Insulating 2x8 Ceiling Rafters

[Bull]

I want to insulate and rock the 2x8 rafters that currently form the ceiling of the lower level of my barn. That way, when I am running the heater down there, the heat will not be so able to collect in between the rafters, which is the highest point for the warm air to go.

2x8s measure almost exactly 7" in depth, right? So, R-19 seems a little too small but the next step up from that at the box store was R-30, which is too large. Am I better off with the undersized R-19, or compressing the R-30 into the space?

 

[cyamaha2007]

I was told compressing the insulation really hurts the r value.

 

[Bull]

I was told compressing the insulation really hurts the r value.

There was a pretty heated thread on here a while back in which this issue was debated, and on each side were people claiming to know what they were talking about. For an average shmoe like me, I didn't know what to believe.

 

[cyamaha2007]

From what i learned in school batt insulation works by making air pockets. If you compress it or if it gets wet it transfers the cold or heat instead of insulating it. Hope it helps Chuck

 

[70redbee]

Are you insulating the room above?

 

[Mike Miller]

When the insulation guys came to do mine they used a friction fit R21 batting between the 2x8 rafters and covered it with a white faced foil backed vinyl. Very comfortable quiet and bright.

 

[NUTTSGT]

A 2x8should measure 7 1/2". You should be able to find some R-21/22 depending on the brand. They also make a R-25 but I think it requires 8"of clearance. I'd search online for what's availble at some of the big box stores close to you.

 

[BigGMC]

Anything you do will be a huge improvement over what you have.
2x8's are generally 7.25-7.5".
R19 is what you'll want. In that cavity it will fluff up to nearly fill it. R19 is marketed for 2x6 walls but the material specs out at 6.25 inches so you compress it when installed in 2x6 walls. The fine print on the package actually mentions a reduced R value when in 2x6 construction.
There is also R-21 but it is actually thinner (approx 5.5"). In a 2x6 wall it gives an R21 rating.
I find it interesting that there are those who debate "stuffing" the cavity will not negativly affect the R value. Look on any R-19 package and you'll see the mention of reduced R value when the product is installed in 2x6 walls. That's the manuf. coming right out and saying it......

 

[Kevin C]

Compressed R30 will get you about R25 in your 2x8's. If your at 16" OC the rolls of R30 are really cheap and it makes sense. For some reason the bats cost 2x as much ( $.87 per sq foot vs $.38 per sq ft).

For $.32 per sq foot you could add a layer of 1" EPS foam at R4 on top of the rafters to reduce thermal bridging.

I would consider putting in vents.

 

[Doug B]

There was a pretty heated thread on here a while back in which this issue was debated, and on each side were people claiming to know what they were talking about. For an average shmoe like me, I didn't know what to believe.

http://www.garagejournal.com/forum/showthread.php?t=162146&highlight=compress+insulation

Lots of good data on compression in there...for your situation,I would agree with R-21 as others have said

 

[Kevin54]

Bull....what are you going to finish the ceiling off with after insulating it? Drywall or OSB? The reason I ask is that if you finish it off with 1/2" drywall, and you have stuffed insulation, it may make the drywall bow slightly.

Personally, if it were mine, I would go with R19, cover is with visqueen, then drywall or OSB.

 

[NUTTSGT]

Bull, are you closing off the stairway ? If you are not closing it off, I think the R19 (readily available) would be enough.

 

[St-rider]

how high is your ceiling? if you don't mind losing a few inches of height you can add some 2x2's to the joist and use thicker insulation.
if you drywall the ceiling you can trim the 2x2's thickness as needed to create a flatter ceiling.

 

[BigGMC]

http://www.garagejournal.com/forum/showthread.php?t=162146&highlight=compress+insulation

Lots of good data on compression in there...for your situation,I would agree with R-21 as others have said

You mean 19?

 

[Falcon67]

I would put the R19 and go on. Make the ceiling tight and it'll work better than you might thing. If you want to completely fill the space, you could use 1" of hard foam insulation (R5) plus the R19. As above, the R30 in rolls 15" wide UNFACED is cheap. You could always peel an inch or so off that but you'll be adding in a vapor barrier under it.

 

[Kevin C]

http://webcache.googleusercontent.c...ompressionChart.xls+&cd=1&hl=en&ct=clnk&gl=us

Insulation compression chart

 

[JakeKohl]

The actual fibers in fiberglass insulation are poor insulators. They actually conduct heat pretty well. The insulating values come from the air trapped / slowed between the fibers. If you compress the glass fibers, you increase the contact each fiber has to the next as well as reduce the amount to trapped air and you seriously start to reduce the insulation efficiency.

 

[38Chevy454]

Are you putting a floor on top of the 2x8s? If not I would go with even more insulation, like R-38. The insulation relative cost increase is low vs R-19 or similar thickness, labor is about the same for install. More insulation will never hurt.

 

[Doug B]

You mean 19?

Johns Manville makes a 6" thick R-21 unfaced roll insulation.

 

[Kevin C]

The actual fibers in fiberglass insulation are poor insulators. They actually conduct heat pretty well. The insulating values come from the air trapped / slowed between the fibers. If you compress the glass fibers, you increase the contact each fiber has to the next as well as reduce the amount to trapped air and you seriously start to reduce the insulation efficiency.

The chart I posted has the actual compression VS R value. FYI, if you want to get the maximum R value with fiberglass for a given cavity you will probably end up compressing thicker bats.

For 2x8's, 10 1/4" R38 C will get you R30. As noted, putting sheet rock over this could be an issue. Always test before you go too far. Worst case is you have to use furring strips at a 90° Angle to nail the sheet rock to.

 

[Al Bundy]

Why not drywall first and just use blown in to fill between the rafters?