Good resources on designing attic insulation?

[Deleted]

DH and I are tossing around the idea tonight of DIYing our attic this summer.  Currently, the whole thing is finished with drywall (c. 1975 we think).  We can see under the drywall from the unfinished portion, and there is insulation in some places but not in others.  Where there isinsulation, it is applied directly to the roof; there aren't any of those plastic forms that allow airflow under the insulation.  The plan is to rip everything out and redo it, building in shelving systems, putting in a safety railing around the stairs, etc.  We're currently discussing what the insulation envelope should be. 

I insulated a house with my family, and we used the plastic forms underneath insulation to insulate between the rafters because we had vaulted ceilings everywhere in the finished space.  I'm thinking we should do the same thing here since we want to re-finish the attic space with drywall and I would like it to be not miserable up there, even though it's not living space.  DH is advocating for ripping up all of the plywood attic flooring and insulating all of our ceilings, leaving the attic completely outside the insulation envelope and allowing for free airflow all over the roof, at least as much as it will be free underneath the drywall.  His argument is that if we put insulation in the rafters, only the six rafter bays that have soffit vents in them will have free airflow.  Also, that would mean we are heating a smaller space, not leaking heat into the very large attic.  I guess third option would be insulating the ceilings and the rafters. 

Thoughts?  Are there any good publications or books on the best way to go about insulating?

[FoxInFiji]

LOL I love the title of this post. Only home geeks like you and me would "design" attic insulation!

OK so your current set up sounds really strange. I don't know if that's a regional thing but that's definitely not the norm. I'm guessing the previous owners planned to built it out to be a finished space but never got around it if it's insulated that way.

Your DH has it right if it's staying an unfinished (no drywall) space. You should insulate the ceiling of the spaces below as well as you can and let the entire attic be a big vented space for your new pretty roof. I'm trying to remember your venting setup but normally the outside air would enter from the soffit vents and leave out the ridgevent or side vents at the top of the roof. Here's a good link:

energy.gov/energysaver/articles/where-insulate-home

Then to use the space...that would never happen around here BTW because we require too much insulation up there...lay plywood over the new insulation and build a shelving system that attaches to the top and bottom of the trusses. Depending on how much you need the insulation might go past the ceiling joists so you might have to build up the joists to support the plywood. Like I said up here in our colder climate we need to insulate over the ceiling joists because it creates a better thermal break but in your climate I know a lot of people don't do that.

*Dork Alert* Here's a picture of our attic while we were adding insulation. Note that huge double chimney is gone now (yeah!).

The attic had existing blown in that was only 6" tall. So we added a layer of batt between the joists (which was taller than the joists) and a second layer running perpendicular to that to create a better seal/cover gaps.

[Deleted]

That is a lot of insulation

Here's our space currently: pandce.proboards.com/thread/114633/spirit-januarys-challenge-bad-pips

We've heard they did intend to turn it into living space, but it's just not practical (or possible to do within the code). That, and I'm fairly certain they were drunk when they did it. "Let's see, stick some insulation there; wait, not too much. No, fool, that spot doesn't need any insulation. Looks great; now let's slap up some drywall!"

It's completely awesome for storage though. The new venting is the reason we are worrying about this. We had 6 soffit vents added and a ridge vent, but they currently aren't going to function very well because of the existing insulation.

We'd love to put the space back as it currently exists because I think it's nice to have finished space, but the question is how to make our venting work properly with this setup.

[FoxInFiji]

Yeah it is.  We went from an R-11 to an R-49 by adding the two layers of R-19 unfaced batt.  It was the cheapest, easiest improvement we've ever done with the best pay back. 

Whoops I completely misread the first paragraph of your OP. I didn't realize it already was a drywalled space.

Well there are two camps when it comes to attic venting in a vaulted space. One is to still try to vent along the roof with the foam baffles and just compromise on the insulation thickness you can use. Two is to closed cell spray foam the whole thing and not vent the space. There is less research on the second one and it costs a lot more so people usually gravitate toward the first option. Another take on the first option is to still vent the space but have spray foam installed on top of the vent blocking. So you get a higher R-value while still keeping everything the same thickness. See the cathedral ceiling option here: www.icynene.com/residential/applications/all-applications

Here's some good reading for you on the topic:

www.chandlerdesignbuild.com/files/fhbFebMarch09.pdf
www.greenhorizon.com/files/JLC%20-%20Insulating%20Unvented%20Attics%20With%20Spray%20Foam.pdf
www.advancedinsulationinc.com/newconstruction/sprayfoam/default.htm

[Deleted]

We did consider not venting the new roof when you suggested a hot roof, but that would have severely impacted the shingle warranty, so we decided to go ahead and build in the vents while we were at it and see what we could fix re: the insulation. Thanks for the extra reading!

Perhaps the best thing would be just to leave it unfinished and insulate the ceilings since it's unconditioned and not living space. But it's cute and fairly comfortable to be up there right now for sorting, filing, etc., and I hate to give that up!