Helpful? 0

Is there an alternative to the under-slab rigid insulation system in passive house design?

I think I saw a detail somewhere showing an insulated suspended floor over a crawl space in lieu of the slab on rigid normally used.

Asked by Janet Digby
Posted Fri, 11/02/2012 - 19:09

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14 Answers

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1.
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Here is a detail of such a connection that we at foursevenfive.com have been working on. It is a draft (comments welcome) but gives you the idea how to get away from foam - 9t indeed lifts you insulated volume from the ground by introduction of a crawspace.

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475 (draft) wall adjacent joist_X_basement.pdf 107.19 KB
Answered by floris keverling buisman
Posted Sat, 11/03/2012 - 00:33
Edited Sat, 11/03/2012 - 00:33.

2.
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Thanks - is it attached or do I find it on the website?

Answered by Janet Digby
Posted Sat, 11/03/2012 - 00:37

3.
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Thanks - looks interesting.

Answered by Janet Digby
Posted Sat, 11/03/2012 - 00:39

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Janet,
In this article, you can read about the Freas house -- a Passivhaus that has an unconditioned crawl space foundation. The floor insulation consists of 24 inches of blown-in fiberglass installed in 24-inch-deep floor trusses: More Passivhaus Site Visits in Washington State.

Answered by Martin Holladay, GBA Advisor
Posted Sat, 11/03/2012 - 07:37

5.
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How is the outer section of the wall assembly supported? Nothing on the cross section seems to give any info.

Answered by Richard Clark
Posted Mon, 11/05/2012 - 00:08

6.
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How is the outer section of the wall assembly supported? Nothing on the cross section seems to give any info.

Answered by Richard Clark
Posted Mon, 11/05/2012 - 00:08

7.
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Floris: First, I am not a pro. I see no real reason for the I-joists if the 2x4 wall is structural. Why not just use ply bracing and a 2x3 or 2x4 exterior wall like Chlupp and others do? Less "heat pumps" in the wall that way, and I would imagine far cheaper. Too, isn't 3/4" ply gross overkill, and spendy? Just thoughts.

Answered by John Klingel
Posted Mon, 11/05/2012 - 02:37

8.
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We are building a no foam passive house in Colorado http://rm-ph.com/projects/martak/ and by far the trickiest part was the foundation. We considered perlite below the thickened slab, then a perlite slab. With the new PHUIS climate data that was turning to be a unwieldy and expensive design. Adding cellulose insulation on top of a thin slab and a I joist floor is much cheaper and much lower environmental impact. The stem wall is isolated and modeled using therm modeling.

In Europe they have a very dense mineral wool board which can support a finished floor. That would be ideal for us.

Answered by Andrew Michler
Posted Mon, 11/05/2012 - 14:32

9.
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Andrew,
I assume you are describing a crawl space, right?

How much room did you leave between the slab and the bottom of the I-joists?

Answered by Martin Holladay, GBA Advisor
Posted Mon, 11/05/2012 - 14:39

10.
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John - you are right, there is nothing that stops you from replacing the I-joists with larson trusses.(the heat conductivity of a I-joist flange is pretty minimal, but a larson truss is better - it does cost time to build...)

Richard, the outer section is hung from the structural 2x4 wall. That is what is supporting it.

Answered by floris keverling buisman
Posted Mon, 11/05/2012 - 15:42

11.
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@Martin it is actually not a crawl space, if it was then we would just dense pack the TGI with 18" space to a dirt (plastic floor). We are suspending an OSB layer separated by some CCA studs onto the concrete floor. That is our air barrier and then lose filled cellulose on top. This is for a r-50 floor so at 3.5 an inch that is a more than 14 inches. As the PHPP model adjusts we can add more insulation which I know you say is crazy, but a lot cheaper not being foam. Also does great evacuating Radon with a small fan.

Answered by Andrew Michler
Posted Mon, 11/05/2012 - 19:38

12.
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Janet, you may be thinking of the recently-published article in Fine Homebuilding by Steve Baczek, called "A Practical Approach to Passive House". The slab has some rigid ABOVE the slab, then a conventionally framed floor (with densepack) above.

Answered by John Rockwell
Posted Thu, 11/08/2012 - 18:35

13.
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Andrew,
You wrote, "We are suspending an OSB layer separated by some CCA studs onto the concrete floor." I can't visualize what you are describing. But it sounds like you have a slab floor with wood components above, without enough room for a person to enter the crawl space between the slab and the floor framing. I don't like that set up -- I think you should have included enough room above the slab for inspection.

Answered by Martin Holladay, GBA Advisor
Posted Fri, 11/09/2012 - 06:52

14.
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Thanks for the great discussion!

Answered by Janet Digby
Posted Fri, 11/09/2012 - 12:06

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