Basement Insulation

Discussion in 'Remodel Forum & Blog' started by K2, Jan 11, 2012.

  1. Dana

    Dana In the trades

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    Unless it has a shiny foil or plastic facer on it, the foam is not a vapor barrier, but rather a vapor retarder. Type-II EPS would have a vapor permeance of about 1.2-1.5 @ 2" thickness, which is more vapor retardent than latex paint, but 3x as vapor permeable as an asphalted kraft facer. It's fine to go to the ceiling with it. Compressing an R19 into the 4-4.5" space between the foam & poly vapor retarder delivers an effective R of about R15-R16 at that density (compressing it down to 3.5" delivers R13- it's essentially a "fluffed" R13 batt).

    If you did it that way you'd have R8.4 of foam to R15.5 of fiber (averaged) for R24, plus the R11 in the studwall, for R35 inside the vapor barrier, R19 on the exterior for about R54 total center-cavity, with a 65% ratio. You could also go with R13s in the new studwall for R56, and a 66% ratio. Either of which would be fine if you're actually in the blue. The most important thing would be air-sealing the foam. If you wanted to stop the R13s in the studwall to stay at R24 foam + compressed batt for a huge dew point margin (a 56% ratio) that would be fine- hardly noticable in the actual energy use. Put an air-barrier at the top of the cut batt in the studwall to limit convective draw.

    You can't reliably fill 7.5" of space with two R11 batts- there isn't sufficient guaranteed loft. It can fill 7" well though.
  2. jrensink78

    jrensink78 New Member

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    Just when I think I've got it figured out...

    OK, so if I should stick with the rigid foam floor to ceiling, it would be easier to put the foam up against the exterior studded wall than straight up along the interior studded wall from the prospect of getting everything nice and sealed. Not sure if it matters from a design standpoint, but I will be going with XPS. So how does this look?

    basement wall option 1.jpg

    Keep the 6mil poly on the inside of the exterior wall. Put the 2 inches of XPS in front of that. Fill in the remaining 5.5 inch cavity with R19 unfaced batt. Then frame the interior wall up in front of that. This also gives me a nice thermal break for the sill and then all interior fiber batts are only touching XPS and the interior framed wall.

    Here are the calcs for internal insulation percentages.

    If I did R11 on the interior wall floor to ceiling...

    R19 on exterior wall + R10 XPS + R19 in the cavity + R11 in the interior framed for a total of 59. 40/59= 67.8%

    And if I only do R11 in front of the concrete and not on the top half...

    R19 on exterior wall + R10 XPS + R19 in the cavity for a total of 48. 29/48= 60.4%

    I am in a blue county that boarders a purple county. So I'm at the extreme north of the blue. I'm thinking that I'll want to be closer to 60% than 70%. So I'd omit the R11 on the top. Then put a block between the uninsulated top half and insulated bottom half to limit air flow between the 2 sections.

    Do I finally have a viable plan?
  3. Dana

    Dana In the trades

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    With the foam up against the poly, the interior surface of the foam then becomes the condensing surface, not the poly, which is fine. That 2" of XPS is only about 0.6 perms- not enough moisture can reach the poly to ever matter, as long as you air-seal the seams & edges of the foam.

    That means you have (R19+ R10=) R29 outside of the condensing surface, and with maybe R25-R30 of fiber (compressed R19 + R11 full-loft) on the interior side even if you go floor to ceiling (recommended- empty stud bay cavities invite air-currents, and become fire propogation paths) in the new studwall. That means center-cavity you'd be at about R60 max for a total making it a roughly 50% ratio, which has plenty of margin.
  4. jrensink78

    jrensink78 New Member

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    I plan on spray foaming and taping the seams. So I'll be sure to get those sealed nicely. In this scenario, you'd still recommend leaving the 6mil poly up?

