Replies

1. 
   WayneL5 | Feb 29, 2004 06:09am | #1

   I don't know the code in California.

   But, what you propose can be quite beneficial and effective and you should be able to sell the inspector on it.  If you put unfaced fiberglass between the rafters, and face the inside with the polyisocyanurate screwed right across the bottoms of the rafters, you'd have the following advantages.

   First, you'd have a radiant barrier facing the ceiling, which would reduce heat loss in cold weather.  Second, you'd have covered the rafter faces themselves with insulation, which reduces conductive heat loss or gain through the rafters themselves.  Third, with the thinner fiberglass between the rafters you'd have better air movement from the eaves to the ridge, which reduces the roof temperature, cutting heat gain in the summer and extending the life of the roof, and would reduce ice damming if you lived in an area that got snow.

   It is possible to mix insulations the wrong way by getting the vapor barrier on the wrong side, but in your case you have it just right, and it's a lower cost solution, too.

   There's likely not a prohibition in the code against mixing insulation, because doing so with proper regard to vapor movement can bring additional advantages.  But, one can never be sure of anything in California.

2. 
   BillHartmann | Feb 29, 2004 09:02am | #2

   Why not just blow in celulose?

3. 
   Mongo | Mar 02, 2004 07:49am | #3

   If you want to mix batts and PI, use R19 batts between the rafters. Then skin the face of the rafters with 2" PI.

   More details if you're interested.

1. 
   amaruoka | Mar 03, 2004 07:22am | #5

   I suppose I could do it that way. It would lower the ceiling a couple inches, though. Is this documented anywhere that you know of (code-wise)?

   The guy at the building dept said, "You can't really mis different types of insulation." When I countered with skinning the house with ridgid insulation and using batts in the wall, he said, "that's differen't". I may have to go down there in person to figure this out with them...

   Andrew

1. 
   Mongo | Mar 03, 2004 10:22am | #6

   I can't think of any restrictions regarding mixing insulation. Ask him where in the code is it prohibited.

   It's done all the time. Cells over FG on attic floors. Rigid and batts, rigid and cells. Sprayed foam covered with batts or cells. And yes, it's done with batts and rigid in between the framing members as well.

   The only leg I see he has to stand on would be if you tried having two vapor barriers. Say, faced FG batts covered by the PI. Or FB batts covered by a poly VB, then covering that with PI. You only want one vb.

   If you do have exposed PI, it'll have to be covered in the living areas of the house due to fire concerns.

4. 
   rjw | Mar 02, 2004 08:17am | #4

   >>I ran this by the building dept, but they didn't seem too keen on "combining different kinds of insulation".

   Did they say why? I've never seeen or heard any such concern

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5. 
   fortdh | Mar 04, 2004 04:59am | #7

   Not aware of any code restriction, and have seen many combos of insulation mixes. The closed polyurethane sprayed foam/fg batt would get the R-30, and probably as cheaply as possible (considering time as money). It is an excellant package. If you want an added thermal break between rafter and sheetrock, http://www.IntegrityGasket.com makes a foam tape for the edge of the 2x. Paul

   Energy Consultant and author of Practical Energy Cost Reduction for the Home
6. 
   Piffin | Mar 04, 2004 05:20am | #8

   You want we should go down to his office and thump him out of his eighteenth century ignorance? There's a dozen ways to combine insulation types to achieve a better package.

   I would probably either use urethene spray foam to get up to R49 in tha space, or blow cellulose over one inch iso or icy board spanning acraoss the rafters with strapping on it.

   The spray foam will be the most expensive, the best, and will give you the most headroom.

    

    

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