How do you connect windows to a flash and cellulose application?
It’s clear that Flash and Cellulose will work best without a poly sheet vapor barrier on the inside face of studs, but is there any special precaution to be taken when flashing window openings?
This project is in climate zone 5. The foot print of the attached 2.5 story garage and living space above is approximately 1140 SF.
The local building department recommended 1″ spray foam on the inside face of sheathing followed by 4.5″ of un-faced batt insulation in the 2×6 stud cavity and 4 mil poly vapor barrier on the inside face of studs.
I am concerned about potential moisture problems.
The typical wall assembly is Hardyboard clapboard siding, possibly a dimpled paper by Hardyboard to allow back-venting of the siding. Sheathing – possibly Zip Systems zip wall. 1″ spray foam. Unfaced fiberglass batts (although it sounds like loose cellulose would be a better choice). 4 mil poly vapor barrier attached to the inside face of studs to run continuously from plate to plate. 5/8″ type ‘X’ gypsum wallboard.
The garage ceiling/2nd floor assembly would be 20″ LVLs/TJIs. A hardwood finish floor. I think 22/32 Advantech subfloor. The 20″ deep joists mentioned above. Strapping. Type ‘X’ GWB ceiling. The question here is should this be all sprayfoam instead of flash and batt and where should vapor barrier go?
The attic floor/2nd floor ceiling would be 2×8 or 2×10 joists. Have not specified only surface mounted lighting yet. Type ‘X’ GWB on strapping.
There are also stair walls between the unconditioned garage and the upper level living space.
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2011-02-16 updated 2:39pm EST
For the floor of the stair landing, part of the landing is only 3 feet above the garage floor and the clearance between the bottom of the joist and the top of the concrete slab would be approximately 2 feet. Is it reasonable to expect the sprayfoam contractor to be able to apply sprayfoam to the inderside of the subfloor? If so would you recommend a flash and batt strategy in the stair landing floor?
Should a vapor barrier be used anywhere? Just under the concrete floor of the garage, right?
Thanks.
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Replies
KH,
Your local building inspector is wrong. Interior polyethylene should never be used on a flash-and-batt job.
See the recent Fine Homebuilding article on the flash-and-batt system. The article noted, "Do not use poly sheeting, foil-faced rigid foam, or any other class-I vapor retarder on the inside of the wall assembly, or you will create the dreaded double vapor barrier."
Martin, thank you for the link to the article. I had skimmed through it before to determine whether 1" of spray foam was adequate, but I missed the dreaded double vapor barrier admonition.
Unrelated to the vapor barrier question, would 1.5" thick spray foam be a safer bet over 1.0" spray foam in Massachusetts?
Some of the other postings at GBA included responses that suggested that blown-in cellulose offered superior performance to fiberglass batts. What are your thoughts?
KH,
Q. "Unrelated to the vapor barrier question, would 1.5" thick spray foam be a safer bet over 1.0" spray foam in Massachusetts?"
A. Yes, it would be safer.
Q. "Some of the other postings at GBA included responses that suggested that blown-in cellulose offered superior performance to fiberglass batts. What are your thoughts?"
A. Blown-in cellulose ALWAYS provides better performance than fiberglass batts.
KH,
There is no single answer to the question, "What insulation is best?" A great many products will do a good job. You need to choose a product that you can afford, with environmental characteristics that you approve of.
For any assembly (floor, wall or roof) separating a conditioned space from an unconditioned space, you need the following two elements:
1. A good air barrier.
2. Lots of insulation -- at least equal to minimum code requirements, and more if possible -- in direct contact with the air barrier.
Hi Martin and fellow readers, I have posted a PDF of the design progress to aid in the Q/A.
Q. At the second floor framing (garage ceiling) given the following: 20" deep joists, the desire to do a flash and loose cellulose applicationAt the second floor between the unconditioned garage and the living space above would 1.5" sprayfoam to the underside of the subfloor and loose cellulose to the bottom of joists be pratical to install and a proper design for handling vapor drive from a heated space above to an unconditioned space below?
Q. At the attic floor/2nd floor ceiling could flashing with 1.5" of sprayfoam at the top side of the GWB ceiling with loose cellulose on top of the sprayfoam work to the top of the 2x10 joists produce a correct design/installation- or is that 'inside-out'?
Thanks again.
Hi Martin I revised the questions about floor insulation questions in the post with the PDF attachment. Sheets 3.3 and 5.2 show a building section (PDF page 10) and a wall section (PDF page 13)
-KH
After speaking with a BASF technical representative It sounds like using 2.5" of closed cell sprayfoam in the 2x6 walls with the remainder of the cavity filled with 3.0" cellulose will yield R-27 insulation in the 5.5" wall cavity. Closed cell sprayfoam 2.5" with an R-value of 6.7 per inch of thickness provides R-16.75 and 3.0" of cellulose with an R-value of approximately 3.5 per inch of thickness provides R-10.5. The total cavity insulation would be R-27.25 for closed cell spray foam flashing and 3 inches of cellulose to fill the remainder of the cavity.
To insulate the floor-ceiling assembly between the unconditioned garage and the living space up above I am considering 3.0” of closed cell sprayfoam (R-20.1) and 17” of cellulose (R-59.5) for nearly R-80 total.
Q. Can cellulose be applied to the bottom of a floor that has sprayfoam on it already – before a GWB ceiling is installed over strapping on the bottom of the floor joists?
Q. Does R-80 make any sense?
Q. Do you agree that 1.5" thick closed cell sprayfoam or greater makes a class II vapor barrier?
Q. If you rely on the thick application of closed cell sprayfoam to provide your class II vapor barrier are you able to connect you windows and flash the window openings to the vapor barrier properly?