Replies

1. 
   csnow | Feb 09, 2005 01:06am | #1

   You will likely get conflicting opinions calling for fiberglass, air channels, soffit, and ridge vents.  Folks will talk about keeping the shingles cool...

   My opinion is that a tightly sealed rafter cavity with dense pack cellulose is better in most cases.  For more cost there is spray foam.  For really good thermal performance, another good option with such shallow rafters would be cellulose, covered by rigid foam under the drywall.

1. 
   DgH | Feb 10, 2005 09:19am | #3

   When I do mine in the summer and fall I am going to use cellulose mainly because of the low cost.My house has 2x6 roof supports. After putting in the appropriate vent holders... I will built depth of the ceiling down an additional 10" and put up a heavy duty vapour barrier material. Then strapping underneath that to hold it in place. I should be able to blow in 13" to 14" of cellulose over the whole area, plus the knee walls and the end walls and the extra lumber plus the insulation won't cost me more than around $1500 (Canadian dollars). This is probably going to be alot of work. I can probably even get an insulation contractor to come and do the spraying for not a whole lot more than my cost for the insulation.I got this concept from an article in the Journal of Light Contracting website. Thought it looked interesting.If not for this, it would have been polyeurethane on the whole thing.Last year I didn't know what any of this stuff meant.

1. 
   csnow | Feb 10, 2005 06:54pm | #4

   "My house has 2x6 roof supports. After putting in the appropriate vent holders... I will built depth of the ceiling down an additional 10" and put up a heavy duty vapour barrier material. Then strapping underneath that to hold it in place. I should be able to blow in 13" to 14" of cellulose over the whole area, plus the knee walls and the end walls and the extra lumber plus the insulation won't cost me more than around $1500 (Canadian dollars). This is probably going to be alot of work. I can probably even get an insulation contractor to come and do the spraying for not a whole lot more than my cost for the insulation."

   If it were me (and I know this is controversial), I would skip the vents; seal the cavities tight at the eves and the ridge. Skip the built-up depth and the vapor barrier.

   Put up a 2 inch layer of rigid foam on the rafters. Seal all the seams tight with gunned foam or foil tape. Put drywall over the foam, and fill the cavities with dense pack cellulose.  Less work, thermal break, and better cost/benefit ratio IMO.

   Why no venting?  Look at it this way;  What would you be venting?  Warm moist air? Not if the details are right.  Driven vapor?  How much will get through really?  Uncontrolled air movement within cavities should be viewed as a potentially bad thing.  Moist outside air can condense within the cavity at times just as it can serve to dry the cavity at other times.  Depends upon ambient conditions.  But why let it in in the first place?

1. 
   JoeyB | Feb 10, 2005 07:49pm | #5

   Thanks for the replies. I had a feeling that there would be many different approaches to insulating this space.Coming to you from beautiful Richmond, Va.

2. 
   hurnik | Feb 12, 2005 10:38pm | #7

   And sometimes your local building inspectors will require venting as well. At least mine required a ridge vent and "proper vents" in the ceiling.Keep in mind this was a complete renovation of the upstairs.In hindsight, here's what I would've done for the 2x6:Installed proper vents (which I did as I was required to do)
   Install 1" foamboard (I used the pink stuff from Owens Corning) over that and had cellulose blown in between. Basically I should've used cellulose instead of fiberglass, but I ended up using Owens High Density fiberglass batts (I think R-21)--Kevin

2. 
   MAsprayfoam | Feb 09, 2005 03:23am | #2

   Your best bet Joey is to use spray foam directly on the roof deck and then sheetrock over it.
   Because this space will be lived in this is the best way to keep the heat out in the summer and in during the winter. It will cost more up front but will pay back in 3 yrs or less with energy savings alone.

3. 
   rez | Feb 11, 2005 04:27am | #6

   Greetings Joey,

   That subject has been addressed here on Breaktime a number of different times in the past.

   Best thing to do is study the varying data.

   If you scroll down in the lower left corner of your screen there is a search function that will take you to previous threads dealing with whatever you type in the search bar.

   If you type in 'cathedral insulation' or other keywords of the subject matter you'll get a good supply of data from those old threads.

   Knock yerself out. :o)

   Cheers

   "Live Free,
         not Die"