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Does a viable (performance and cost effective), non-foam option exist for under slab insulation?

First, I'd like to thank the many pros and contributors here at GBA. I have been reading and learning for quite some time. Thank you all, first for caring about green building, and second, for being willing to share your time and expertise.

My husband, four year old daughter, and I are preparing to build our first (and hopefully last!) home in western North Carolina. Our goal is to build a home that is healthy for both people and planet. Working within the constraints of our budget, we are trying to source the most toxin free and energy efficient products/systems possible. We have been all over the gamut on wall design (hempcrete, Durisol, Liteblok, double stud wall) getting close to settling on 2 x 4 walls with Air-Krete insulation (R-21), along with two inches of Roxul ComfortBoard IS (R-8) on the exterior to reduce thermal bridging. This should give us a nominal R-30 (including sheathing/wall board), though I'm concerned that the effective R-value may not be enough for our Zone 4 climate. Our target is the following recommendation from Alex Wilson (http://www2.buildinggreen.com/blogs/how-much-insulation-enough):

{Moderate climates: Zones 3-4
For these locations, I recommend intermediate insulation values between those for cold climates and hot climates. I suggest a 4-5-10-30-60 rule: R-4 windows, R-5 under slabs, R-10 foundation walls or slab perimeter, R-30 above-grade walls, and R-60 in the attic or roof.}

For those interested, our planned wall, exterior to interior is: fiber cement siding, rainscreen, weather resistive barrier, 2" of Roxul ComfortBoard IS, 1/2" Magnesium Oxide Board sheathing (with air-tight approach), 2 x 4 stud wall (16" o.c.) with 3 1/2" of Air-Krete, 1/2" Magnesium Oxide Board, and zero VOC paint (we plan to have "outie" windows). We are also strongly considering switching to 2 x6 walls with 5 1/2" of Air-Krete (R-33) which would increase our nominal R-value to R-42 (about an extra $3,000 in cost).

Our single level, approximately 2100 sq. ft. home will have a slab on grade foundation and is oriented to the south for passive solar (with polished concrete floors providing thermal mass). Our roof will be hipped and our walls will be nine feet tall with flat ceilings (no can lights in ceilings). Our attic space will be non-conditioned (no mechanicals or ducting will be in attic) with Air-Krete sprayed on the attic floor. We are planning to use ductless mini-splits and fiberglass casement windows (we're currently looking at Alpen and have also considered Inline and Marvin Integrity).

We will also be putting Roxul ComfortBoard IS or DrainBoard on the slab edges. My problem is in deciding what to do under the slab. Understanding that foam may be the most cost effective for performance value, I'm trying to determine whether we have any other options that do not greatly increase the price or decrease the performance value. I'd love feedback on the following:

1. Roxul DrainBoard. In searching the web, there seem to be a few folks who think this would be okay under the slab. However, there are also a few who seem to feel it is too early to know whether Roxul is a good choice under the slab. This leaves me feeling uneasy.

2. Perlite. Please see: http://www.perlite.org/library-perlite-info/insulation-perlite/Perlite-u...

3. Pre-crushed Durisol chips with a quarter inch layer of Portland cement and landscape filter fabric (idea from "Breathing Walls" by Swanson, Miller, and Federer).

4. Foamglas. I've had difficulty sourcing this and getting pricing. From what I've read, it's expensive.

If none of these options are plausible, I suppose we will be left with EPS (due to more environmentally benign blowing agents than XPS).

If you can shed light on the underslab insulation or if you see any problems with any other details, I would greatly appreciate your help. I continue to learn as much as I can, but recognize that I still have a long ways to go!!

Thanks in advance for any thoughts or advice you may offer!

Asked by Stacey Owens
Posted Mon, 03/31/2014 - 18:56

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10 Answers

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1.
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Stacey,
The short answer is that you should use EPS under your slab.

The long answer is that the best foam-free alternative is Foamglas. The downside is that Foamglas is very expensive.

I would categorize the other options you list as experimental and therefore risky.

If you insist on a foam-free house, one design to consider is to build the house on piers (or to build a ventilated crawl space foundation) and insulate the wood-framed floor assembly with cellulose. This option makes it hard to install a wheelchair ramp.

Finally, I'm mystified by your decision to use AirKrete between your studs and on your attic floor. AirKrete is crumbly and can shrink. Most experts would advise you that cellulose makes a lot more sense in these locations.

Answered by Martin Holladay, GBA Advisor
Posted Tue, 04/01/2014 - 05:40

2.
Helpful? 0

Thank you for your response, Martin! Bear with me as I try to reason through this. Are these other options risky only in terms of thermal performance or would there be other concerns as well? The reason I ask is that in our area most people do not put any insulation under their slab (it's great that slab-edge insulation is now mandated by code though!). Following Alex Wilson's recommendation above, we ideally would like to have a minimum of R-5 under the slab.

I contacted Roxul directly about using DrainBoard under the slab and this is what I received:
"Thanks for the inquiry into ROXUL. I have heard of several people using ROXUL DRAINBOARD under slab but we have not tested it in this application, so I cannot comment on how well it will perform. It may work but a couple of the concerns would be when the board is placed horizontally under a concrete slab, I do not know how the direct compression will affect it or if the area is wet, hydrostatic pressure will force water into it and reduce the R value."

If an air gap membrane (http://www.cosella-dorken.com/bvf-ca-en/products/foundation_residential/...) were used on top of the pre-crushed Durisol chips mentioned above, would that option become less risky?

