How attached are you to that area being conditioned space? Honestly, I would make the space unconditioned. It is easier to get high R-values and an easier project all around. It sounds like this is a knee wall attic space.... here's the steps:
1. Air seal the ceiling plane and any walls to living (conditioned space): use 1-part foam and seal penetrations into the house (plumbing vent stacks, electric wire penetrations, electric box cutouts to sheetrock, weatherstrip access, etc). For higher level, seal the top plates of walls to the sheetrock.
2. If there are ceiling joists bays that run under the knee walls, block them off under the kneewalls with rigid foam sealed in place.
3. Install baffles in the rafter bays of the eaves to maintain low venting. Make sure the you install the baffles so that insulation will cover exterior wall top plates
4. Insulate kneewalls with fiberglass batts, fill the stud cavities
5. Seal all seams on ductwork with duct mastic or UL-181 listed tape
6. Insulate ducts with fiberglass batts (R11-R19) if they are bare metal and won't be covered with blown insulation
7. Insulate attic with R49 blown fiberglass or cellulose
You can make your attic a conditioned space in two ways. Paying attention to details concerning types of materials used and the alignment of your thermal(insulation) and the pressure (air tight) boundaries is critical in both. This can be DIY but not if you want to do full scale spray foam (unless the space is small).
1. Vented roof deck (I wouldn't use this approach):
In this approach, an air space for venting (usually 1") is left along the roof deck. This is accomplished by ensuring low vents are present at the eaves in each of the rafter bays. Baffles can then be installed at the low venting and then insulation (fiberglass batts usually) is installed in the rafter bays so that it leaves a 1" gap between the insulation and roof deck. You need to purchase the appropriately sized batts that leave an air gap. If you are lucky and have 2x10 rafters you can buy high density R30 that is made to be 8.25" thick in order to achieve the air gap (2x10's are 9.25"). If they are 2x12' - Use HD R38, 2x8 - use R21, 2x6 - use R15, 2x4 - use R15 and tear a layer or two off the batts.
There also needs to be high venting present, ideally in the form of a ridge vent. The purpose of this air space is to dry out the inevitable condensation that will occur on the roof deck from time to time as some air from the building contacts the roof deck.
This insulation approach only addresses the thermal boundary. You need to also make the attic air tight to the exterior. You will need to install an air barrier. This can be sheetrock, exposure rated rigid foam, or even housewrap. The details here are important and it is difficult and labor intensive to achieve a good pressure boundary. It needs to be complete with no gaps from the peak to where it intersects with the ceiling material of the floor below the attic.
You could also install rigid foam into each of the rafter bays leaving a 1" gap along the roof deck and and then sealing the rigid foam in place with 1-part expanding foam. This will insulate and create and air barrier in one step. Rigid foam usually can't be left exposed in living/conditioned/mechanical spaces. You would need to cover it with a thermal barrier (sheetrock) or purchase exposure rated rigid foam (R-max TSX-8500 or similar). Check with you local code authority.
Main downsides of this approach:
- R-value is limited depending on the size of your rafters and you are limiting that further by adding a 1" air space. You can add more r-value and eliminate thermal bridging at the rafters by adding rigid foam to the face of the rafters after filling the cavities. You could also fur out the rafters to add depth and use cheaper fiberglass to gain R-value before installing the air barrier. But this is more time and $. Installing rigid foam to the face of the rafters eliminates a pretty significant source of heat at each of the rafters.... isn't very good.
- Difficult to make the boundary to the attic air tight when you also have to purposefully add venting to each rafter bay.
- Might have to add a lot of venting if it isn't already present.
2. Make it a hot roof (no venting):
Insulation that is air tight is applied directly to the roof deck. Closed cell rigid or spray foam. All low venting is eliminated if it is currently present. At least 2" of closed cell foam is applied directly to the roof deck, from the peak down to the top plates of the exterior walls. This creates an air and thermal barrier. Because there is no air between the roof deck and the insulation, it is no longer a condensing surface and no venting is needed. Colder climates might require more than 2" of foam in order to keep the bottom of the foam from being a condensing surface.
2" of poly-iso foam will get you about R12.5. You could add more rigid or spray foam to get more R-value, or fill the cavity with fiberglass batts. You will get more R-value from foam (~R6 per inch) than fiberglass (~R3 per inch). Some spray foams may be rated for exposure and your code authority may not require that they be covered with a thermal barrier as listed above.
Spray foam at this scale is not a DIY project. Installing rigid foam against the roof deck could certainly be DIY, you will go through a lot of 1-part foam sealing at the gaps and junctions. As with the other approach, you could add more foam to the face of the rafters or fur them out and fill with fiberglass in order to achieve higher Rvalues. I'd shoot fro R30 total at a minimum, but R38-R49 would obviously be better.
I wouldn't leave exposed fiberglass in the space if it is to be used for storage. I would cover with housewrap or wmp paper. In the hot roof approach, this layer wouldn't need to be air tight, just a covering to prevent exposure to fiberglass.
Downside:
- R-value is limited unless you add dept the the rafters or add insulation to the face of the rafters
- spray foam/rigid foam is expensive compared to fiberglass batts and sheetrock/tyvek
Upside:
- it seems to be easier to make the attic air tight with reference to outside using this approach
- if you re-roof in the future, you can add rigid foam to the top of the roof deck at that time in order to gain more R-value. You can't do this with the vented option because the air space makes any insulation above it worthless.
