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Passive Project Foundation and Slab Insulation

by Chad Ludeman on June 11, 2009 · 37 comments

in envelope,foundation,Passive House,passive project

Many have been waiting for us to start posting on specifics of our Passive Project which contains two homes – one of which is seeking the stringent Passive House certification for extreme energy efficiency. We have been waiting to post until we have completed each section of the build. This ensures that we post once on exactly what we ended up building rather than what we hope to build.

Passive Project Slab Curring

Now that we have finished pouring our foundation and have begun putting up our first SIPs panels, we can dig into the nitty gritty of our Passiv Haus construction details. Let’s start with the foundation wall/slab construction and insulation details.

Our Passive House consultants from PHIUS (Passive House Institute of the US) gave us the following basic design rules for our foundation and slab insulation:

  1. Under slab insulation of R-50 (10″ of XPS Rigid insulation)
  2. Outer foundation wall insulation of R-10 (2″ of XPS Rigid insulation)
  3. Separation of the floating slab from all foundation walls with a minimum of 1″ (R-5) of rigid XPS Insulation

I’ll spare you the details, but we did not get to this design in one day. Once these reqs were finalized, we came up with our own version of how we would build the foundation and had it approved by PHIUS. Below are the three main types of foundation details we needed to cover. The standard foundation wall is how most of the perimeter of the foundation is built. The new party wall is the foundation wall in between the two new homes we are building. Finally, the existing party wall is the detail for the party wall next to the existing home on the site next to our Passive House.

Standard Passive House Foundation Wall Detail


Passive House New Party Wall Foundation Detail


Passive House Existing Party Wall Foundation Detail


There are some air sealing details in the above diagrams that we will talk about in more detail in another post coming up. The basic main issue that was difficult to pull off in our foundation, was ensuring that the slab was completely floating and isolated from the foundation walls with at least 1″ of rigid. Typically in Philly, the slabs will rest right on top of the foundation walls for extra support. In our case that would cause too much thermal bridging from the slab to the foundation.

Below are a few more pics of the actual construction.

Passive Project Foundation

Passive Project Foundation

Passive Project Foundation

If you enjoyed reading this post I can promise you'll love our new writing over at Postgreen Homes. Yeah, we know that's the same thing your favorite band said and their new album is nowhere near as good as their early stuff, but seriously, we are actually still getting better.

There also isn't much conversation to be had here . . . at least not with us. So come on over to the Postgreen Homes Blog and tell us what you think of our new(ish) digs and crazy ideas. We will be sure to tell you what we think of your opinion.

{ 3 trackbacks }

Passive Project Under Slab Air Sealing | 100K House Blog
June 24, 2009 at 2:53 pm
Passive Project Under Slab Air Sealing « the latest architectural news
July 29, 2009 at 9:53 am
Passive House Air Sealing – Lessons Learned | 100K House Blog
October 28, 2009 at 4:36 pm

{ 34 comments… read them below or add one }

1 tlynch June 11, 2009 at 7:56 pm

How does the cost of this foundation compare to the cost of the more traditional slab on the first two houses? I guess I am asking about total cost up to the sill plate.

Did you use the same contractor? Was there anything with this foundation that they were not 100% comfortable with? Increased risks?

2 cduff June 11, 2009 at 9:34 pm

Is your 2×8.25 Sill Plate attached to the foundation wall with a traditional anchor bolt, or a Simpson type embedded strap anchor? Regardless, was this determined by the SIP Manufacturer, or your local building code?

Congrats on pulling this detail off. I would shake your contractor’s hand personally, but I live/work in Indiana. Explaining the concept of thermal shorts and bridging to contractors around here can be a challenge.

Keep up the great work!

3 chad June 12, 2009 at 10:31 am

tlynch – I’m not 100% positive of the cost difference, but out current estimate is about $5K more or $2500 per home. The rigid alone totaled about $6K…

We did use the same contractor. They were comfortable with everything from a risk standpoint. It was all new to them and they have been building foundations for 25+ years so they were not always happy during the process. The hardest detail to pull off was the separation between the two homes at the new party wall. We will most likely do this differently next time.

cduff – We actually used both. I’m not sure what code is in our area. The SIP manufacturer did not determine how the sill plate was attached, but did make sure we used 2x material instead of the traditional treated 1/2″ ply that would normally be used. This is due to the SIPs cantilevering over the foundation walls and the need to support both sheets of OSB on the SIPs.

I forwarded your comment on to our GC who will be glad to hear that.

