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Community and Q&A

Insulation type, etc.

learner15 | Posted in Energy Efficiency and Durability on

hi all,

i have a semi-detached brick 2-family home in brooklyn. 20′ x 65′. tenants are on the top floor and we occupy the first 2 floors. our ground floor is about 3 feet in ground on 3 sides and at grade in the rear. we are renovating and intend to put 2 full bathrooms and 3 bedrooms there, in addition to the laundry and utility room. we recently install a navian ncb240 which currently serves our dhw and baseboard radiators for both floors (tenants have separate boiler).

we are preparing to remove the current rat slab and repour a new slab on the ground floor. we will install a french drain, vapor barriers on both the walls AND floor. additionally, we will lay 2′ POLYISO INSULATION beneath the new slab. finally, i want to lay tubing before the pour.

throughout my research, i’ve taken note of the following considerations, of which i am still trying to firm-up:
1) 1/2″ pex w/ oxygen barrier,
2) 12″ center (though some say closer, depending on heat loss analysis),
3) keep loops same length (prob around 220′ for me), what is margin of difference? 5′?
4) mixing stations (later, i will staple up tubing to heat upper floor, so higher temp needed),
5) remote manifold toward center is recommended because boiler is at end of floor.
6) sensors?
7) glycol?
8) how far do i lay tubing from interior walls, which are not up yet, and plumbing for 2 bathrooms?
9) is polyiso the best option to insulate below the slab?
10) what am i missing?

this site was recommended as a place to get contentious and informed advice. ALL IS APPRECIATED.

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  1. charlie_sullivan | | #1

    I'll offer an opinion on one item: I would recommend EPS over polyiso under the slab. EPS absorbs moisture than polyiso, and loses very little of its R-value when that happens. whereas polyiso can lose a lot of its performance in that case. And EPS is substantially cheaper per unit R-value. The only downside is that EPS takes more space for a given R-value, so you'll need to remove perhaps an additional inch of soil vs. what you'd need to remove with polyiso for the same nominal initial R value. But for the same final R-value taking into account possible degradation, you don't need any more thickness of EPS than you would polyiso.

  2. GBA Editor
    Martin Holladay | | #2

    Charlie is right; you can't use polyiso under a slab. Either XPS or EPS will work. Of these two types of insulation, EPS is more environmentally friendly. (XPS is manufactured with a blowing agent that has a high global warming potential.)

    If I were you, I wouldn't try to use rules of thumb to design a hydronic heating system with in-floor tubing. Unless you are experienced at designing hydronic systems, you need to have your heating system designed by a mechanical engineer or an experienced, competent contractor.

    Remember: It's OK to change your mind about the PEX tubing, and use a different method of heating your lower floor. But it's not OK to skimp on subslab insulation. The insulation is more important than the PEX tubing.

  3. learner15 | | #3

    thanks for the info, though i'm not entirely clear. if i have a vapor barrier, why would the polyiso degrade? at what rate does it degrade? i can't really clear out more soil than already planned, because of cost. r-13 in 2 inches sounds great. should i really sacrifice r-value to use the EPS?

  4. GBA Editor
    Martin Holladay | | #4

    Q. "Should I really sacrifice R-value to use the EPS?"

    A. You're not sacrificing anything -- you're just using the right material for the job. Polyiso is not rated for below-grade use or soil contact. Polyethylene will not keep buried material dry. (If you want to conduct an experiment, wrap a cardboard box of corn flakes in polyethylene and bury it. Dig it up in a year and see if it is dry.)

    If you want XPS instead of EPS because the XPS has a higher R-value per inch, that's your call. But you can't use polyiso.

  5. Dana1 | | #5

    Over the full lifecycle of a slab XPS is not higher-R than EPS of equal density. It has a performance edge for the first few decades, but as it loses it's blowing agent it's performance drops to the that of EPS.

    Polyiso is mildly hygroscopic- it wicks and hangs onto moisture. With a PERFECT moisture barrier between the polyiso and the soil it won't take up ground moisture, but during the pour it will take on moisture from the concrete, and any water that lands on the concrete thereafter, drying only very slowly. If you put the vapor barrier between the foam & concrete, it'll take on ground moisture, and become saturated. It's fine to use it for insulating the interior side of below grade foundation walls, but not the exterior, and never under slabs.

    The R-value of saturated polyiso is well below that of even pond-soaked EPS, so yes, even at only 2" you're better off with EPS under the slab. But if you're heating the house with the slab 3-4" would be better, depending on your climate / location. But in any climate, the additional energy savings of the additional short-term R1.6 advantage you would get with R10 / 2" XPS vs.R8.4 / 2" Type-II (1.5 lbs nominal density) EPS will never be "paid off" by the ever so slightly lower greenhouse gas emissions from the heating system in a basement slab in the time it takes for the HFC134a to leak out, doing it's damage. (It might for some slab-on-grade in very cold climates with that little insulation.)

  6. charlie_sullivan | | #6

    Building on what Dana said...even just considering the first few decades, even though XPS has higher R-value per inch than EPS, it has _lower_ R-value per dollar spent. So for a given amount you'd be willing to spend on insulation, you'd be sacrificing R-value by using XPS, and maximizing it with EPS.

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