Foam, lot of foam!

Last week, we successfully set the foundation walls. Two days later, Peed Plumbing came and completed the ground works in 100+ degree weather!  They did an excellent job despite the heat.

Ground Works

On the same day, Universal Foam made the first of their deliveries.  The driver and I unloaded the foam.  This is probably about half of what we will eventually put on the house.

We have three different specifications. 4″ EPS, 2″ EPS and 1″ XPS (The blue/green stuff).

Here is the idea.  We will insulate under the slab with the 4″ and 2″ to achieve a 6″ under-the-slab insulation.  This foam is treated to guard against termites.

With the 6″ slab insulation, we still don’t have a continuous insulation from the walls to the slab because there is no foam underneath the wall panels.  The 1″ XPS will be laid on top of the concrete slab all the way into the cavities of the Superiorwalls to connect with the wall insulation, thus forming a continuous insulation layer.

See this drawing for our insulation details:

Wall Junctions Slab to Ceiling-02

So, there you have it.

We know there are debates going on about insulation choices.  Here’ re our thought as related to this project:

1. Should we use foam at all?

Some people would prefer not to use foam at all because after all, it is still a product of the oil industry.

The alternatives are:

  • Cellulose (R-3.47/in): recycled newspaper, blue jeans
  • Stone wool (R-3.33): rock-based mineral fiber and recycled slag from steel and copper industries
  • Perlite (R-3.04): mined material, essentially expanded glass
  • Strawbale (R-1.45-2.18)

These all are great insulation choices but cellulose is probably a better material for indoors, we don’t want a house covered in newspaper (no one likes wet newspaper). Although, we may consider it as additionally insulation indoors.  Stone wool and perlite are both mined materials that might not necessarily be better for the environment than foam.  Additionally, we are not sure how we can keep it in place on the walls.  Strawbale is a great one too but perhaps when we actually build a strawbale home one day.

There are obviously others but there are few that matches the versatility of the rigid foam boards.  For our project, we need something rigid that can be used in outdoor and underground environments that can withstand moisture without physical deteriorations or drop in R-values.

2. What type of rigid foam?

We decided to use both, although mostly EPS.  XPS typically has a higher R-value than EPS (5.72 v.s. 4.17).  However, studies have shown that in long-term underground applications, XPS has the tendency absorb water and lose R-value.  EPS absorbs some water too but is able to maintain R-value.  This is why we decided to use EPS for all exterior and under-slab applications.  Above the concrete floor, we wanted a rigid foam board to break the thermal bridge along the entire slab edge and preferably one with a high R-value without the thickness to preserve basement floor height.  XPS seems to fit the bill.

From a sustainability perspective, XPS foam is worse for the environment because it uses hydrocarbon-based gas as blowing agents.

Anyone who is more knowledgeable about this, please feel free to comment.


4 responses to this post.

  1. other alternatives for subgrade insulation: high density EPS, perlite, schaumglas/foamglas and foamglas granulate. all have higher embodied energy and CO2 emissions, but aren’t made from petroleum-based products.

    sub-grade, we’re specing high density EPS over XPS due to less GWP. but lately have begun to shift to crawlspaces to avoid the issue altogether.

    walls present a number of greener options – hopefully one day we’ll have half the products the EU has, at comparable cost.

    in case you’re interested, we recently ran the numbers on R-10 equivalents for a variety of insulation types:


  2. Cool, we seriously thought about perlite and foamglas too. Funny you mentioned perlite, I kept trying to find applications for it such as adding it to concrete to pour into ICF forms or to pour a slab with it.

    I even met a guy who owns perlite mines in China who is developing perlite embedded wall panels that he wants to import to the U.S.

    In the end, I simply could not get people on the team to get interested in this.

    Foamglas, for a long time last year, it was impossible to find any suppliers in the U.S. In fact, that was the reason that led me to look at perlite. I hope it is easier now. I like the granulate especially, if you can use it to replace gravel and build a pad, then you have your continuous insulation.


  3. biggest issue we’ve seen w/ foamglas right now is expense – it’s running a bit more than XPS right now per SF, but if oil continues to climb – they could reach parity on cost. a project in portland (rob hawthorne’s duplex) was slated to use perlite to cut down on usage of petrol-based foam. though on some level, i wonder if it really saves that much, given the extraction and transportation required for the perlite.

    i think it does solve a lot of the negative issues surrounding, say, a 14″ subslab foam!!

    the granulate i like a little more than perlite – but it also has significantly higher embodied energy and CO2 emissions than other sub-grade products.

    in lieu of perlite-embedded wall panels, i’d prefer affordable AAC block!


  4. Another issue that shouled be considered about perlite is the potential of radioactive material. I’ve heard that some mines were contaminated in some of the major perlite producing countries. Even though every passivhaus will have a robust airtightness layer, I see this as a potential problem too on a indoor air quality side of things.

    AAC blocks are nice. I saw one built in Arlington last year. The builder gave me a sample which I am still amazed at how light it is.


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