4 cduff June 12, 2009 at 1:47 pm


I’d be curious to know what you and/or the contractor found difficult about the party wall detail. My initial reaction might be that in order to get the seams taped on the vapor barrier, they probably had to assemble the second wall on the ground, attach the drywall, and then tilt it into place. This would give the “sandwhich” effect of the air barrier between the 2 layers of gyp that the detail illustrates. What is the anticipated STC (sound Transmission Class) rating of the party wall? Were the studs staggered in each of the two stud walls to help with sound transmission?


5 chad June 12, 2009 at 3:16 pm

Your getting a bit ahead of this post cduff, but I like your enthusiasm. The difficulty was in pouring the slab with a 1″ piece of rigid separating the two homes. We set a thin bed of concrete and then placed the rigid in that. Then we poured both slabs around it.

I’m not sure about the STC. That’s a bit over my head…

6 John Semmelhack June 18, 2009 at 9:49 pm

Top notch stuff here, guys. Just curious on the SIP and exterior EPS foam board detail – did you consider a 6″ SIP with 3″ exterior foam? This would give you much improved R-value at the various top/bottom plates and window/door rough openings. Is there a tradeoff I’m missing?

7 Emily June 21, 2009 at 6:15 am

This is great stuff! I’m really interested in your work. Where about in Philly are you building the Passive Houses? I’d like to bike by one day and get a first hand look. My gratitude for creating ethical solutions within sustainable design.

8 Kevin D June 27, 2009 at 3:07 am

Two things are bothering me slightly:

1. The exterior wall will be about R35. This is your protection from a design temperature difference of about 60 deg. F. Fine. Why then, would you put (R50) under the whole slab when that design temperature difference is only 15 deg. F. The same calculation that gives you R35 walls would say that the slab insulation only needs to be R35 * (15/60) = R8.75.
With XPS costing $0.31/board foot, I’m calculating that you spent $1500/unit too much on slab insulation.

2. A couple of the exterior bearing walls are bearing on 1″ of foam. That foam WILL compress, and will compress even more when the house is loaded with people dancing upstairs at the housewarming party. The other exterior bearing walls aren’t resting on foam, so there will be a significant discontinuity, enough that the slabs will crack a few inches away from the party walls after a few years. A lot of fiberglass in the pour may help, but then you have to deal with a hairy slab.

9 Harris July 18, 2009 at 11:48 am

Regarding “Kevin D 06.27.09 at 3:07 am – Two things are bothering me slightly:”…
The site mentions elsewhere that PassivHaus US approved the design. But I’m not sure how since the SIP walls need to be 16″ thick, with an R60 for envelope (typically 12″ I-joists installed upright with blown fglass + 4″ rigid on outside). Had this home been designed with such walls, it may have justified 10″ of rigid underslab. Otherwise, it is fairly lopsided from an insulation standpoint.

10 Kevin D July 19, 2009 at 11:53 am


I’ll take a quick stab at the question. The PassivHaus specs. have been a moving target. The overall goal is zero energy. Postgreen is providing a high efficiency heat pump and grid-tied photovoltaic panels to help power it. I’ll speculate that the marginal cost of R60 walls is higher than the marginal cost of the added PV to make up for the difference in energy consumption of R60 vs. R35.

As I mentioned above, I do agree that this insulation scheme is “lopsided”. The only harm, however, is an almost insignificant 1% penalty to the budget.

11 Chad July 19, 2009 at 4:05 pm

I admire the enthusiasm. I will try to get a new post explaining these issues tomorrow.

12 Harris July 19, 2009 at 7:56 pm

Kevin – there was no question in my post. You’ll notice that there were no question marks anywhere. Simply comments to keep the conversation going.

Online chats frequently come with naysaysers… not me. I simply enjoy discovering – from a positive outlook – what goes into the decisions made during construction. I also know how difficult financing can be when it comes time to appraise such projects (another post entirely!) and this can influence design decisions on the fly… man is finance a pain in the @$$!!

I don’t agree with your assessment that some bearing walls will compress XPS board (rigid board) and lead to structural floor racking. SIPs have very good shear strength… the outside OSB is not going to tear away from the styrofoam and start pushing down on the XPS below. If the renderings above are correct, the bottom plate will spread the load just fine.

I’m curious tho: was there an energy/design modeler in on the planning/design phase? It is a relatively simple process to inoculate the project against errors. My HERS Rater is also a modeler, and he is the best insurance I have to prevent 1% cost overruns. At $1,000, modeling is well-spent money. If you do this long enough, you rapidly realize that scientific thinking usually prevails. It is well spent money. My guy brings harmony to a project that I simply cannot provide managing everything else. And let’s not kid each other: overruns add up. Fast.

My hope is that the project is planned to the extent that few, if any, build decisions are left to the supers in the field. Not dissing field guys, but if they were left out of the design stage and discussions, they can’t possibly be in “the know”.

