Replies

1. 
   wjrobinson | Mar 10, 2015 11:46am | #1

   David just add the drywall with longer screws.

2. Expert Member
   Dana Dorsett | Mar 10, 2015 11:54am | #2

   Ceiling & floor designs start on p 122:

   https://law.resource.org/pub/us/code/bsc.ca.gov/sibr/org.gypsum.GA-600-09.pdf

3. 
   wjrobinson | Mar 10, 2015 12:57pm | #3

   Dana, 50,000 drawings listed, which applies ???????

4. GBA Editor
   Martin Holladay | Mar 11, 2015 08:57am | #4

   David,
   For a garage, you'll want to install 5/8-inch drywall. A.J.'s suggestion is a good one -- just use longer screws.

   For more information on this topic, see How to Insulate a Cold Floor.

5. 
   dibbinartius | Mar 11, 2015 02:30pm | #5

   Thank You for the rapid response. Some one has including the drawing detail I was referring to. I have not found the exact drawing in GA-600-09. I have structural wooden I joist 9-1/2 inch deep with 1-1/4x1-1/2 flanges and 3/4 web. The floor above is 3/4 OSB and 3/4 hardwood. The truss are supported with concrete block wall. The question is; to obtain the fire rating, should the drywall be placed over the polyisocynaurate with vapor barrier or under it? That is does the vapor barrier need to protect the drywall as well? Thanks to all who have helped with this issue.

6. GBA Editor
   Martin Holladay | Mar 11, 2015 02:37pm | #6

   David,
   The drywall goes on the garage side (under the rigid foam) to protect the rigid foam from ignition in case there is a fire.

   You don't need a vapor retarder or vapor barrier. For one thing, the top of this assembly is presumably the warm-in-winter side, not the bottom of the assembly. But there won't be much vapor drive through this assembly in any case. Finally, in case you're curious, the foil-faced polyisocyanurate is already a vapor barrier.

7. 
   dibbinartius | Mar 11, 2015 04:03pm | #7

   Martin,
   Thanks again for your response. According to the article by Lstiburek, it seems that a vapor barrier necessary to prevent moisture in the cavity between floor and ceiling. Assuming that is correct, and that is the reason for the polyisocynurate with foil, the addition of fire barrier drywall begs the question does the drywall need to stay dry? Would the drywall, being exposed to a moist environment from underneath as in summer time, absorb moisture and become saturated and perhaps moldy? I have high moisture in summer time because of a very cold year around basement floor and humid summer air. Some time the floor even sweats. so that is why I asked about order of materials.

8. GBA Editor
   Martin Holladay | Mar 11, 2015 04:19pm | #8

   David,
   The drywall in your garage is unlikely to be moldy. If you have mold on the drywall in your garage, you need to investigate the source of the moisture and mitigate it.

9. 
   wjrobinson | Mar 11, 2015 09:19pm | #9

   David, call some contractors.