
Eliminating warm moist air from the cold side of the insulation is the primary reason for venting. The roofing material provides a barrier that is impossible for moisture to travel through from above (obviously) and below. If you insulate tight to the bottom of a roof surface, any warm moist air that penetrates through the insulation will reach the underside of the cold roof surface and have nowhere to go. The latent moisture in this warm air will condense and drip back into the insulation. You need to have an inch and a half of air space between the top of the insulation and the bottom of the roof deck. This air space needs to have relief in the form of ridge and soffit vents to promote a chimney effect that keeps frsh air circulating to prevent any condensation issues. The situation you describe with temperature swings well below dew point and little heat on the roof from sunlight makes this a roof system that will condense moisture quite easily.


OK, got it. Then how about using sheets of styrofoam insulation? It seems that would make it easy to get a 1.5" gap, since it comes in 2" thickness and I've got 5.5" of space to fill. If not styrofoam, where do I get 4" thick fiberglass insulation, or do I just use thinner stuff, or squeeze in the thicker stuff (which will certainly reduce the R value...)? Thanks.

Scuba_Dave 07-31-2009 04:19 PM

With 2x6 rafters I'd use R19 & rafter vents
If you want more insulation then put foam board over the rafters, then drywall

Foam board is between R4-R8 per inch of thickness
How many inches is there on the roof?

http://roofing.owenscorning.com/imag.../raftrmate.jpg

Gary in WA 07-31-2009 08:38 PM

I would run this patio conversion to living space by your local Building Department. I presume you are just framing right over the existing slab, which could easily give you problems after finished. Door binding from settling with extra weight added, cracks in slab due to loads with no footing, whole slab settling lower than existing house at transition area, not to mention frost heaving if in a cold climate. Your window will look odd that much lower than the door.

The electrical connections, switches, and receptacles are condensing moisture as the building is not closed-in before wiring. A safety thing, in all the codes in the States. The switches are live ? ready to shock someone when the dew/moisture covers it and the hot wires. Wires at boxes require a staple within 8" of all boxes. At the very least, get an electrical permit and inspection so you don't kill someone.

The rafters may require 2x8 thickness for required R-value for your local area. Hopefully you used a pressure treated bottom plate and an IC (insulation cover) can light. Most building departments require a solid header at your exterior door/window unless shown otherwise on the approved plans. Heat is required in this room.

Take another picture from a different angle, I just want it safe for you and your family. I am trying to help you meet the very minimum safety code required, for a conversion without a permit. Be safe, G

Jeremy Hillary Boob, PhD 08-01-2009 10:23 PM

Quote:

Originally Posted by Scuba_Dave (Post 309131)
With 2x6 rafters I'd use R19 & rafter vents
If you want more insulation then put foam board over the rafters, then drywall

Foam board is between R4-R8 per inch of thickness
How many inches is there on the roof?

http://roofing.owenscorning.com/imag.../raftrmate.jpg

Thanks. So, I've seen the recommendation that for this application, unfaced insulation should be used, with a plastic vapor barrier. Is that correct?

Scuba_Dave 08-02-2009 08:19 AM

Do you have any vapor barrier at all right now?
You only want one
I prefer to just use the faced insulation on anything like a cathedral ceiling

My joist are for R38-C insulation (10.25" thick) - the C is for cathedral ceilings

They also have R30c insulation - 8.25"

Per Owens Corning:
Reg R30 is 9.5" thick
Reg R38 is 12" thic


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