13 Kevin D July 19, 2009 at 10:49 pm

Harris, I agree with you 100% on all you say about the energy modeling. I too am enjoying the discussion and what’s to be learned.
I also agree that the SIPs are in no danger. But they are transmitting the entire weight of the structure to the edge of the thickened edge slab, and the thickened edge is bearing on 1″ of foam. That foam will compress, and the slab will crack where it goes to 4″ from 8″. I’ve never ever seen a detail that puts a bearing wall on foam.

I’m a 100k fan… These guys have totally nailed what the market just recently discovered it wants.

14 Harris July 19, 2009 at 11:34 pm

Ah – I missed what you were saying about the loads. And I agree: the apex of the angle at which the slab turns from 8″ thick to 4″ is the weak spot. Assuming there are no center loads on the plan (there are no lolly column footings) all the weight will be on the perimeter… right smack down on the 1″ foam. I can see clearly now that the expanded polystyrene is going to compress, and the slab is likely to snap at the change in thickness.

I too am a fan of affordable sustainable construction, but, I’m not quite getting the “100K” bandwagon fever pitch. I’m left reconciling the nobility of pushing the envelope on green building with problems that could have been prevented. My hope is that there is some set-aside for concrete repairs, and all that comes from severe building settling. Could it be that Dow Chemical has specs on per-square-inch compressive load capacity of its foam boards?

15 chad July 20, 2009 at 10:22 am

Let’s keep in mind that these designs were reviewed and approved by our structural engineer. The rigid insulation is rated for 25 pounds psi, so any compression that might occur would be very minimal…

I am not a structural engineer, so I can’t really add anymore to the discussion than this. The post is about insulation only and we will most likely never cover structural issues in this blog. We hope that everyone gaining inspiration from the blog that is building, will hire the appropriate structural engineer for all of their projects…

16 Jesse August 13, 2009 at 9:17 pm

Foam insulation requires a lot of energy to produce, I’m talking beaucoup CO2. We really need to be smart about where we place our foam. Imagine a Passive House that took so much energy to build (embodied energy) that it was really no better than a typical house. This nightmare could be closer to reality than you think.

17 barry September 29, 2009 at 3:23 pm

4 points:
why not a 10.25 SIP instead of the 8.25 SIP + 1 XPS? It’d be cheaper and get a higher R for just an extra inch thick.
2. how deep is the foot? if as i suspect, it’s not a shallow one, why not?
3. why XPS instead of EPS? Aren’t there moisture issues with XPS and even if not, is the extra cost worth it over the EPS?
4. why r-50 under slab when walls are r-40? you lose more heat through walls than foundation, so you’d need less, not more with slab.

18 chad September 29, 2009 at 3:42 pm

Barry – I’ll try to answer your questions as best as I can. Keep in mind I’m not the Passive House consultant though, so I don’t have all of the answers.

1. The 1″ of exterior rigid was an attempt to further reduce all thermal bridging from what few studs would be in the SIPs for windows, doors and corners. In the end, it turned out we didn’t need the 1″ of rigid to meet the Passive House standard and therefore we eliminated it. Also, 10.25″ SIPs would be very heavy for us on these tight lots. The 8.25″ ones were bad enough…

2. I assume you’re asking about the footer. We have very bad soil conditions in all Philly lots as there are basically old buildings dumped in on their own foundations. We always have to go down 5-10′ to get to good soil or an old foundation solid enough to build on.

3. I’m not aware of moisture issues with XPS and we achieve the R-value needed in a much more compact footprint with a product that is rated for higher compression that we need in our foundations.

4. You’d have to ask the Passive House consultants about this one. This is a controversial topic that is being debated more and more these days. The slab is concrete which has the ability to suck heat out of the house so maybe that has something to do with it.

19 GreenBuildinginDenverdotcom October 1, 2009 at 11:23 am

The slab insulation question was probably just answered by Dr. Straube of the BSC in a new paper*. For North America, I think Dr. Straube is dialed in a little better than Dr. Wiest and the PHPP

“Typical BSC BA low-energy house

Numerous BSC-designed Building America prototype homes have been built in cold climates (Zone 5 and higher) that compare well to the Passivhaus standard in terms of their primary/source energy consumption. Typically, these houses use a minimum of R-5 (U=0.2) windows (triple-glazed, low-e coated, warm edge spacers), R-10 sub-slab insulation and R-20 wall insulation in a conditioned basement, R-40 above-grade walls and R-60 ceilings (the”5/10/20/40/60″ approach).

That’s about $2k savings per house on that blue Styro.

