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Question regarding basement subfloor options. Amdry panels vs Rigid Foam Panels with Plywood

sepa82 | Posted in General Questions on

Greetings. I’m a long time reader of this website and a first time poster. I’ve learned so much from this community, but now I’ve got a question that I’ve been unable to find the answer to so far just by reading articles or Q+A posts.

I’ve been slowly working towards finishing my basement. Starting with insulating. My goal is to essentially turn the basement into a styrofoam cooler and then begin the “finishing” part once it’s sealed and fully insulated. To date we have put 2 inches of closed cell spray foam on all rim joists, added two inches of rigid foam board on all walls. The boards have staggered seams that are taped, but also bonded with spray foam. The foam goes from floor to rim joist. The crawl spaces are sealed in the same manner, except that I also put 2 inches of foam board on the floor with staggered seams that are taped and foamed.

As of yet the main basement part still has a concrete floor. The home was built in 1992. I can’t be sure that plastic sheeting was put under the slab so I am assuming that it was not. In a previous article (https://www.greenbuildingadvisor.com/blogs/dept/musings/new-green-building-products-june-2013) Martin seemed to give a favorable review to the AMdry Subfloor system. It would provide adequate insulation beneath the subfloor material, but I’m leery of it’s ability to stop moisture vapor from coming up between it’s joints. I don’t see how it’s connector system could possibly be vapor impermeable. After that initial write up from Martin I haven’t heard anything on this website about AMdry panels as a viable solution for basement subfloors and that raised my suspicions about them.

Obviously rigid foam panels glued to the floor with sealed seams and plywood over top tapcon’ed 6-8x per panel would be as perfect as one could hope for, not to mention cheaper. But my basement is is large and my the thought of attaching 50 full sheets of plywood to concrete with a hammer drill is daunting considering my elbows are already injury prone.

At the end of the day I want to do the best job, but I do have to weigh the installation time and joint damage. Do you feel that AMdry panels can provide adequate protection from adsorption on their own or would I be taking to much of a risk by foregoing the tried and true rigid foam route. I should note that I will not be carpeting the floor. It would most certainly by LVL over the subfloor.

Thanks for any and all help.

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Replies

  1. Nick Welch | | #1

    The Dricore/Amdry approach just seems fundamentally flawed. The seams expose the OSB to the exact conditions that we all know we need to protect organic matter from in a basement.

    I've only done a small test section of my basement (2 4x8 sheets worth) floor so far, but using 23/32 OSB, a fastener in each corner (so 4 per sheet) seems sufficient. Might end up needing an occasional extra here and there as needed. I'm trying to get by with the minimum and this seems to be working for me.

    I had a bad time with tapcons. No matter how thoroughly I tried to clear the dust out of the holes, they would bind up, not go in all the way, strip, break, etc. Even when I drilled all the way through the slab. I ended up switching to split pins with a flat countersunk head. You just hammer them in. They seem to go in with fewer problems.

    Also, OSB seems to be more consistently flat than plywood, which is a significant part of why I chose it.

  2. Roger Berry | | #2

    Sepa82,

    Before you commit to all that work, you should do a simple test to see how much moisture is actually in the slab. Especially if they didn't put plastic under the pour or if they also skimped on the stone under the plastic. I am assuming there is no insulation under the slab as well.

    Take a two sheets of clear plastic about 3 feet square and lay them on the floor - one in the middle of the biggest area and one in/near the basement corner with the highest ground elevation outside. Tape them all the way around tightly and wait a week or two. If moisture collects under the plastic in any visible amount, that is what will be under your floor. If you have a sump pump already, probably plan on seeing lots of moisture.

    The waffle bottomed floor panels ostensibly allow airflow underneath to avoid excess moisture build up. Much like too thin ventilating paths for roofs, at 1/2-3/4" waffle, not much will ever move under the floor without help from a fan. The bigger the room, the more likely the middle air will never transport except up through the gaps like Nick said.