    Thank you profusely for all of your help. I don't think I'll spend as much time figuring anything else out about my entire house as I've put into finding a solution for the basement insulation. If you want to PM me your email address, I will totally send you some beer money. You more than deserve it.
  5. Dana

    Dana In the trades

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    Leave the poly in place, lest you accidentally create an path for air (and air transported moisture) to move all the way to the exterior sheathing of the upper floors to create a problem. There is nothing to gained by removing the poly in your new-improved stackup. If a tiny air leak in your foam layer allows moisture to condense onto the poly it's not a big deal, but if moisture adsorbs into the structural sheathing it could be, depending on just how much moisture it its. You can never have enough air-barriers in the stackup, even if moisture vapor barriers need to be carefully planned. Poly sheeting detailed to be air-tight is both an air & vapor barrier.
  6. bostondiyer

    bostondiyer New Member

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    Location:
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    Hi Dana,

    I think at this point I've probably read all of your posts on this forum - great stuff, I've learned a lot. I'm partially finishing a basement in a house built in 1930 and I have some questions about insulation and a subfloor that I couldn't find answers for.

    At this point I've laid down many of the bottom plates (PT with foam gasket underneath, no poly) and many of the internal stud walls are up, but not the external ones. I have at this point basically committed to having the bottom plates for the walls on the slab, not on the subfloor. I am building the walls and then putting them on top of the PT bottom plate, though - so basically a double bottom plate. I am using Ramsets to attach the bottom plates to the floor.

    First, some questions about the walls. My plan was to have 2" XPS foam attached to the wall, then a 1/2 inch gap, then 2x4 stud walls with unfaced fiberglass batts. The purpose of the 1/2 inch gap between the two was to 1) facilitate easy electrical, coax, telephone, and ethernet wiring, and 2) allow me to properly plumb the finished wall in case of any problems in with the underlying concrete block wall. I understand that this gap is bad insofar as it allows air to circulate behind the wall, harming the effectiveness of the fiberglass batts. My question is, would stapling Tyvek to the back of the walls before I lift them into place solve this problem? And is it really that significant? My primary motivation is to prevent condensation and mold - a slight loss in R value is not a huge concern. If this would still be a problem, could I add 1/2 inch of EPS/XPS to fill the gap (and cut channels in it for my wires) thus having a total of 2.5 inches of XPS?

    My other question about the walls is, how do you recommend handling windows? I would think you'd have to have some wood-on-concrete there, so I assume PT, but I'm not sure of the proper way to frame and insulate around them.

    Next, regarding the floors - my initial plan was to use Delta-FL as the subfloor, but your enlightening posts about dew points and condensation have made me see the light and I realize I need to insulate. However, I am extremely limited in height - slab to joist is about 81.5 inches, and I'm going to have drywall ceilings. What is the minimum you would recommend to avoid mold problems? And is it possible to Ramset rather than use Tapcons for the subfloor, or to not attach them at all? If I'm install a floating laminate, could that eliminate the need for plywood? I'm 6'1", so I'm trying to gain any height I can, as well as keep things as close to code as possible (80" in MA for basements) - but I'm not pulling permits, so if it's not exactly to code them so be it - I'd rather it be done right and avoid mold.

    Thanks a lot in advance.
  7. Dana

    Dana In the trades

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    A half inch gap between insulation layers is a thermal by pass big enough for a hurricane to pass through!

    Having the Tyvek air-barrier on the back side of the studwall is better than not having it- it will preserve the performance of the batts. But impeding all air movement (==moisture movement) paths is important for the overall performance & resilience of the assembly. Adding the half inch of rigid foam is a better choice than leaving it open.

    If you promise to never put down a rug you can probably get away with R2 (half-inch EPS) under the subfloor, but R3 (3/4" EPS) would be better.

    It's possible to "float" the subfloor, but if you do you'd need to use a double-layering with staggered seams on the subflooring to avoid "potato-chipping" curl of the subflooring and lifted corners from normal seasonal moisture cycling. I've never seen subflooring done with Ramsets rather than screws. How well it works may depend on the condition & thickness of the slab. A lot of circa 1930 basement slabs are merely nicely troweled rat-slabs, highly variable in thickness, with sections less than 2" thick. (I have sections in my circa 1923 slab that are only about an inch thick, and other places where it's 4".)

