Replies

1. 
   davidmeiland | Sep 29, 2007 05:30pm | #1

   You can get rigid foam at R7 per inch. Not sure of the cost diff, when we do that it is supplied by the insulator. The Foamular products are what we use under slabs.

1. 
   nash | Sep 29, 2007 08:59pm | #2

   thnaks for the response.

   the reason I was interested in using foamular 250 is that it could be used as a vapor barrier and it is ridged enough that it should provide a good base to scrw the sheetrock thru.

   which ridged insulation product would you recommend for my needs

   thanks

1. 
   Piffin | Sep 29, 2007 10:37pm | #3

   You don't mention what thickness you are planning to use of foam. Your R-value estimates make me assume 1-1/2"Thefoamular is rated at about R5/inch.
   There are foil faced plyisoanurate faom panels that are rated at R-7+/inch. Thermax is one. Celotex has another.
   if you tape the seams and any teaars in the foil surface, it will function as a Vb also.I have used it suessfully several times almost as you have detailed for attic spaces. FG batts first, then the Thermax, then strapping to provide a goo dway of holding it by screwing through the strapping, foam, and into the rafters. This also gives a space for wiring and makes a far better surface to hit with SR screws. It can be fairly frustrating to try to hang sheetrock with 1-1/2" of foam between it and the rafter. Then when you miss the rafter, you have more holes in the VB.An inch and a half of Thermax polus the R19 batts will give you just a bit over R29. That spoace will also let the radiant barrier of the foil function to some degreee. 

    

   Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

1. 
   nash | Sep 30, 2007 04:51am | #5

   thanks for your reply.

   I plan on using r-21 5 1/2 inch fiberglass batt insulation. and a 3/4 inch formular board r-4

   http://www.owenscorning.com/around/insulation/products/pfgi.asp

   I'm trying to save as much room as possible amd would like to attach the sheetrock directly to the foam board and screw thru to the rafters without using strapping on top of the board.

   My main question would be does anyone know if the Formular 250 3/4 inch board is ridged enough to be a good backer for the sheetrock ?]

   Also is there a better performing ridged board out there that cost about same that would suit my needs better ?

   thanks

1. 
   Piffin | Sep 30, 2007 05:20am | #7

   Yes it is rigid enoughI just named two other kinds that are better - did you read that last reply I made?

   Welcome to the
   Taunton University of Knowledge FHB Campus at Breaktime.
   where ...
   Excellence is its own reward!

   Edited 9/29/2007 10:21 pm ET by Piffin

2. 
   BillHartmann | Oct 01, 2007 07:38am | #12

   "Celotex has another."Not exactly.Another brand/trademark problem with a bankruptcy thrown in.Celotex was the comapny name and made among many other products Thermax polyiso.Celotex went bankrupt, IIRC it was over asbesto claims.Dow bought the Thermax business.I think that OC got the shingle business.Rmax is similar sheet product.Here is a list of an association of manufactors. http://www.polyiso.org/ContentPage/Manufacturer.html.
   .
   A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.

   Edited 10/1/2007 12:39 am by BillHartmann

1. 
   Piffin | Oct 01, 2007 12:44pm | #13

   Thanks for that correction.Ya know - I have always used the Thermax andit came labeled as Thermax.Last time around, I ordered Thermax and the label on what was delivered was DOW. I used it and marked in my mind to be more specific next time, because a lot of those sheets were not square. Either the quality slipped, or I got sent some seconds. 

    

   Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

2. 
   basswood | Sep 30, 2007 12:58am | #4

   I like to use the Owens Corning Insulpink (same stuff--XPS--as Formular, but it comes in 2'x8' sheets that are formed for flush installation of 1x3" firring). The stuff is sold as a product for finishing basements, but I use it for attics too.

1. 
   nash | Sep 30, 2007 04:55am | #6

   that looks pretty good for my needs.

   but the 1 1/2 thickness is a little to thick( i'm trying to save as much space as possible) I might be crazy but 3/4 inch does make a difference when I do a mock up on the wall

   thanks

    

3. 
   RedfordHenry | Sep 30, 2007 06:48am | #8

   Assuming you are going to paint it, your drywall will serve as an acceptable vapor retarder.  Since you plan to vent the rafter bays (I'm assuming you have ridge and soffitt vents, and are planning to use Propavent (or similar), any vapor that does get into the rafters will go staight up and out.  If it were my building, I'd sacrifice the headroom and use 1.5" foam (polyiso) along with strapping as Piffen has described.  The rigidity of the foam has no relevance to the drywall.  If you are concerned the foam compressing when you attach the drywall, don't worry, it won't.

1. 
   nash | Sep 30, 2007 04:54pm | #9

   thanks for the replies

   i do have a ridge vent and soffit vents.

   combining the fiberglass batt and now I'm leaning towrds using the insul-pink boards I will achieve an r-value of 28.5. I'd be pretty happy with that.

    

   Question.

   What tape can I use to help seal thre boards ? should I just use the tape manufacturer sells or is there a more cost effective alternative ?

   thanks in advance.

1. 
   RedfordHenry | Oct 01, 2007 04:04am | #10

   Venture tape is pretty good, if you can find it.  About the same cost as Dupont's Tyvek Tape.

2. 
   basswood | Oct 01, 2007 05:55am | #11

   I seal the joints with a generous bead of PL300 foamboard adhesive on the edge of each course of Insulpink and embed the next course in it. A small amount of "squeeze out" along the length of the seam indicates a complete seal. PL300 along all edges produces a strong, airtight assembly.Another tip: I rip my own 1x3's out of 3/4" plywood...straighter, less splitting, etc.