    If you are fortunate enough to have good perimeter drainage around the house and sufficient stone under the slab (thick enough to keep seasonal ground water levels at bay), you may find only a faint imprint of the plastic square after the two weeks. If so, great. Consider leveling any dips in the concrete slab with leveling compound and laying out the locking edge vinyl floor planks directly to the slab AFTER you seal the whole floor. Yes it will be slightly colder than with an 1" of foam, but vinyl always feels cold to touch.

    I needed to replace 40 year old tiles in a midwestern ranch with sketchy water control. I can't recall what product I was given at the tile store, but it had two purposes. One to seal the floor and to provide bonding to the left over adhesive film still stuck to the floor. Worked on both accounts and far as I know the floor is still stuck down nearly 20 years out. Locking vinyl strip floor would be even easier without the mess of glue. If you really want to be belt and suspenders safe, level and seal the floor and then possibly put down plastic under the vinyl. I would check with your local flooring people on the ins and outs of that.

    The prospect of fastening 50 sheets of plywood over foam is quite daunting and I agree with Nick that Tapcons suck. Even with the foam, putting any wood product over a floor that is transpiring moisture is asking for long term mold issues. Picking rotten plywood off bathroom floor joists is miserable enough. Picking it off concrete while clearing Tapcons (the ones that don't break off) sounds like a whole 'nother level of misery.

    If you have the head room and a nearly dry floor, you might consider sealing the floor overall, then laying down 2x4 sleepers 16" O.C. and attach the plywood as a floating floor assembly. No Tapcons. Plastic shims to level as you go. Some leveler compound for the final true up and vinyl of choice. The air gap and inherent R value of the wood sleepers will be close enough to the foam given vinyls normal "feel".

    Hope that gives you some useful food for thought.
    .

  3. Expert Member
    Malcolm Taylor | | #3

    Nick,

    Tapcons can be immensely frustrating. I think you will find these a lot more forgiving.
    https://www.grkfasteners.com/products/concrete/caliburn-concrete

  4. Nick Welch | | #4

    Malcom: I actually was using the Caliburn screws. I was just referring to concrete screws generically as tapcons.

  5. Expert Member
    Malcolm Taylor | | #5

    NICK,

    That's too bad. i find tapcons almost unusable, but have had good luck with the GRK screws. That said, any time I can avoid an assembly that relies on fastening to concrete I do. With the exception of epoxying rods, they all seem hit or miss.

  6. Sean Cotter | | #6

    I have looked into these products pretty extensively. I have limited head height in a 90+ year old basement (certainly no plastic underneath, 3-5" slab). I get vapor drive in some areas, none in other areas. A small dehumidifier keeps us under 40% with minimal work during the spring/summer.

    I need to pour SLC to level some small areas and mapei suggesting their epoxy to seal existing, then the SLC. I found a good rated product (want to say Armour Shield or something?) that was 1/3 cost of the Mapei (and you didnt need to buy a ton of it). I applied a small area, no vapor drive there now. Eventually I'll do all the areas and SLC where need be, then rigid core vinyl planks.

  7. sepa82 | | #7

    Thanks for the input guys. I've done the moisture tests you spoke of and had nothing noticeable on the plastic. I was under the impression that foam on the slab was always the go to for basement floors to avoid moisture/ mold.

    I also hate Tapcons. Thanks for the advice on which other fasteners to try. I'm not familiar with the split pins that Nick mentioned. The GRK's sound pretty good as well.

    So when you say sealing the floor before the sleepers are you talking about taping down plastic or something else?

    OSB is typically flatter, but in my experience it doesn't seem to respond to moisture as well as plywood.

  8. Jon R | | #8

    If there might ever be water under the floor, then I'd want it to freely flow to a sump/drain without wetting the OSB/plywood.

    Don't put wood between two non-permeable layers.

  9. Roger Berry | | #9

    Sepa82

    I was suggesting sealing with a paint like product. Epoxy would certainly be good at sealing, but if you have nearly 1600 sq ft to do, it will be very costly and smelly. I sadly cannot recall the brand of sealant the floor store sold me. It was definitely water based and most likely an advanced (for its time) acrylic of some type. I suspect there must be similar types today. Putting plastic sheet over the top of the floor isn't quite the same thing as bonding a paint film directly to the concrete.