    Putting a layer of EPDM flashing between the wood & concrete (and extending all the way to the interior side under the sill) does a world of good at window penetrations. Pressure treated would survive, but the dimensional changes from moisture cycling the wood leads to air leakage. Some of the box stores carry self-adhering EPDM tapes (designed for sealing membrane roofs) that can work, but you may have to lap them to get the necessary depth if it's not wide enough for the whole shebang, eg: http://www.homedepot.com/p/Quick-Roof-EPDM-Self-Adhesive-Rubber-Flashing-Tape-FT910/202267106 . (This stuff is 9" wide stretchy-tape- probably wide enough for your application without lapping it.)
  8. bostondiyer

    bostondiyer New Member

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    Location:
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    Sounds good, I'm thinking I'll measure the plumb of the wall and if it's square I'll plan on eliminating that half-inch entirely. Is it a bad idea in your opinion to use Tyvek anyway (or, does it add anything)? It strikes me as easy insurance against any mistakes in airtightness, but I just want to make sure it couldn't cause any counterintuitive problems a la vapor barriers in basements.

    You're right about the slab. I hadn't heard the term rat-slab, but after looking it up, that's exactly it. I couldn't understand why it was so unsound, there were areas with exposed dirt and >1 inch concrete, tons of exposed aggregate, and the prior owner had put lots of surface patches of varying quality over it. Any unsound areas or patches that could be broken up with a breaker bar I already broke up, then dug down ~6 inches and filled with hydraulic cement, then I cleaned it extensively and applied several layers of acrylic. It's as solid as it could get at this point, and there will be 1/4"-2.5" of SilFlo nailable self leveling compound on top of it. Would Tapcons work with such a slab?

    With regards to the windows, good advice on the EPDM flashing, I'll definitely use that on the one I'm replacing. For both, though, their depth is only about half the depth of the concrete wall, so I guess my question is, what is the proper way to insulate around what will be an 8.5+ inch window sill - that 3-4 inches of concrete block depth plus the other 2 inches of XPS depth and 3 1/2 inches of 2x4 depth? I can use my intuition and figure something out, but I was just wondering if there are any particular tips you have about how to handle it. I was thinking XPS foam as thick as I could fit sealed with closed cell foam then painted PT wood as the actual interior sill.

    Thanks
  9. bostondiyer

    bostondiyer New Member

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    One question I forgot to ask - do you see a benefit to a product like Delta-FL in basements that have had very minor water infiltration issues in the past? Basically, there's lots of good reviews for it online, I'm just wondering if, all other things held constant (aka, no tradeoff in terms of reducing XPS foam insulation or reducing ceiling height) the concept behind it is snake oil or if there's a legitimate purpose/benefit to having it that exceeds the cost.
  10. Dana

    Dana In the trades

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    As long as you have 3/4" inch of concrete (or SilFlo) to bite into a rat-slab is more than sufficient for holding down subfloors with TapCons or similar. (Tip: Be sure to vacuum out the concrete dust before installing the screw.)

    There's no need to use PT wood for the interior window trim if you have XPS &/or EPDM between the wood and the concrete.

    If you've covered the slab with SilFlo and are laying down even a thin layer of foam with 6mil poly there's no particular advantage to the Delta FL approach. The Delta product is a good capillary break- better than a half inch of foam (which is by itself at least a so-so capillary break) but the SilFlo has much lower capillary draw than standard concrete, and 6 mil poly is a very good (and cheap) vapor barrier.
  11. bostondiyer

    bostondiyer New Member

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    6
    Location:
    Westwood, MA
    Hi Dana,

    That's great, it looks like I'm going to go with a poly vapor barrier and 1 inch XPS. Thanks for all the help.

