unfinished attic - new insulation

I have a 1940's house with an unfinished attic. I would like to add insulation to the roof and use the space as an office.
First, my roof rafters are a non-standard spacing of 20" on center, instead of the 16 or 24 that the rolls are sold in. Does this mean I will have to cut each roll on length??
Second, the rafters are 2*6 (actual 5.5 wide). If I am to maintain an inch for ventilation between the insulation and the roof, I can only add 4.5 deep of insulation. Is that true? For rolled insulation I would get an R-15 max. I am looking into the rigid foam board to use since it provides higher R per inch.
ANy insight would be appreciated.
Geoff Oldham ATLANTA, GA
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Can you use blown in?

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A couple of thoughts.
You should be able to get 24" wide rolls and cut them back 4"
You do need vent space behind the insulation.
Most, if not all, foam requires a fire resistant (drywall) covering.
You may be coming close or crossing over the ability of the roof to support all that extra weight and have you checked that the floor can support the weight of an office.
Also have you checked local codes for problems doing what you want?
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Joseph E. Meehan

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Only in cold climates. In hot climates like Atlanta, there is no issue there. There are no ice dams in Atlanta.

That's a good point. If he's going to do blown-in, he will need to have some kind of covering to keep the cellulose from falling out. If he wanted to do foam, that covering would need to be drywall rather than something easier to apply.

I doubt there will be a problem with the roof. 1940's houses were seriously over-built by modern standards, and 6" of insulation is not a whole lot of weight. This is especially true if the roof was re-sheathed in plywood at some point rather than having the original 1x6 plank sheathing, plywood is considerably lighter than 1x6 planks.

The weight of an office will be a different story, since that could be significant static weight (in the form of heavy office furniture and filing cabinets). Attics aren't designed for significant static weight.

The problem with local codes is that often they're set up by damnyankees(tm) who haven't the foggiest notion what proper building practices are for Southern climates. The codes used by the folks who enforce codes were written by damnyankees, the colleges they attended used books written by damnyankees, and I can't figure out how, when damnyankees account for only 40% of the population now that Texas, Florida, and California are three of the four most populous states, the building codes continue to cater to damnyankees and totally ignore the needs of folks in the Sun Belt. Attic ventilation to avoid ice dams, indeed. (What a crock of BLEEP in Atlanta!).
That said, an architect or engineer's stamp and a variance application will typically handle the code issues. Code here in Phoenix AZ says attics needs X cubic feet of ventilation, but that has not stopped builders here from applying modern building science to build houses that are better than code (by saving the 20% of the heat gain caused by heat gain through the attic-based A/C ductwork and air handler by putting them in conditioned insulated attic space rather than in a ventilated attic), it just requires more work on their part getting that engineer's stamp and variance.
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I suggest that instead of attempting to put fiberglass, you put up drywall (unfinished) or thin paneling or that new-fangled cardboard looking cellulose sheathing stuff, and blow in cellulose. That's a lot easier than dealing with oddball widths of fiberglass. The downside is that when the roof develops a leak, the whole assemblage may come crashing down in the area of the leak.

Ventilation between insulation and roof is only necessary in cold climate areas where ice dams can form on unventilated roofs. See http://www.buildingscience.com for more information. In your case, if you can do dense-pack cellulose, it will also serve as the vapor barrier in your climate (Atlanta), so you are free to pack the cellulose as densely as possible for the maximum insulating value.
I would suggest that you incorporate a radiant barrier too, since R-21 (what you'll get from the dense-pack in 6" of space) isn't a whole lot of insulation for your summer heat. There are thin foils that can be stapled to the bottom of your paneling. The shiny side should face down into the air space. This information is from Oak Ridge National Laboratory's insulation labs, since I've been blasted before by morons who can't see how the shiny side facing down will keep heat out of the house (hint to said morons: Good reflectors are also poor radiators).

One thing to think about is whether the attic joists in this attic space are sufficient to support the weight you're thinking about adding to them (in the form of the room). Depending upon how the house is built, it may or may not be easy to add additional reinforcement. I am also assuming that this house did not use trusses (few did in the 1940's) and that you will not have to disrupt any of the rafter support in order to put your new room into the attic space. That said, most 1940's houses were seriously overbuilt by modern standards (they had to, they did not have modern materials science to tell them what they needed to meet minimum standards, so they used rule of thumb and then rounded up), and should not have any real issues here.
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goldham ruminated:

A note on the radiant barrier. Yes, it is a very good idea to incorporate it. There is nothing about the fiberglass that stops the infrared energy. In the recent past I asked a scientist at my local university about plain old aluminum foil. He told me that it is the oxide that's on it's surface that does the job. And it is on both sides. It's also a tough oxide. The building industry has styroboard with foil on both sides. It is so lightweight and cheap it's easy to overlook where it's value as a thermal barrier. Some that I had purchased before had foil only on one side. It also serves as a moisture barrier, which should be between the insulation and the living space. Hope this helps.
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Owens-Corning makes a product called rafter-mate. It is a styrofoam spacer that you tack to the underside of the roof sheething before installing the insulation between the rafters. It maintains the needed gap the sheething and the insulation. My rafters are also 2X6. I used the rafter-mates and R-16. The R-16 compressed somewhat but it's worked well. Check the link below. http://www.owenscorning.com/around/ventilation/raftrmate_attic.asp

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