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Flash and Batt and moisture issues…

user-869374 | Posted in Green Building Techniques on

I am 2/3 way through a major remodeling project and insulation time is fast approaching. I am looking hard at sprayed foam insulation and have prices closed cell. I Iive in the outskits of D.C. so the climate is zone 4 with 5 not too far off and humid.

I have four areas to insulate:
1. First floor -existing Brick outside wall, airspace, 4″ block wall and treated 2×2 lathing. Proposed 1 1/2 pink foam board glued to block wall between lathing and then drywall over it. Q – is this a good plan and do I need to worry about a vapor barrier?

2. Second floor – new construction -Hardly board siding, tyvek house wrap, OSB sheathing and 2×6 studs. Proposed 1/2″ blown-in closed-cell foam insulation with 5″ of fiberglass batting (flash and batt) then drywall. Q – Is 1/2″ of foam the right thickness based on my region and moisture permeability or is 1″ or even 2″ a better plan? Is a moisture barrier needed on the inside house face of walls? Is blown-in fiberglass on top of the cc foam a better plan than the fiberglass batting? Or is open-cell the way to go for walls?

3. Attic – new construction – Insulation to be put on attic floor after drywall ceilings are installed on second floor. Proposed (following the drywall ceiling installation) – 1/2″ blown-in closed-cell foam insulation with 11″ of fiberglass batting (flash and batt). With the closed cell insulation barrier on the warm side of the house is this sufficient or should I look at 1″ or more closed cell?

4. Crawspace – existing – 2×10 floor joists on block wall with plank subfloor and hard wood flooring. Proposed – 1/2″ blown-in closed-cell foam insulation with 9.0″ of fiberglass batting (flash and batt). Once again is 1/2″ sufficient or should I look at 1″ or more in this area?

Thank you for any insights…

Scott

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Scott,
    Joe Lstiburek recommends that flash-and-batt jobs include a minimum of 1 inch of closed-cell spray foam in Zone 5. If you're in Zone 4, there is no minimum foam thickness, because condensation problems are unlikely in Zone 4.

    That said, it's hard to spray an even 1/2-inch layer of spray foam. Anyone who tries probably ends up with an uneven layer that varies from 1/8 or 1/4 inch to 3/4 or 1 inch. You might be better off asking for 1 inch -- it's likely to be a better air seal.

  2. user-869374 | | #2

    Thanks, after reading a number of articles it was looking like 1" was a better plan than 1/2"... any thoughts on the foam board insulation on the block wall?

    Thanks for taking the time to respond!

    Best,
    Scott

  3. Riversong | | #3

    First problem is that 2/3 through a major renovation project is not the time to think about insulation. Any building or renovation project needs to start with all the design goals and specifications during the initial design process and before any deconstruction or new construction begins, so that the entire thermal and structural envelope is properly integrated.

    1. First floor -existing Brick outside wall, airspace, 4" block wall and treated 2x2 lathing. Proposed 1 1/2 pink foam board glued to block wall between lathing and then drywall over it. Q - is this a good plan and do I need to worry about a vapor barrier?

    The color of the foam is irrelevant, it's the type that matters. IECC insulation requirements for zone 4 are R-13 walls. Use the 1½" foam between the strapping and add an additional 1" XPS inside of the strapping before drywall. Tape the seams and you have an air/vapor barrier and will meet the minimum R-value standards.

    2. Second floor - new construction -Hardly board siding, tyvek house wrap, OSB sheathing and 2x6 studs. Proposed 1/2" blown-in closed-cell foam insulation with 5" of fiberglass batting (flash and batt) then drywall. Q - Is 1/2" of foam the right thickness based on my region and moisture permeability or is 1" or even 2" a better plan? Is a moisture barrier needed on the inside house face of walls? Is blown-in fiberglass on top of the cc foam a better plan than the fiberglass batting? Or is open-cell the way to go for walls?

    As noted, 1" closed-cell is the minimum for effective flash & batt or flash and fill. Blown insulation is always superior to batts, which are always installed in a less-than-optimal manner and don't have sufficient density. Cellulose is far superior to blown fiberglass.

    All that's needed, or desired, on the inside surface is a 1 perm vapor retarder primer.

    3. Attic - new construction - Insulation to be put on attic floor after drywall ceilings are installed on second floor. Proposed (following the drywall ceiling installation) - 1/2" blown-in closed-cell foam insulation with 11" of fiberglass batting (flash and batt). With the closed cell insulation barrier on the warm side of the house is this sufficient or should I look at 1" or more closed cell?

    If the ceiling drywall is air-tight, there are no can lights, and all penetrations are sealed, then there is no need for spray foam. Install loosefill blown cellulose - far superior in every way than fiberglass.

    4. Crawspace - existing - 2x10 floor joists on block wall with plank subfloor and hard wood flooring. Proposed - 1/2" blown-in closed-cell foam insulation with 9.0" of fiberglass batting (flash and batt). Once again is 1/2" sufficient or should I look at 1" or more in this area?

    If there is any ground moisture in the crawlspace or if the crawlspace is vented to allow ambient moisture in, then the air/vapor/radiant barrier should be underneath the joists, not at the subfloor.

    Use bluejean cotton batts between joists and install foil-faced polyisocyanurate rigid foam board underneath the joists as an air, moisture and radiant barrier.

