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Thread: Basement Foam Board Question

  1. #1
    DIY Junior Member BrianC's Avatar
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    Default Basement Foam Board Question

    I'm finishing my basement. I plan to glue 1" XPS to the exterior concrete walls. Then I will build 2x4 walls against the XPS, fill with roxul and cover with drywall. All of the XPS seams will be taped or foamed sealed. I have read a lot about how important it is to make it air tight. My problem is that one of the concrete walls has ductwork and wiring running right up against it for about 10 feet at the ceiling. There is about 1/2" between the duct and the concrete. I can't move the duct. Do I need to get that section of the wall covered in XPS or can I leave it bare concrete? Maybe slide a 1/4 sheet in there? Does it matter? Is it ok for the XPS to be in contact with the ductwork? There will be a drywalled bulkhead built around the duct, so I'm not concerned about the XPS being exposed to flame, just heat.
    I don't know if it matters, but the house is in Western Maryland. It's built into a hill so the back wall is below grade and the side walls are only partially below grade (bottom back corners). The wall in question is pretty much above grade except for the bottom couple of feet or so.

    Thanks.

  2. #2
    DIY Junior Member BrianC's Avatar
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    222 views and not a single reply? Is this just an unclear question or did I stump the panel?

  3. #3
    In the trades Dana's Avatar
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    The temperature of the duct matters- upper service temp of XPS is ~160-170F, but shrinkage can occur over time even at lower temps. If you can slip in some half-inch foil-faced polyisocyanurate it's rated for use up to over 200F (and won't deform until ~300F).

    It might be easier to make it air-tight by insulating the gap with FrothPak (1.5lb density polyurethane), which is good to about 240F. At a half-inch of thickness a 12 board foot kit from a box store would cover ~24 square feet, and you can use any extra to seal your rigid foam to the foundation & foundation sill, etc. Any spray polyurethane will pretty much glue your duct to the wall though (it's a similar chemistry to Gorilla Glue.)

    If you can use 1.5" EPS rather than 1" XPS it's usually a bit cheaper, and is more environmentally benign, since EPS is blown with pentane which has about 7x the greenhouse potential of CO2 rather than HFC134a (used in nearly all US XPS) which is something like 1400x CO2. At 1.5" it's ~R6 (any density), and has at least as good an up-rating curve as XPS with rising temp. When it's below 25F outside 1.5" of EPS on the above grade portion (which is most of your wall in question) will be performing at ~R7 in your stackup. With R15 Roxul and 16" o.c. studs, with the thermal bridging of the studs factored in you're looking at about an R15-R16 "whole wall" value.

    Alternatively, if you can find a source for reclaimed roofing iso (polyisocyanurate with fiber or paper facers), that stuff is about R5.5-6 per inch and denser than the foil faced goods you find in box stores. At 3.5" you're at R20, and you can hold it in place with 1x furring through-screwed to the foundation with 5" Tapcons 24" o.c. on which to hang the gypsum, and you'll get pretty much the full R20. Reclaimed iso is typically 25-35% the cost of virgin stock, and would come in at about the same cost as virgin stock 1" XPS. I have several vendors in my area advertising reclaimed roofing foam of various thicknesses on craigslist, but I also have a local outlet for insulationdepot.com (which will sell small lots if you have your own truck, but will ship nationwide in larger quantities.) It can even be cheaper per R per square foot than R15 Roxul.

    It comes in a number of standard thicknesses, but you'll need to dig up a source for longer TapCons than what's usually available at box store at foam thicknesses over ~2.5"/R15. (Online vendors, other local hardware stores, etc.) The exposed edge of iso needs to be 1/4" or so off the slab if you're not sure about the moisture content of the concrete & subsoil, since it can potentially wick up moisture over time, whereas EPS/XPS won't. (A single layer of 3" roofing iso was the retrofit solution at my house- it took more than 15% off my heating fuel use!)

    With your studwall approach, cut some 3.5" strips of the foam to put under the bottom plate of the studwall as a capillary & thermal break, and through-screw the plate to the slab with TapCons. That will keep the framing drier & warmer no matter what your sub-slab conditions, and you won't have to use pressure treated timber.

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    DIY Junior Member BrianC's Avatar
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    Thanks Dana for the detailed response. While I was at Lowes I saw some stuff in a roll called Reflectix, which is basically a thin layer of bubble wrap encased in foil. It has an R value of 3.7 and is only 3/8" thick, so it fits in the gap perfectly. What do you think about using this product for my application? Will it provide a benefit? More importantly, will it be counterproductive?
    http://www.lowes.com/pd_13353-56291-...=#BVRRWidgetID

    Thanks.

  5. #5
    In the trades Dana's Avatar
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    Reflective insulation only achieves it's specified R-values when air gaps are applied on both sides. Most of that bubble-wrap crap is exactly that- overpriced underperforming crap. Read the fine print on any ASTM test reports an you'll find that the "as tested" configuration will have a very different amount of air-gap than any practical "as installed". Any time you see waffling discussions about R-values and/or pictures of space shuttles, references to NASA etc, hold onto your wallet and keep on walking. (Unless you're building your house in a high-vaccuum high-radiant heat transfer environment such as the surface of the moon.)

    Radiant heat transfer between radiating & absorbing surfaces is relative to the difference of the fourth power of their absolute temperatures (relative to absolute zero). When temps are very close together, even though it's a 4th-power difference it does nearly nothing. They work best at very high temperature differences, with isolating air spaces between the radiating and absorbing surfaces. They might be able to be effective in reducing the heat transfer between say, a 140F roof deck and an uninsulated 50F air conditioning duct (a 90F temperature difference), but it's not necessarily the "right" solution even there, nor does it usually have the best bang/buck ratio. At the anticipated temperature differences you'll see between your studwall and concrete it's completely useless without air gaps, and almost useless even with air gaps.

  6. #6

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    Dana nailed it...as usual.

    Leave the Reflectix on the shelf. Stick to the rigid board.

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