Question about insulation of rafter baffles

[Adam Baker]

Getting ready to insulate the roof of our new garage, and I have a question. We're using Owens Corning Raft-R Mate rafter baffles running to the peak, w/ a vented ridge cap installed. Rafters are 24 in on center, top chord of the rafter is a 2x6, and the rafter baffles are approximately 2" thick.

We plan on fiberglass insulation, should I get insulation meant for 2x4 walls or 2x6 walls? I'm assuming 2x4 so that the insulation doesn't get compressed to staple in place, but I'm having a very difficult time finding insulation meant for 24" OC for 2x4 walls. I'd like to go w/ roll insulation so that it's a single piece of insulation from soffit to peak, but most of what little I can actually find, are in pre-cut lengths either 48" or 93" long

 

[K'ledgeBldr]

“Insulating roof…”
Open ceiling?
Is this going to be “conditioned” space?

There are specific requirements for insulation in this situation- the amount is based on geographic location (???). Or, is this just a way to reduce radiant heat?

fill-in some blanks and there will be answers.

 

[Adam Baker]

The whole garage would be considered conditioned space. There is a loft above the main garage space that is used for storage and my mom wants that area to be temperature controlled. Located in North Carolina.

 

[The Cobbler]

Neither R-12 (2x4) or R-20(2x6) is really suitable for roof insulation. it will help some, but you should come up with a plan for higher R value in my opinion .

 

[Adam Baker]

Neither R-12 (2x4) or R-20(2x6) is really suitable for roof insulation. it will help some, but you should come up with a plan for higher R value in my opinion .

Any suggestions?

 

[Toolfool]

You say, "top chord of the rafter is 2x6". Do you have rafters or trusses ? Different situations. Photos would help.
Also, the baffles you referenced are designed for blown-in insulation.

 

[nmk_61802]

suggestions

R13 x 24" craft faced rolls are typically used on the bottom chords of trusses as a starter, prior to topping with blown insulation to the desired R value.

I don't know if Menards extends to North Carolina:

https://www.menards.com/main/building-materials/insulation/insulation-rolls-batts/r-13-ecoroll-trade-r-13-kraft-faced-fiberglass-insulation-roll-23-x-32/510495/p-1444437011375-c-5780.htm?tid=-3757476578405557690&ipos=5

It sounds as though you are extending the baffles from the soffit to the peak for a continuous air channel, then applying the insulation to the under the roof deck.

It appears that North Carolina's energy code requires R38 (with some exceptions) when conditioning the space. If you are insulating below the roof deck, my suggestion would be to do 5.5" of closed cell spray foam at 6.5R per inch for 35.75R. Spray foam does not require an air channel as it does not allow air to pass thru and condense.

If not conditioning the space, or not regularly I would not worry too much about it, and do whatever fits within the budget. Just remember, you can't go back and add insulation easily, and it is one of the few things in a build which will eventually pay for itself.

 

[K'ledgeBldr]

Suggestions?
I'll assume you're referring to attic trusses and the top cord is 2X6. Even if they're on 24" centers you can still get rolled batts in 23" widths.

There's two ways to achieve a code compliant "vaulted ceiling"- you can scab onto the bottom of the top cord to achieve a depth that will accommodate a baffle vent and enough batt insulation to get to R-38; or use a hybrid system that uses materials with higher R-values.

With some cost restraints I would do a hybrid system as follows- with the 5-1/2" cavity add baffles in every cavity from the kneewalls to the ridge, then UNFACED R-13 batt (unfaced because you don't want a double vapor barrier scenario), then over the top (bottom) of the cord install 4" foil-faced (to the inside-conditioned space) polyisocyanurate panels. Foam the **** connections and seal the seams.

You can install drywall right over the polyiso panels with glue and 5" screws.

The combination of airspace, batt insulation, the polyiso, and the drywall, will yield almost a R-40. You can obviously yeild a higher R-value with thicker panels- but they get real expensive. The only thing that would help from a radiant heat gain perspective would be having radiant barrier decking.

I'm not a fan of spray foam directly to roof decking- if for some reason you were to "spring-a-leak" you wouldn't know about for probably years. Guess what's happened to your framing in that time!???

 

[CombatNinja]

Any suggestions?