  4. user-869374 | | #4

    Thanks for the detailed info. I have been working the insulation question for a while, there are many options and lots of varying opinion. Pink foam board I have is XPS fortunately. Thanks for the tip on the additional 1" and taping. For the attic, I do have recessed lighting planned but all are IC and air tight, so I don't think the blown cellulose will be a problem. As for the crawlspace, I take it the moisture barrier on the bottom of the joists are to protect them? The crawlspace does have 6 mil plastic over the exposed dirt, but it is vented. Thank you very much for your insights, all makes sense...
    Best Regards,
    Scott

  5. Riversong | | #5

    Scott,

    "Air-tight" can lights are never fully airtight and no responsible builder uses them in the thermal envelope. It's a mistake. Use surface fixtures or can lights and use caulked polypans around every electrical outlet in the thermal envelope.

    And, by the way, I appreciate your Freudian slip in calling Hardy Board siding "hardly board", since it's hardly worth using and yet immensely popular (kinda like filterglass insulation).

  6. Danny Kelly | | #6

    Scott – allow me to suggest another option. I would be most concerned with moisture issues and not having the required R-Value on your first floor – sounds like a solid masonry wall. I would furr out your 2x2 another ½” or ¾” with either wood or rigid foam if you want a thermal break and install 2” of closed cell on the masonry wall. Most closed cells are about R-6.5 per inch so you will have your R-13 and the closed cell will solve a potential moisture issue. Simply installing pink foam between each furring strip leaves a potential moisture problem every 16”. Better yet, reframe this wall with a gap between the masonry and the framing to allow the spray foam behind it. You may spend a little more on this floor than you were planning so lose the foam on the 2nd floor and the attic to keep your bedget in line. If you have a 2x6 wall – use a dense pack blown-in insulation and you can achieve just as good results as the spray foam. Do as Robert suggested in the attic and crawl.

    As a side note, I would use 15# felt or DOW housewrap in lieu of Tyvek. Have had some issues with Hardie as well - try to avoid butt joints between pieces if you can.

  7. Riversong | | #7

    I wrote "Use surface fixtures or can lights", but meant "use surface fixtures or track lights".

  8. user-869374 | | #8

    Many thanks to everyone for their assistance. I am planning on some changes including trying to improve the building envelope by trying to improve the seal on the recepticals and light fixtures. I have also cut back on can lights opting for surface and track lighting. A question I have is where I still have can lights in the second floor ceiling, is there a reason that installing the can and then spraying closed cell foam over the ICT can from the attic side is a problem? Is it better to build a box over the can and seal that? Same kind of question for those dastardly blue boxes on outside walls. I understand the idea to seal them, isn't sealing of the cavity with cc foam (1") sprayed on the OSB and on the box sufficient? (realizing a dedicated box sealed would be better).
    Unfortunately the house wrap and hardie are already installed, just have to keep my fingers crossed. I'm assuming my contractor should have taped the window flanges after installing the windows... didn't happen if it should have. I an only try to pick with the best plan for where I am at. I am looking for sellers of the blue jean insulation Robert spoke of, any suggestions? Also any thoughts on a product called Prodex? Not sure if I should believe what they are saying but looks like it might be good over the cotton insulation in the crawlspace. I think I am going with the 1" foam with blown cellulose (wet or dry?) for the walls on the second floor (but may just go with the blown cellulose without the foam - just not certain about the air seal and vapor barrier of this technique - I have to check with the city code folks to see if the primer Robert recommended will meet their requirements. Any opinions on cellulose DIY? The ceiling thinking about spray foam over any cans and perimeter with blown in cellulose for the rest. Any issues with cellulose dust? I will have to use the attic for storage due to the small size of my house and frequent moves with the military (although this will be our final stopping point).
    Downstairs now planning on 2" cc foam on the block wall on the first floor. In the crawlspace if the vents are sealed or better yet removed and blocked out and there are no real moisture issues is there a problem with foaming the underside of the floor? Just curious...
    I really appreciate the help. I have been doing a lot of reading to get a better handle on the insulating system for my house and really want to do it right the first time...

    Best,
    Scott

  9. GBA Editor
    Martin Holladay | | #9

    Scott,
    1. Don't use cotton batt insulation in the crawl space. The only insulations suitable for use in a crawl space a rigid foam board or spray polyurethane foam.

    2. Don't spray foam directly against the back of the can lights. First, build a box to cover the light out of gypsum drywall or rigid foam (taped or tacked together with dots of spray foam adhesive); the box should leave enough room for air all around the fixture. This technique should be familiar to your spray foam contractor.

  10. user-869374 | | #10

    Thanks Martin, I will build the boxes as suggested and seal them. Robert recommended the vapor barrier under the trusses instead of spraying the subfloor with foam. I don't want to create moisture issues, should the cc foam be sprayed on the subfloor face and trusses? and then put the foil backed foam board on the truss face and seal that? would have two moisture barriers, actually three counting the 6 mil plastic on the ground in the crawlspace...

  11. GBA Editor
    Martin Holladay | | #11

    Scott,
    1. You always want a vapor barrier covering the soil on the crawl space floor. Whatever you do, don't omit that.

    2. If I had to insulate the ceiling above a crawl space, I would use rigid foam board attached to the underside of the joists. If I was unable to buy thick enough foam to achieve the desired R-value, I would use two layers of foam with staggered seams. Seams between the foam board panels should be taped with housewrap tape or sealed with canned foam.

    That said, it's always best to seal up crawl space vents and insulate the crawl space walls, not the crawl space ceiling, unless there is an important reason why a sealed crawl space won't work.

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