I'm looking for recommendations on cost effective ways to insulate a home that I recently purchased. The house is a modular home that was assembled in 1995. The ceiling rafters are 2x6 on 24 inch centers in the main portion of the home, and then 2x6 on 16 inch centers in the bonus room above the garage. The bonus room currently does not have collar ties installed which will be installed mostly to give me a nice flat surface to install ceiling fans and recessed lights. I'm trying to figure out the safest and best way to properly insulate a home like this without it leading to rot from condensation, etc.

I believe that the zone I'm in (Zone 5 a or b) requires at minimum, R38 for a cathedral type ceiling, please correct me if I'm wrong. I am contemplating adding 2x4 furring strips perpendicular to the existing 2x6 rafters on 16 inch centers, but the catch is that I'd like to install them with the 2x side touching giving a total rafter depth of 9 inches. I could also get by I believe with 2x2 furring and then install 2" xps foam board to the underside giving me a 7 inch cavity and a start of R10 with the foam board.

I would like to bring the entire attic roof assembly into the building as conditioned space. My main reason for that is ease of air sealing, not having to deal with insulating the kneewall, and because I have storage built into parts of the kneewall that would be tough to properly insulate.

As for insulation I've read so much regarding vented and unvented roof assemblies on story and a half homes, but there hasn't been a definitive answer on the proper path to take. If it's advisable, I'd love to use wet spray cellulose insulation in the walls and ceiling. I've had a few spray foam quotes for both open and closed cell foams, but they are fairly pricey and may not be in my price range.

Thank you very much for your help, I really appreciate it!

Here is a general outside shot of the home in question.
http://i.imgur.com/ItQjxHm.jpg

This is a photo of the upstairs main portion of the home with the existing collar ties. Rafters are 2x6, 24" on center.
http://i.imgur.com/sF85ffV.jpg

This is a photo of the upstairs bonus room above the garage. Collar ties will be added. Rafters are 2x6, 16" on center.
http://i.imgur.com/4wgSxB9.jpg

First, forget about recessed lights; they are the worst thing you can do in that room by far. Buy low LED surface mounted lights instead - they are readily available now. Next, increase the depth of your rafters to 16" and install a good air channel from the eaves to the ridge. Fill the bays with cellulose insulation and install a "smart" vapor barrier like Membrain. Tape the edges and seams of the smart membrain. Insulate from the eaves to the ridge along the slope of the roof.

Bob,

Thank you for your response. I know recessed lights are not at all the best choice for this space, but they are I/C rated and I do plan on installing 1 inch foam baffles around the lights and air sealing them to the installed drywall. I plan on installing them in the flat area that is created by the collar ties. If there is absolutely no way to properly install can lights in such an arrangement, then I wouldn't mind looking into track lighting with cfls or LED lighting. Can you link me to specific LED fixtures that you would recommend? I'm willing to look into them also, but cost is a concern.

As for the depth, because of the configuration of the home and existing walls and door ways, I'm not able to increase the depth of the rafters to 16". Anything more than another 3.5" (2x4 turned on it's side) would cause issues with door ways to two of the 3 bedrooms up stairs. If an air channel from eaves to ridge is require/recommended, I figured that I would use xps foam to create them by using 1"x1" strips against the roof decking and side of rafters, then attaching a full 1" thick slab from eave to ridge on the bottom side of that leaving a true 1" air cavity. The vent chutes that I've seen for sale at the home improvement stores are so flimsy they look like they would crush if cellulose were to pack too tightly next to them.

In my situation, would an unvented roof assembly work properly? I do have a foam contractor who recommended 6 inches min. of open cell foam without venting. His reasoning for only 6 inches is because he believes that the r value of foam at 3.6 per inch is significantly better than a similar r value for fiberglass batt.

OP - if you are going to install pots, use a vapour hat like this http://www.ndrelectric.com/_prod/vp99-en.php . Seal it to a poly membrane or as Bob suggest a smart membrane. If you cross block the location of the pots, you can tack the vapour hat flange to that. When you run the wire into the hat, do it in a corner, then squeeze a glob of acoustic caulk into the inside of the corner around the wire. You then grab the outside of the corner and squeeze it like a bag, choking it against the wire. Now slip a zip tie around the part that you have squeezed, pull it tight and you have an air tight fitting. Seal the flange to the poly ceiling barrier with two rows of acoustic caulk and you pot light will be tighter then any other portion of your ceiling.