If adding insulation seriously affects the use of the space, you probably need a dormer.

Some energy codes allow the use of less insulation in the roof if the insulation is full depth above the wall plate.

Though the question wasn't posed, it's important to check that the flooring structure of the attic is sufficient for occupation as living space before converting the use.

Thanks guys. I did a Google search on "thickness of r-38 batt insulation" and it said it would be approximately 6.25" thick. Everywhere else I saw estimated 12" for batt material. I knew something was off or I misunderstood. It looks like spray gives the highest R-value at the lowest thickness.

Regarding foam board, is there a cross section photo that I could reference to see how it's installed? I'm not sure I understand. Could it not be installed directly below the sheathing, between the rafters and then sprayed with foam?

I've got at least 7' headroom over probably 75% of the space but am concerned about dropping the ceiling further over the shorter knee wall areas as those will be under 7'. There are 3 window dormers located along the knee wall so should help a little bit.

Yeah, the flooring joists are all 2x6 so will definitely need to be beefed up.

If the roof is at or near the end of its useful life, it will be easier and cheaper to add insulation at the top than messing around with the interior.

However, adding rigid foam board below is also possible. Extruded polystyrene (XPS) at R5 per inch and polyisocyanurate (ISO) boards R6 per in. are what would be used. Layer them irregularly using shiplap boards if they're available and tape the seams with Tyvek Tape or equivalent.

If you're investing in spray foam, it would work better going in first, as it will cover all the irregular gaps; then follow it with friction fit batts, or netted insulation.

Spraying closed cell foam (R value 6.5/inch) directly between the rafters (without using foam board) is fairly common these days when you want to keep the attic within conditioned space (not a vented attic).

In most areas you must cover the foam with sheetrock or other fire/heat barrier material, but you will need that anyway if its finished space. I would get some estimates for spray foaming the ceiling if that thickness of foam will meet your needs. It will be relatively expensive, but seems to work very well.

Bruce

If headroom is the issue I would add 3/4" strapping (you need it to level the rafters anyway) and then spray foam the rafter cavities flush with the strapping for R40+. The foam will be more efficient than R40 fiber batts.

Make sure the rafters and floor joists are adequate to support the new loads.

The best method is Worthy's suggestion of insulative sheathing on top of roof deck. The worst would be batts as they allow air movement, are rarely installed to fill entire cavity and do not address thermal bridging of rafters. JDS's suggestion is a good middle ground because it would fill the cavities, address air leakage and provide a small thermal break at the rafter edges.

The biggest problem with closed cell under roof deck is that leaks can be more difficult to detect, trace and fix. It's also rare for closed cell to be trimmed flush with furring or framing which means multiple lifts or incomplete cavity fill, resulting in below code minimums levels.

Kudos for researching and striving for code minimum roof insulation! It's sad that so many building new homes are skimping on this detail.

http://www.southface.org/wp-content/uploads/2016/07/georgia-code-iecc-appendices-2009-1.pdf

page 6 shows detailed information on air sealing attic kneewalls

and between floors.

this is the ticket to making this bonus room in the attic livable

and affordable to cool in our hot hot summers.

these bonus rooms are surrounded by extreme attic temps on

sides, top and bottom. by foam insulating the roof line, you are

controlling one part of the issue.

by detailed air sealing as defined in pdf file attached, you can

continue air sealing & insulating walls and between floors.you can diy or opt to foam insulate walls & between floors

of the bonus room.

if you foam insulate, price the entire roof line of the house (creating an unvented attic), vs just foam insulating the

bonus room roof line/knee walls etc. if entire roofline

of house is insulated then knee walls will not require

additional foam.

while adding foam sheathing on top of roof is very cost effective,

it is only so if roof is being replaced, as stated by worthy.
itrw, roofing companies (at least in my area) would have no clue as to

how to properly do this, so an education process would be

included in this scenario. this education process also involves

a learning curve of installers, usually resulting in me having to

teach by example of working with the crew to achieve desired

result and trouble shoot problems. (this is my reality...lol)

I've never had a client opt to go the foam sheathing

on top of roof decking route, vs just foam

insulating the roof line of the house.

my standard info for my clients is open cell foam not closed

cell. granted less R-value per inch as compared to cc foam,

but when the roof leaks, the oc will allow the moisture

to exit, instead of trapping it next to roof decking.

best of luck OP and if you decide to diy or have

any questions please post them.

Thanks everyone for the replies. Our city was hit by a massive hail storm last spring. We got a new roof out of it. I wish I'd known then about trying to pay extra to have the roofers take the time to install foamboard insulation above the roof. That option isn't possible now.

Anyway, based on the suggestions above, it sounds like the following:

1. Use either oc or cc spray foam (not sure which to choose) directly against the roof sheathing to fill all areas/spaces between the rafters.

2. Add 3/4" strapping to the rafters to help with leveling the rafters and provide some thermal bridging. Make sure the spray foam extending level with the outermost edge of the strapping.

3. Drywall.

A couple more questions. Is there any reason to try adding some foam board in there somewhere or just all spray foam? What type of strapping material should be used (i.e. 1x3 spruce lumber)?

As mentioned previously, the rafters are all 2x6. There is one significant portion of the roof that has several 2x4 support boards nailed to some of the rafters about halfway up that extend down to a load bearing ceiling plate on the opposite side of the prospective space. Obviously, those will need to go. In that case, I'll need to beef up the rafters to add that support back. I'll likely be sistering more 2x6. Doing that though will cut down on the space available for filling in with spray foam insulation. I may just try taking and posting a photo.

Use either oc or cc spray foam (not sure which to choose) directly against the roof sheathing to fill all areas/spaces between the rafters.

According to Building Science Corp.'s Dr. Lstiburek, either will work in your warm climate. One doesn't cause more problems with finding leaks than the other.

Other methods mentioned--such as flash and fill with batts or loose fiber or foam boards may be cheaper on a diy basis but will be less thermally efficient.

Thanks Worthy for the informative links.

As I'd mentioned in my previous post, I'm going to need to beef up my current roof rafter system. Not only are the rafters all 2x6 but they're spaced at 24" oc AND supported by multiple kickers place roughly halfway up the roof pitch. The first photo below shows what I'm trying to explain. When I remove these kickers, I'm planning to add more 2x6 arranged at either 12 or 16" oc.

However, when I add more rafters it decreases the available insulation space and also increases thermal bridging. Both of those factors will decrease my possible R-factor right? Is there anything I can do about this?

I attached a second photo for the heck of it. It shows the underlying roof from another angle and also shows the knee wall, which also has kickers I'll need to address.

The only way to stop thermal bridging is to take the roofing off, add a layer of rigid insulation, plus strapping or plywood and renail new roofing.

There's various options for strengthening your 24" OC roof framing for the span: 1) add rafters at 12" OC; 2) sister deeper rafters based on span. Plan on proper bracing until new rafter are in place.

Given your situation, sprayed insulation looks to be the best alternative for any given R-rating if you can afford it.

Sistering obviously reduces bridging. Now that I see it, I agree that spf is the best approach.

Your safest bet on the framing changes--roof and floor--would be a stamped plan from a structural engineer/architect. This is not a small project.

As well, you will have to make changes to the HVAC system to ensure proper air handling.

However, I wonder if this change of use is even permitted under Dallas zoning. Though I suppose you could claim it's for storage, not living space.