Will you speak to the following from "Breathing Walls" as it has me concerned about using EPS along with a vapor barrier: "Conventional slab construction uses a polyethylene vapor barrier (retarder) with or without extruded polystyrene as insulation, which can produce mold growth on both sides of the barrier within a few months of construction. This is because of dampness in the soil, which is always trying to escape and "breathe" to the air above. Conventional construction tries to impede this natural flow of moisture, which creates problems. You see evidence of this when you crack open a conventional slab and examine the underside, often revealing patches of mold growth."

We are trying so hard to get this right ~ thanks again to you, Martin, and to anyone else willing to help!

Answered by Stacey Owens
Posted Wed, 04/02/2014 - 23:06

3.
Helpful? 0

Martin

Do you worry about the durability of the air krete in the walls. Will it lose it seal to the studs. I am a believer in redundant air barriers. You dont know where a leak will be and if there is another layer that helps reduce the leakage from a primary air barrier. Joe L says that dense pack is close to being an air barrier,,,,,it doesnt replace as air barrier. But it would back up the primary air barrier,

What is the cost of Air krete. I assume it is more expensive than dense pack. Would it be better to use either dense pack cellulose or fiberglass and place the savings into thicker walls

Answered by Robert Hronek
Posted Thu, 04/03/2014 - 01:53

4.
Helpful? 0

Stacey,
Q. "Are these other options risky only in terms of thermal performance or would there be other concerns as well?"

A. Even the manufacturer of Roxul insulation informed you, "I have heard of several people using ROXUL DRAINBOARD under slab but we have not tested it in this application, so I cannot comment on how well it will perform." If that statement from the manufacturer isn't enough to dissuade you, it hardly matters what I write. If you want to experiment with this approach, go right ahead.

Q. "Will you speak to the following ... as it has me concerned about using EPS along with a vapor barrier: 'Conventional slab construction uses a polyethylene vapor barrier (retarder) with or without extruded polystyrene as insulation, which can produce mold growth on both sides of the barrier within a few months of construction. This is because of dampness in the soil, which is always trying to escape and "breathe" to the air above. Conventional construction tries to impede this natural flow of moisture, which creates problems. You see evidence of this when you crack open a conventional slab and examine the underside, often revealing patches of mold growth.' "

A. I disagree with this analysis. The reason that you want a vapor barrier and insulation at this location is to create a barrier between the warm, dry interior and the cool, damp soil. Encouraging moisture flow from the damp soil into your home would be nuts.

Answered by Martin Holladay, GBA Advisor
Posted Thu, 04/03/2014 - 12:00
Edited Thu, 04/03/2014 - 12:01.

5.
Helpful? 0

Robert,
Q. "Do you worry about the durability of the Air Krete in the walls?"

A. I know that it is crumbly and can shrink, which is why I wouldn't use it.

Q. "Will it lose it seal to the studs?"

A. Possibly, yes.

Q. "What is the cost of Air Krete?"

A. Call up local contractors and ask.

Q. "Would it be better to use either dense-packed cellulose or fiberglass and place the savings into thicker walls?"

A. Yes, in my opinion.

Answered by Martin Holladay, GBA Advisor
Posted Thu, 04/03/2014 - 12:04

6.
Helpful? 0

Martin,

Forgive me, I should have been more clear. I have absolutely been dissuaded from using the Roxul DrainBoard under the slab. I was trying to glean whether using the air gap membrane over the pre-crushed Durisol chips would be effective. I think I have made my peace with the environmental concerns surrounding the EPS and plastic vapor barrier, however, I was looking for reassurance regarding the mold concerns. I certainly agree with you that we do not want to encourage moisture flow into our home. I am trying to determine the best way to mitigate the moisture without breeding any mold.

I do very much value your opinion as I have been reading your informative blog for a long time. Thank you!

Answered by Stacey Owens
Posted Thu, 04/03/2014 - 12:41

7.
Helpful? 0

Stacey,
Durisol is made from wood chips mixed with Portland cement. I think that using Durisol chips as sub-slab insulation is risky because it is experimental. If you like to experiment, go ahead and install the Durisol chips.

I think that investing in the Delta-MS Underslab Sub-Base Course & Vapor Retarder (the product that you linked to) is not a good use of your money for a residential job. Plain old 6 mil poly should work fine.

Answered by Martin Holladay, GBA Advisor
Posted Thu, 04/03/2014 - 13:05

8.
Helpful? 0

Pebbled pumice is good for about R2/inch.

So is EPS-loaded concrete, eg:

http://www.epscement.com/en/about-eps-cement1.aspx

http://www.greenconcreteus.com/epscrete.html

http://foamliteconcrete.com/

http://www.styrocrete.com/

It hasn't really taken off as a building material in the US, but is a pretty good use for the piles of scrap EPS bead found at most EPS molders. (It has to be compressed to it's pre-expanded density to be reasonably recycled as polystyrene.) There are a lot of DIYers taking this approach but it may be tough to get a DIY batch to fly with inspectors without an actual product spec.

Answered by Dana Dorsett
Posted Fri, 04/04/2014 - 17:05

9.
Helpful? 0

Sorry, Dana, I missed your response somehow. Thank you for taking the time to share those websites.

We have decided to pursue the Foamglas if we can swing it in the budget.

Answered by Stacey Owens
Posted Tue, 04/15/2014 - 16:13

10.
Helpful? 0

Stacey,
The answer to your question should be a simple YES. And in your list of alternatives you list what is probably the most cost effective under slab insulation. Martin chooses to disparage Perlite by tagging it as "experimental". While it is not popular and no giant petrochemical company, with advertising dollars to spend, promotes it, it has been used in numerous passive house projects in the US and many more in Europe.

Answered by Jerry Liebler
Posted Sat, 04/19/2014 - 17:35

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