Yeah, as K'ledgeBldr suggested, just add furring or 'scabs' to the bottom of your top chord to gain depth. If it is framed with 2" x 6" already, adding a 2" x 4" on edge would give you roughly a 9" cavity to work with. If you really are conditioning this space, you need to get it to at least R38, preferably higher. Anything in that R20 and lower range will be a waste of time. Lots of good suggestions here, especially K'ledgeBldr above with the polyiso panels. Big drawback there is cost. If you want to do this economically, rolled fiberglass is the way to go. also agree with him on avoiding spray foam. It is expensive, toxic and you will never know if the roof is leaking until massive damage has occurred.
I don't know the scale of this project, but strongly consider making your own baffles on site with 1" thick rigid foam board. It provides vastly better airflow than those cheap commercial baffles and you can get R6 from it right off the bat. I did it in my 45' x 13' bonus room when I finished it off and it cost me a day and a half in time but it works so much better and allowed me to get to R36.

 

[jkuro]

I believe the OP said he has rafters not trusses. A top or bottom chord is found on trusses. If the Op does the work himself a permit may not be necessary. He could then add r13 with the rafter baffles to the existing ceiling.
https://www.homedepot.com/p/Owens-C...-Insulation-Batt-23-in-x-93-in-BF11/202585859
If you scab a 2x2 onto each rafter you could use r19 with the rafter baffles.

Owens Corning R- 19 Faced Fiberglass Insulation Roll 23 in. x 39.2 ft. (1 Roll) RF41 - The Home Depot

Visit The Home Depot to buy Owens Corning 6-1/4 in. x 23 in. x 39.2 ft. R19 Kraft Continuous Roll Insulation RF41

www.homedepot.com

This may not meet code but some insulation is better than nothing.

A more expensive option would be to put up a ceiling in and blow r38 in. Put some flooring in the attic space and use it for light storage.

 

[Voi]

I don't know the scale of this project, but strongly consider making your own baffles on site with 1" thick rigid foam board. It provides vastly better airflow than those cheap commercial baffles and you can get R6 from it right off the bat.

I agree. Site built baffles are often recommended when needing a continuous run from eave to ridge. It is also often recommended that they be deeper than the stock baffles when running from eave to ridge.

Four of the baffles the OP linked to are about ten bucks and more or less equal to about what one could build with one sheet of rigid foam.

It would be more labor but it's the only way I would do a continuous ventilation channel.

I bought a sheet of high density expanded polystyrene recently. The stuff rated for ground contact. It was inexpensive and nice stuff. I use it as a base when cutting plywood with my track saw. I think the 2" thick was like $15 on sale. I'd look into something like that.

Or maybe even re-claimed polyiso.

 

[rebelranger]

After R19 it's a point of diminishing returns. Sorry look at the EPA charts. If this is your forever home and you use everyday, get the highest R value as possible anything else, just get to R19 and air seal.

Code for insulation is so behind vs technology it's not funny.

 

[K'ledgeBldr]

After R19 it's a point of diminishing returns. Sorry look at the EPA charts

Is this real disinformation or misinformation?

 

[rayra]

Is this real disinformation or misinformation?

lol, you asking it about the poster, or the garbage that the EPA promulgates?

 

[rebelranger]

I'm no expert but here is a great link. https://www.energyvanguard.com/blog/the-diminishing-returns-of-adding-more-insulation/

I also agree the EPA is questionable at times

 

[dfiler2]

That article is only dealing with walls, are you suggesting it's the same for ceilings? Most of your heat loss is through your ceiling and it does require a lot more insulation.

 

[K'ledgeBldr]

I'm no expert but here is a great link. https://www.energyvanguard.com/blog/the-diminishing-returns-of-adding-more-insulation/

I also agree the EPA is questionable at times

That article is specifically about walls. The OP is dealing with a “vaulted ceiling” scenario.
Allison Bailes phd, “is a royal pain in the ***!”; according to Joe Lstiburek. HE is THE leading authority on this type of subject matter.

I have actually met/know Allison from back when I built a few homes that required “Earth Craft Home” certification. It was an energy conservation program here in the ATL. I believe Allison is still here in the Atlanta area.

 

[Steve W.]

I get the impression that the OP is trying to do pretty much what I did.

My 24x30 has attic trusses that give a room that is 10x30. Top and bottom chords of the trusses are 2x6. I used similar baffles, then kraft-faced R-13 insulation (not fiberglass) and OSB to cover. I know it's not code compliant for a living space, but it's mostly used for storage. I have another layer of R-13 in the bottom chord, so there is some additional protection for the lower level. Walls in the lower level are just 2x4, so they have R-13 insulation and 7/16" OSB on the inside and outside.

It's far from perfect, but it's a LOT better than my house, which was built in 1957.

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