    Sounds like you are very fortunate to have well done slab work if the plastic squares test didn't produce moisture. Still I would suggest considering the floating floor method and avoiding the fuss of drilling into the concrete. Maybe with the exception of a few to keep the first sleepers in place. You might want to consider using the Advantech-Gold Edge type or other brand of wet resistant subflooring. It is pricey like plywood, but I could not believe how well it stands up to wet and abusive weather. The "temporary" steps sat in the elements for nearly two years and I still re-used the landing top panel for a firewood platform. Once the slab is sealed, the amount of moisture under the floor will more likely reflect relative humidity for your location. Excessively humid air settling into the basement could create that memorable "basement smell" over time, but I think you would feel a kind gross level of moisture in the furniture long before the sleepers and sheeting were in danger.

    You haven't said where you are, so if it is Houston or New Orleans I take that last bit back.

  10. sepa82 | | #10

    Roger, I'm in South Eastern Pennsylvania. Philadelphia suburbs. Currently the basement floor is painted. I'm under the assumption that it was a drylock type of paint. I only say that because the walls had been drylocked prior to purchase. I was under the impression that any chemical sealant used to retard moisture/ vapor penetration would only be able to stand up for so long before eventually failing and letting in moisture which would begin the mildew/ mold cycle on whatever organic material was within reach.

    I'll definitely consider what you've mentioned about the sleepers and the advantech. I think I might still do a layer of rigid foam (sealed and taped) before the sleepers. I'll lose an additional inch of headroom, but I won't have to worry about if/ when the foam bond fails like I will with a liquid coating. An example of what I'm talking about was discussed by Todd Fratzel (http://www.homeconstructionimprovement.com/how-to-insulate-a-concrete-floor/)

    I guess I'm looking for the easiest, yet bulletproof basement floor insulation/ subfloor option. I guess from all of the posts the one thing in common is that the prefabricated amdry panels and equivalents aren't really up to snuff. Thanks for all the answers and Happy Easter

  11. Roger Berry | | #11

    Sepa82,

    I want to mention UGL products at this point, now that I have taken the time to go look them up again. I didn't use them 35 years ago because I felt that Thorocrete would be better for my first major basement rehab. It was harder to get in the city since it was primarily used for sealing silage towers and the like. Also was a major PITA to mix and apply, but it did finally stop all the water from weeping through my 30's vintage block foundation. I never attempted to seal the floor which I knew to be a shambles of batches poured over the dirt directly.

    It seems UGL has been busy in the ensuing years, so a trip to their site to look at all the new improved products might be worth the trip. UGL Extreme does allow placement over existing coatings albeit without warranty. If you think the existing coatings are actual Drylock brand materials by UGL you might find another of their products would be even better suited. Might go a long way toward explaining why you don't see moisture, too. My now rather outdated mindset about UGL was based on its formerly needing a cleaned open concrete surface to bond too. It was pretty fussy about that requirement in those days, as I learned during my time working after school in a hardware/paint store. It also took more coats than they wanted to admit, too.

    Had a bit of a laugh when I checked out your link in prior post. You probably think I was just cribbing Mr. Fratzel. Alas, I learned most things the hard way, or with help from others who went before me. I would agree that a tongue and groove edge helps a lot for underfoot stability. Sheeting on sleepers can feel a bit "squirmy" under foot if seams are not locked or sitting on the sleepers. I would not bother with treated 1x stock unless the quality you see in your part of the country is better than what I see here in Colorado. The cleanest 1x4 will save a lot of screw splits that make the anchoring less than optimal.
    I also personally prefer XPS over EPS regardless of a certain contributor's feelings. Cuts cleaner, hangs together better, and tapes way better.

    I will wager that your humidity is no worse than Chicago or Boston where I lived in the past. Best of luck with the project and may all you eggs be Cadbury's.

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