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How do I get my stick-framed house in Tampa, Florida, to breathe and keep the heat out?

user-7098403 | Posted in General Questions on

We live in the Tampa Florida area (latitude 28, zone 2) in a Manufactured Home. The house is 46 feet long. All of its front is facing due east with absolutely no shade. The exterior wall temps range from 110 to 120f in the very long summer. Manufactured homes are built to HUD standards and as such are framed differently than a normal site built home. I have provided a pdf of the typical framing anatomy for our home. Ours is slightly different in that we have two 3/0 windows in the bedroom (the one pictured has one.) and a smaller bath window higher ours is also wider. Item #13 is 2x4s not 1x4s. Item # 9 is 3/4 plywood. and item # 18 should follow #17 and is a typo in the description. That said, plans are to remove aluminum siding, remove 2″ fiberglass insulation in all stud bays and install Roxul comfortbat in lieu. new cladding to be 5/8″ double sided HDO 4×8′ sheets installed vertically. The HDO is 100% water barrier with a vapor perm of less than 0.1. It shall be attached to meet new wind load uplift etc. required. The interior of the home shall be structurally sheathed in APA rated 3/8″ sheathing staggered horizontally (except first 2′ at every corner shall be continuous vertical panel), nailed and glued for shear. The home sits on concrete pier (blocks) which are 4′ OC on top of 17″x 22″ ABS base pads on what was tested and assumed still is 1000 PSF soil conditions. Every 5′-4″ OC is a galvanized earth anchor as well as 4 longitudinal anchors at each end of the house + three roof over straps and earth anchors.

The objective is to feel safe in hunkering down during a cat 3 hurricane and cutting our energy bill significantly while increasing comfort. My dilemma is how to prevent thermal bridging (studs), provide R 16 or better insulation (total wall except windows) and let the interior water vapor escape the walls. With the way manufactured homes are framed, bridging control and vapor control are the real bugbear for me.

The other two pdf are self explanatory. form APA.

Best Regards to all in advance for my laughable situation,
Bill Taylor

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Replies

  1. User avatar GBA Editor
    Martin Holladay | | #1

    Bill,
    I wasn't familiar with the abbreviation "HDO," so I Googled it. I refers to High Density Overlay plywood. Evidently this type of plywood can be used to make siding. News to me.

    Walls don't have to "breathe." Some people use the term "breathe" to mean "leak air," and in that sense you never want a wall to breathe. Some people use the term "breathe" to mean "have a high vapor permeance," and it's safe to say that a high vapor permeance is a double-edged sword. It can help you or hurt you.

    If you spend more months of the year operating an air conditioner than a heating system, then choosing a low-permeance siding (as you evidently have) may make sense. It will certainly preclude the problem of inward solar vapor drive. However, low-perm siding isn't a good idea during the winter. I imagine, however, that your winters are mild enough that you can get away with a low-permeance siding.

  2. User avatar GBA Editor
    Martin Holladay | | #2

    Bill,
    If you want to improve the thermal performance of your walls in conjunction with a siding replacement job, it's odd to open up the stud bays from the exterior to install mineral wool batts. It would be easier (and more effective) to simply install a continuous layer of exterior rigid foam as you replace the siding. That way the rigid foam will address thermal bridging through the studs.

    You might want to read these two articles:

    How to Install Rigid Foam Sheathing

    Roofing and Siding Jobs Are Energy-Retrofit Opportunities

  3. user-7098403 | | #3

    Apparently my pdfs did not post that show the construction. I will try again here. We are removing the interior (decorative, non structural) paneling as well and trying to create an envelop of strength. My initial plan was to just install polyiso board on the outside and re clad with the HDO. FBC will not allow any vapor barrier or retarder with a perm of less than 1.0 to be installed on the outside with no coatings or covering on the inside wall to be less than 1.0 as well. In response to the first answer. We run the central air about 300 days a year.
    Humidity is very high. Average summer day time dew-point is in the 70s.

    Thanks to all
    Bill

  4. User avatar GBA Editor
    Martin Holladay | | #4

    Bill,
    You wrote that the Florida Building Code "will not allow any vapor barrier or retarder with a perm of less than 1.0 to be installed on the outside with no coatings or covering on the inside wall to be less than 1.0 as well."

    Then why did you choose to install a low-permeance siding?

  5. user-7098403 | | #5

    Trying again to post the pdf.

    Thanks to all
    Bill

  6. User avatar GBA Editor
    Martin Holladay | | #6

    Bill,
    Can you cite which part of the Florida Building Code "will not allow any vapor barrier or retarder with a perm of less than 1.0 to be installed on the outside with no coatings or covering on the inside wall to be less than 1.0 as well."

    Where does it say that? What is the exact wording of the code requirement?

  7. user-7098403 | | #7

    Sorry about the double reply.I just figured out what I was doing wrong on the pdfs.
    Here are the other two for reference.
    Bill

  8. user-7098403 | | #8

    Martin

    I will do so this evening when I get back in front of the books. FBC 2017 is what we are using.

    Best Regards
    Bill

  9. User avatar GBA Editor
    Martin Holladay | | #9

    Bill,
    It's easy enough to choose exterior sheathing materials and exterior siding materials with a vapor permeance that is greater than 1.0 perm, if in fact the Florida Building Code actually requires such a choice. (Note that plywood and OSB are "smart" vapor retarders with variable vapor permeance. For more information on the vapor permeance of plywood and OSB, see "Water Vapor Permeance of Wood Structural Panels.")

    But, once again, I have to ask why you want to install a low-permeance siding.

  10. User avatar
    Jon R | | #10

    I think FBC will allow low perms on the exterior. But if not, then you could put a rainscreen behind your low perm siding.

  11. user-7098403 | | #11

    Martin,
    Manufactured homes do not have a foundation that the rim is supported by. Weight then plays into siding choice. We have been in this home since 2003. HDO or MDO is the only material we know at this weight to strength ratio that can take the direct sun at this lat and still hold paint. And yes Jon. The building official wants weeping board on top (outside) of the polyiso board. XPS foam might work without?

  12. User avatar GBA Editor
    Martin Holladay | | #12

    Bill,
    Do you intend to use HDO plywood as the sheathing, as the siding, or as combination sheathing/siding (kind of like T-111)?

    You wrote, "Weight plays into siding choice. We have been in this home since 2003. HDO or MDO is the only material we know at this weight to strength ratio that can take the direct sun at this lat and still hold paint."

    This is confusing. Siding doesn't need a "weight to strength" ratio -- it has no structural function whatsoever. Lots of siding types, including vinyl siding, don't need paint.

  13. User avatar
    Jon R | | #13

    Strips of foam, Comfortboard and EPS insulation are examples of > 1 perm insulation that can address thermal bridging.

  14. user-7098403 | | #14

    The HDO will be structural siding (Like T1-11), We will add batten boards to create a pseudo "board and batten" look. At 64 pounds a sheet it is stronger than an APA 3/4 sheathing in both directions. In hurricane protection we want to stop a 9 lb 2x4 at roughly 150 feet per second, as well as uplift on the panels.The forces are quite large. Nail field for 4x8 sheet on 16" is nearly 100 8d per sheet. Which is why we are also sheathing the inside walls with APA 3/8 sheathing glued and nailed. We want a very strong wall without re-engineering the frame. Hardiboard is too heavy and not as strong in any direction as the HDO. Its a compromise I know. Using 6" wide strips of foam over the studs is one of those we are considering along with Tyvek house wrap or something vapor permeable like that.

    Ill code to the code later tonight I hope. This

    Thanks to all
    Bill

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