On a side note, BSC doesn’t push SIPs because they think they are generally too expensive. For the simple forms that Postgreen is doing, I believe SIPs will save enough framing labor and material to make up the difference. Chad’s IE background will be helpful in making that happen.


20 October 13, 2009 at 10:41 pm

For the very nerdy among us, Dr. Straube has elaborated on why the PHPP is indeed using too much sub-slab foam.

21 Adam Jacobs October 15, 2009 at 10:45 am

Would you be willing to ask your structural engineer to comment on the foam compression issue and post it?


22 chad October 19, 2009 at 12:05 pm

Adam, Our engineers rated the building with 25ppsf XPS. I’m not sure what else to say. They check the blog from time to time, but I try not to send extra questions to avoid extra bills unless needed…

I just returned from the Passive House conference and Straube’s name was frequently mentioned. There are plenty of comments in his posts and also on the Passive House US forums, so I won’t try to add any separate confusion here.

23 Katrin Klingenberg October 20, 2009 at 6:54 pm

The proposed numbers by Dr. Straube are actually incorrect. There have been multiple posts by now correcting the numbers of his comparison and his conclusions. (see Marc Rosenbaum’s and David White’s post on GreenBuildingAdvisor amongst many others for detailed explanation:

By my own calculations, the primary energy savings of a PH home over Dr. Straube’s proposed BA case are about 40% and site energy is half of what the BA home uses. This does not “compare well”, this exceeds significantly.

24 Gregory Lehman October 24, 2009 at 10:47 pm

“Lopsided” insulation numbers make a lot of financial sense. Thickening the walls adds many costs aside from the insulation itself:

For example instead of a 24′x36′ two-story house with 4″ SIPS walls, going to a 16″ thick cellulose wall by adding 12″ to the perimeter would increase the size of the footprint from 864 sq ft to 988 sq ft increasing the foundation, roof, and property taxes all by 14%. Jacking up the insulation levels in the roof or foundation has minimal effects on these factors.

For this very reason we built our new rental duplex with R31 walls, and R70 ceiling (R20 under slab). As I live in a straw bale house, when I think of how regressive property taxes are against thick walls, I think of it as the “straw bale tax.”

Its time to get legislation in place in which property tax is figured by net living space.

25 Zac April 8, 2010 at 3:45 pm

Regarding the use of foam under structural foundations, check out the topic “geofoam” on wikipedia, or just look at – but it’s completely fine if specified correctly, to give you the short answer.
This pdf gives you an idea of the various compressive strengths of different densities of EPS:

to put these numbers into context, the IBC allows soil pressures of up to 1500PSF (sans soils report) for gravity loads. 1500 PSF = 10.41 PSI, which means you need to use EPS29.

Having said all that, the point is completely moot since all bearing walls in the details above bear completely on concrete footings which in turn bear on the soil, so the foundation here is no different than any other building on a shallow foundation.
Only the slab on grade is bearing on foam, and these loads are relatively light, roughly 0.7 PSI

26 Richard D September 21, 2010 at 6:39 pm

I’m on a team that is considering using XPS for foundation insulation. Our project is in north-eastern California and frost is not a problem. Standard practice for a slab on grade here is to have 4″ of crushed rock, topped by 2″ of clean sand, vapor barrier and then the slab. The sand layer is used to run the hot and cold water and then day lighted or “stubbed up.” Sewer is trenched 18″ below the crushed rock. How did you handle the water supply lines? Do you cut chases in the XPS for your lines?

27 Greenbuildingindenver September 21, 2010 at 7:38 pm

Watch out, Building Science Corp. says NEVER use the sand.

28 John O'Brien October 5, 2010 at 3:42 pm

They actually say not to use sand above the poly, as it wreaks havic with the moisture loading below the slab.

29 Kris December 6, 2010 at 9:07 am

Just curious if you have ever considered using Thermomass (T-Mass) for the foundation walls. The key is needing an experienced installer.

30 passivhaus June 7, 2011 at 5:04 am

Passive design is not an attachment or supplement to architectural design, but a design process that is integrated with architectural design. Although it is mostly applied to new buildings, it has also been used for refurbishments.

31 Teo Muntean October 3, 2011 at 10:21 pm

It is possible to use also EPS300 instead of XPS ? thank you.

32 Chad Ludeman October 4, 2011 at 8:03 am

Yes. Yes it is.

33 Green Building October 5, 2011 at 1:17 pm

That’s so cool that you are doing a slab insulation. What a great way to help keep your energy costs low and protect the environment. BTW, I hear that you need the sand to help with the water…

34 Teo Muntean October 6, 2011 at 3:07 pm

Thanks! I am building now a passive house and putting pictures on the website… hope to be a real success!

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