Open beam ceiling insulation


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Old 12-14-10, 07:54 PM
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Open beam ceiling insulation

I have a ceiling in a room that I want to insulate. It's an open beam ceiling. There's 28 1/2" of space in between each 5 1/4" rafter. Should I use rigid fiberglass insulation? What about extruded polystyrene thermal insulation board? Are there any options besides finishing it with 1/2" drywall? I live in Southern California. Should I put vapor barriers above and below the insulation or just below or just above? Would I use 1 mil plastic?
 
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Old 12-15-10, 08:31 AM
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I'm not a California pro, but here are a couple of tips. If you are in a cooling dominated climate, efforts to keep the heat out start on the outside. Now I know these comments may not be applicable, but they may be options somewhere in the future.

Shading is always good. Shingle companies offer products that reflect a portion of the heat before you have deal with it. Some roof options will use several inches of rigid foam board above the sheathing you currently. Even up north, log homes have been using 4" to 6" of rigid foam above the rafters for years.

If replacing the roof is not on the option list, then installing rigid in between the rafters as suggested is an option. Here you must decide if you want to vent from soffit to attic (peak) or seal it up tight and fill the entire cavity. Were you thinking about leaving the rafters exposed or covering over them with the 5 1/4" used for insulation?

Bud
 
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Old 12-15-10, 11:55 AM
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Re-roofing is not an option. I am looking to insulate from the inside (between the rafters). I need to know where to the vapor barrier (above or below the insulation or both) and how to create the vapor barrier (plastic sheets?).

I wish there was some sort of insulation that looks decent and doesn't drop fiberglass that wouldn't require drywall finishing, but I guess rigid insulation w/ drywall is gonna be my only option.
 
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Old 12-15-10, 12:19 PM
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The rule for a vapor barrier is it goes on the warm side of the insulation, but never both. More than one VB will trap moisture. The paradox is the warm side is on the inside when you heat and on the outside when you cool. The solution is to address the primary issue, air leakage. Since 90% of any moisture movement through a wall or ceiling travels with the air, air sealing will be very effective at preventing condensation inside those cavities.

You didn't say if you were covering the rafters or detailing in between them so they remain exposed, but in either case seal as you go. Detailing between would be the toughest as it is almost impossible to establish and maintain a good seal next to the wood. If you are covering the rafters the sheetrock would be a good air barrier.

Now, back to the VB. What do you do most, heat or AC? I'll assume AC, thus your VB goes next to the roof deck (the warm side of the insulation), then your insulation, then the sheetrock. If you use 5.25" of rigid foam board sealed on the edges, that is your VB for all effective purposes. If you have any recessed lights, make sure they are "air sealed and IC (insulation contact) rated.

I'll add a link that discusses VB or vapor diffusion retarders to help you select a middle ground as there is no perfect solution in many homes.
Energy Savers: Vapor Barriers or Vapor Diffusion Retarders

Bud
 
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Old 12-15-10, 02:29 PM
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There's only about 5 really hot days here. I don't even have an AC. For me it's all about maintaining the heat in winter time. So I guess that means the plastic goes up last. Would I run some 1 mil plastic across the whole roof or cut it to go just between the rafters?

Can moisture really travel through drywall? Without plastic sheets could moisture weaken the rafters?

so they remain exposed
Is there any kind of insulation that can be left exposed? I think you're talking about drywalling just in between the rafters, leaving perhaps 1/2 an inch of the rafters exposed.

That would probably save me from having to rent one of those drywall lifts. I wonder if I could get away with using 2.5 X 1.5 studs and 3/8" drywall in an effort to minimize the weight load on the roof.
 

Last edited by I_Will_DIM; 12-15-10 at 04:25 PM.
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Old 12-16-10, 11:08 AM
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"There's only about 5 really hot days here. I don't even have an AC. For me it's all about maintaining the heat in winter time." See, I told you I didn't know anything about ca .

Correct, the vb would then go on the inside, but you should use a minimum of 4 or 6 mil plastic.

Moisture migrates very easily through drywall.

Sealing between the rafters would be difficult. I would plan on a 100% fill and covering the rafters, that's assuming you are comfortable with a non-vented roof. With that space between the rafters, IMO, 1/2" minimum drywall. Getting it up there, that's what friends are for. Do check the load capacity to be sure the new work is possible.

As for exposed insulation, they do make a fire retardant paint for rigid foam, but I have NO experience with it. Check your local building code authorities.

A common ceiling covering for us up here in the north would be 3/4" "V" panel pine, goes well with the log homes. As long as you install a good VB underneath it, I believe it qualifies as a fire barrier, but do check.

Bud
 
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Old 12-16-10, 12:13 PM
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Besides mold, what other damage could occur? Just curious.

I'm guessing that V-panel pine weighs considerably less than drywall. Correct?
Do they make 1/2" V-panel pine that might work as well?

V-paneling would save me some time, not having to build all of the extra drywall framing inside the rafters.

Do check the load capacity to be sure the new work is possible.
Do you mean load capacity of the roof itself? How would I determine that?
 
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Old 12-16-10, 04:47 PM
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UPDATE: I was just at Home Depot and saw this stuff called InsulFoam ( Insulfoam - Versatile, Durable, Recyclable EPS ). It comes in 4' X 8' sheets at 2" thickness and it has an R value of 10. No toxic fiberglass to worry about. I think I can slide it in between my rafters and it will hold in place. I'd space some decorative 1" X 4" wood up there and wahlah.... done. This ought to save myself a TON of time. It looks like this insulfoam even has a built-in vapor barrier.
 
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Old 12-21-10, 03:27 PM
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Stop the presses: This Insulfoam stuff is FLAMMABLE! I found a piece that was coming off some InsulFoam at HomeDepot, brought it home and held a match to it. This stuff burns easily! It wasn't like I had to continually hold the match to it either. It burns on its own no problem. So unless there's a product that I don't know about, I guess I'm gonna have to go with regular rigid fiberglass insulation and drywall over it. Would have been nice to stick some stuff up there that won't burn and then not have to put all that extra weight up there (drywall and wood framing).
 
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Old 12-21-10, 08:06 PM
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Find your location, or City zone class below the map: Chapter 11 - Energy Efficiency

Then read R806.4 for your requirements of foam board thickness and IF you need a vapor barrier or not, from the site below.

If already a vented cathedral ceiling, follow R-806.1,2,3,: Chapter 8 - Roof-Ceiling Construction

The extra weight won't be that much. Depends on the size and span of the beams......

Gary
 
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Old 12-21-10, 08:42 PM
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28 1/2" of space in between each 5 1/4" deep rafter. I'm gonna add extra 2 X 3 studs at 24" mark of the drywall, plus I'll have 2 X 3's on the edges of the drywall. You think I need 1/2" drywall or could I get away with 3/8"?

I don't care much about codes. I plan on living here a long time. I once built a room addition WITHOUT a permit on an old home and later sold the home to a buyer with an FHA loan! Nobody ever questioned the addition.
 
 

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