Looking to finish off an attic space to make it habitable for a bedroom. Has a nice stairway going up to it.
Have a gable roof. Roof rafters are 2×4. Only ventilation going on right now is cross ventilation. There are no vents in the soffits. Very little space for them anyway.
I’ve looked up in one of Taunton’s books. It suggests putting ridgid insulation in with R50 value with no ventilation. I presume that’s styrofoam. None of the styrofoam at HD, etc. has an R value anywhere near that. This is in MN. Cold climate.
I checked in with our local inspector for any clues. He said something about putting in windwash for ventilation (means venting soffits then?) and isoneen (sp?) for insulation.
Attached is a picture of the attic.
General Contractor
Minneapolis MN
http://HandyWerks.home.comcast.net
Replies
I think what you would want is sprayed foam in the rafter bays.
Check out this type of spray foam insulation, it is meant to use with an unvented attic. The r value is 3.6 per inch, about the same as fiberglass but it seals the bays whereas fiberglass can allow air leakage. Where you are in MN I would plan on adding something to the 2x4's to get more space in the bay for insulation.
I looked up the dealers in MN and there are at least a dozen. The cost in my area is about 2 times the cost of fiberglass, mostly do to the cost of the material. Your area may have better pricing since there is more competition. Do a search in this site, you'll find some helpful posts. Also, FHB had an article on foam insulations last year (I think) and I know Journal of light construction has had a few articles over the years.
http://www.icynene.com/
Or try Demilec brand foam at a substantially lower price. There are others. Avoid biobased foams as they are mostly marketing with no actual environmental benefit over the other foam products when all factors are considered.
I think the closest you can come to getting an R50 roof is by two different methods:
1) Insulate from below with Corbond or a similar closed-cell polyurethane. They offer about R6.5/inch. You may have to put in some additional wall studs to get the depth you need, but if you put them in horizontally (i.e. at a 90 deg angle to the extant rafters) You'll also benefit from the thermal break this creates to minimize thermal short-ciruciting.
2) Insulate from below as much as you can with Icyenene or similar products. When the time comes to re-roof, simply add several inches of XPS to the roof deck, then build the new roof on top of that. Once again, you'll also achieve R50. The one thing with this approach though is to take a close look at the average temperatures in your area, the roof construction, etc. to ensure that nothing dews inside your roof. Icynene should minimize water vapor transmission when it's that thick, but I'd double-check
Also, have a look at the many free resources at buildingscience.com, they are eye-opening to say the least.
I'm not sure if your budget would allow this but if you are due for a reroof, look into adding ACFoam panels above the roof deck. The 4.5" version will give you close to an R-30 with very littld thermal bridging. The facsia would need to be redone to make up the additional thickness. The panel is made up of 4" of polyisocyanurate and 7/16" OSB. You can then an R-15 fiberglass batt in the rafter bay to increase the R-value at a low cost. The result will be a system that performs better than most systems that claim an R-30 while disregarding the thermal bridging of the framing members. Check the engineering of adding the additional weight. It is not much heavier than other insulations that you could add for the required R-value. If you can swing the budget for this method it will give you superior performance with minimal risk. There is a lot of science behind why this is the best method to use in your climate but I'm out of time for now.
The foam spray in place system is call Icynene. you can find an local installer on the Icynene(.com) web site.
Thanks to everyone for your input. I really appreciate it.From the responses I have received, it seems that a spray-in foam, such as icynene, is the answer. I will have to add to the roof rafters to get more depth so we can get up to the R value we need. I have also been looking at the national and local codes, and talked with building inspectors.Thanks!!General ContractorMinneapolis MNhttp://www.Handy-Werks.com
A question for the cod officials is. what cod do they follow? if it is the international cods 2000 and up then I can get you the pages that cover the use of foam and how performance of the product out ways the old R-value system. but cod officials are a ruff breed to change the way they think. ( old school, my way or no way) you get the idea.
Edited 8/3/2005 12:18 am ET by FoamMan