    Two final questions:

    1. This basement is about 800 square feet total. The unfinished portion will have a constant-drain dehumidifier going 24/7/365 and the walls will be Drylok'd, while the finished ~400 sq ft will be air conditioned as well. Under these circumstances, could I get away with a single layer of plywood glued to the foam, with the whole thing floating (no Tapcons)? I'm trying to maximize ceiling height and minimize slab penetrations.

    2. Is laminate flooring okay here?

    Thanks!
  12. Dana

    Dana In the trades

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    If you rely on glue alone with no anchors you may see some seasonal "waves" or "potato-chipping" developing in the floor. Even with the dehumidifier running I'd expect at least some seasonal moisture level changes in the wood.

    Laminate flooring would be fine.
  13. bostondiyer

    bostondiyer New Member

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    Location:
    Westwood, MA
    Hi Dana,

    Final question, I promise -

    I'm still very wary about screwing into the slab at this point, as it's still in poor condition even after the Silflo. So I'm going with a floating subfloor, with a double layer of OSB as you recommended.

    T&G OSB is 23/32, while regular OSB is half the cost and only 7/16. What would you think about a double layer of non-T&G OSB with staggered seams, screwed together extensively throughout? So basically, slab > 6-mil poly > 1" XPS > foam construction adhesive > double layer of 7/16 non-T&G OSB, screwed together extensively.

    Thanks

    EDIT: Also - I was thinking of leaving a 1/4" gap at the edges along the walls and sealing with Great Stuff, so if the OSB expands it will have room to do so - good or bad idea?
    Last edited: May 24, 2014
  14. Dana

    Dana In the trades

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    If you double-layer the OSB with seams lapped and bond them together with screws & construction adhesive you'll be fine.

    You'll probably need more than 1/4" of expansion capacity on the edges for a floated subfloor- give it about 1/8" for every 5' of dimensional width (eg: For a rectangular 20' x 40' room give it an inch to expand along the 40' axis, a half-inch on the other.) It won't actually expand that much unless it floods, but it moves more than you might think even under normal cycling.
  15. bostondiyer

    bostondiyer New Member

    Messages:
    6
    Location:
    Westwood, MA
    Hi Dana,

    I want to thank you for all your advice. Everything is built now (only finishing work left) and it turned out great.

    Now that the window is installed and sealed, I have a question about the sill and other areas around the window frame where the concrete block is exposed. It's probably ~6 inches of concrete block plus 2 inches of XPS and 3 1/2 inches for the 2x4 stud wall. As of right now I've dryloked it but I'd like to sheetrock the sides and insulate the concrete block to prevent thermal bridging. Would 1/2" XPS plus 1/4" sheetrock be sufficient? I have limited space to work with so it couldn't be much thicker than that. I'm just worried about condensation on the drywall/concrete block that could facilitate mold growth.

    Thanks
  16. Dana

    Dana In the trades

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    With only XPS between the sheet rock and concrete you won't have huge condensation/mold issues. In that stackup the primary condensation issue is wintertime condensation on the painted interior surface. Half inch XPS isn't much of a thermal break over the concrete from a heat loss point of view, but it's way better than nothing, and should limit the amount of wintertime surface condensation on the painted 1/4" sheet rock- it's worth doing. The thermal performance of half inch XPS is comparable to U0.35-U0.40 window. When it's really REALLY cold outside there can be some surface condensation, but unless you keep the house air unusually humid in winter it won't be dribbling way making puddles every day. If you DON'T install some R there it probably will puddle (when it isn't frost/ice instead.)

    At only 1/4" sheet rock it would technically be a code violation as an insufficient thermal barrier against ignition on the foam. If the total surface area of the XPS covered by the 1/4" sheet rock is small (probably is) it's probably not worth worrying about from a fire-spread point of view though. It takes half-inch sheet rock to meet spec on the thermal barrier. An alternative would be to use unpainted 3/4" -1" fire-rated polyiso (the Dow Thermax with the white facers, not shiny aluminum) and no sheet rock over it.
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