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Thread: Best Subfloor option?

  1. #1
    DIY Junior Member Jallia's Avatar
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    Default Best Subfloor option?

    Hi all,

    I'd like your thoughts on which of these options would be best for a basement subfloor. We recently purchased a house with a finished basement (minus the floor) which was ripped out due to a water softener tank bursting and flooding the basement. When we had the house inspected, no moisture issues were detected in the basement. The walls are finished but the flooring is down to the slab. We are planning on having 10mm laminate installed on the subfloor. Here are the options I've been provided with from our contractor from least to most expensive:

    5/8" T&G OSB over platon with all joints and perimeter sealed

    5/8" T&G spruce ply over platon with all joints and perimeter sealed

    Dri-Core

    5/8" T&G OSB over 1" rigid foam

    5/8" T&G spruce ply over 1" rigid foam

    We live outside of Toronto so it gets cold here in winter. My husband works from home and is in his basement office all day. I want something that will keep the room feeling warm so I'm thinking the 5/8" T&G spruce ply over 1" rigid foam is the best option. From what I've read in other threads, it sounds like laying 6 mil poly under the foam would also be a good idea to prevent any possible moisture issues. However, there is a $700 difference between the least expensive and most expensive options. I want to make sure that the price difference is worth it. Any thoughts on this?

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    If you don't want to put in radiant heat mats in the floor, I'd go with the ply and foam. Dana is very knowledgable on this sort of stuff...he'll probably pipe in later.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
    DIY Junior Member Jallia's Avatar
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    Thanks Jim. I appreciate your input. Looking forward to what Dana has to add in as well.

  4. #4
    In the trades Dana's Avatar
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    Even with Platon as a barrier against ground moisture, the bottom of the sub floor can be cool enough to condense room moisture permeating the OSB or ply, with some risk of mold/rot over time. The basement slab also represents a significant heat loss at Toronto's subsoil temps. (The severity of that loss will vary by soil type and ground moisture.) In Toronto R5 of rigid XPS insulation beneath the subfloor is easy to make an economic analysis work on heating fuel savings alone. And from a comfort point of view it's a no-brainer too. In fact, if you have the head room, even R15 would be cost-neutral in a 15-20 year analysis if you went with cheaper Type-I EPS (lowest density bead-board), but it soaks up 100mm of headroom. Splitting the difference, you can R8 Type-II (medium density) EPS for about the same money as R5 XPS (the pink or blue stuff), about 38-40mm thick.

    If using EPS instead of XPS, absoluted DO use poly between EPS & concrete. EPS is about 5x more vapor-permeable than XPS at a given thickness. Even though it's closed-cell, the interstitial gaps between the beads provides a better vapor path, even though it's waterproof to liquid water. With XPS you could probably skip the polyn an otherwise dry basement- the additional water vapor rejection you'd get with poly is 10x that of 25mm XPS, but the foam itself is a HUGE improvement over bare untreated concrete. Still, the cost of poly isn't much- your call if you use XPS.

    R8 sheets of Type-II EPS with poly facers is available through box stores- if you go that route there's no point to additional poly- just tape the seams with housewrap-tape before laying down the sheet-wood.

    OSB is cheap & effective- it would only represent a problem if the place were prone to occasional flooding. Under highest-humidity OSB grows mold more readily than plywood, but with at least R5 of insulation and even modest vapor-retarders between subfloor and the cold cold ground there's little avantage to going with plywood.


    [edited to add] Do NOT use iso (polyisocyanurate) rigid board under the subfloor. It gives you ~20-50% more R per mm, but it's only a semi-closed cell structure and could saturate with repeated wettings. (It's usually sold with foil facers at box stores, but can be had with fiberglass or asphalt facers at building supply distributors that deal primarily with contractors.) It's great under roofing, where it can eventually dry toward the interior, or on wall structures (in a properly designed stackup for the climate) but on a basement slab there can be issues.
    Last edited by Dana; 11-19-2010 at 12:50 PM.

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    DIY Junior Member Jallia's Avatar
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    Thanks so much for your input Dana. Head room is not an issue as we have nearly 8' between slab and ceiling. With that said, would R15 XPS be your first choice or would you lean towards the R8 Type II EPS? I think I'll have the poly added regardless (unless we use the EPS with poly facers), as the cost isn't really that great in a 700 sq. ft. basement.

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    Nuclear Engineer nukeman's Avatar
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    Make sure that you also think about things like stairs, doors, etc. when looking at subfloor thicknesses. The R15 XPS would be about 3" (~75mm). The R5 XPS will be 1" (25mm). Just something to think about.

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    DIY Junior Member Jallia's Avatar
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    Quote Originally Posted by nukeman View Post
    Make sure that you also think about things like stairs, doors, etc. when looking at subfloor thicknesses. The R15 XPS would be about 3" (~75mm). The R5 XPS will be 1" (25mm). Just something to think about.
    Hmm...very true. I suppose there's some kind of building code regarding stair risers being the same height? We also won't be putting a subfloor in the furnace room so we don't want to step down in to the great abyss whenever we go in there...

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    Nuclear Engineer nukeman's Avatar
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    Correct. I'm not sure what it is in your area, but there is typically a requirement that the riser heights cannot vary by more than 3/8" (9.5 mm) in height between the tallest/shortest.

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    DIY Senior Member dlarrivee's Avatar
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    How much head room do you have?

    I'm a big fan of NOT using OSB in a basement.

    I would use a dimpled membrane if you don't have much head room, and as much XPS as possible if you have a few inches to spare.

    You would have to rework the entire stringer to fix the last riser, or your final step into the basement would be an awkward feeling one.

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    In the trades Dana's Avatar
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    XPS gives you a 25% increase in R value for any given thickness, but it's substantially more money at any given R value. Figure out how much headroom you're going to give up, then see if the modest improvement in R is really worth it. R8 vs. R10 for a 50mm lift isn't really a huge performance gain, but it's a substantial cost savings to go with EPS. But if you're only giving up 25mm, you need all the R you can get- XPS it is.

    OSB is only a disaster in a basement if it's likely to flood (in which case plywood isn't so hot either.) Protected from condensation by the insulation, and from ground moisture by poly (&/or XPS, if you go that route) it's not going to have issues. In summers you'll have to dehumidify to keep the basement under 60% relative humidity during the muggy weeks anyway. If it goes over 70% you'll have more than just moldy OSB to worry about (EVERYTHING will be susceptible.)

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    DIY Senior Member dlarrivee's Avatar
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    Okay well I just don't like OSB anywhere, anytime then.

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    In the trades Dana's Avatar
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    Fair 'nuff- you don't HAVE to like it! ;-)

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    DIY Junior Member Jallia's Avatar
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    We just got a re-quote from the contractor who said his original quote was for a rectangular room and the basement is actually finished with 4 rooms so more cutting, etc. Talk about a price difference! How difficult would you say this project would be for do-it-yourselfers? We've laid about 1000 sq. ft. of engineered hardwood before so I'm pretty confident about the laminate we plan on putting on top of the subfloor, but it's the sufbloor itself that scares me...any thoughts?

  14. #14
    In the trades Dana's Avatar
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    This is not rocket science stuff but it can be time-consuming to cut everything to fit around obstacles & odd corners.

    If you're using 25mm XPS it can be reasonably cut with a utility knife (or a hand-saw), and any moderate dimensional gaps at the edges can be ignored. You'll want to use power tools for the sub-flooring itself, and use a roto-hammer (aka "hammerdrill" in some parts- not sure what they call it in Ontario) to drill through the subfloor & foam into the slab to anchor the subflooring to the slab with masonry screws every 40-50cm.

    Alternatively (or when using thicker material making screw lenghts cumbersome) you cab float the subfloor by gluing the foam to the slab and it to subfloor to the foam with purpose-specific foam-board adhesive (it's solvents won't attack the foam the way standard construction adhesives can.) Use hazelnut-sized blobs of adhesive on a 40-50cm grid, and be sure that the seams of the subfloor do not coincide with the seams in the foam- give it at least a 25cm overlap. You typically get about a 15-20minutes to adjust the position of the sheet-goods before the adhesive sets up- it's fairly forgiving in that regard. Press the foam/subfloor down once it's perfectly positioned as you're gluing it, to minimize any gaps created by the adhesive.

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    DIY Junior Member Jallia's Avatar
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    Ok, stupid question. If I go with the gluing method, what happens if I want to put down 6 mil poly first?

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