Do We Really Need 12 Inches of Foam Under Our Slab?

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Do We Really Need 12 Inches of Foam Under Our Slab?

A last-minute attempt to apply value engineering to our Passivhaus design

Posted on Jan 14 2013 by Roger Normand

[Editor's note: Roger and Lynn Normand are building a Passivhaus in Maine. This is the 21st article in a series that will follow their project from planning through construction.]

After kicking the tires on the Passive HouseA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates. Planning Package (PHPP) results obtained by the Passive House Academy (PHA) on EdgewaterHaus, we have decided to make one design change that, if acceptable to PHA, will save us money and still allow us to comfortably meet the annual heat demand limit set by PHPP.

In a northern climate like Maine, the challenge in meeting the Passivhaus standard lies in achieving the exceptionally low annual heat demand (AHD) of 4.75 kBTU1,000 Btus/(ft2•yr). As my previous blog noted, EdgewaterHaus design had an annual heat demand of 3.97 kBTU/(ft2•yr) and earned a Passive House Design Stage Assurance.

Maybe we can install thinner foam

The first place to look at reducing the cost of the building envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials. was the 12 inches of EPSExpanded polystyrene. Type of rigid foam insulation that, unlike extruded polystyrene (XPS), does not contain ozone-depleting HCFCs. EPS frequently has a high recycled content. Its vapor permeability is higher and its R-value lower than XPS insulation. EPS insulation is classified by type: Type I is lowest in density and strength and Type X is highest. foam insulation below the basement slab, or perhaps some of the Roxul Drainboard insulation outside of the ICFInsulated concrete form. Hollow insulated forms, usually made from expanded polystyrene (EPS), used for building walls (foundation and above-ground); after stacking and stabilizing the forms, the aligned cores are filled with concrete, which provides the wall structure. foundation. Both are more expensive than the cellulose insulationThermal insulation made from recycled newspaper or other wastepaper; often treated with borates for fire and insect protection. used to fill the above-grade wall cavities and blown into the attic, and provide less real insulating benefit.


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The Roxul and EPS are installed below grade, where the soil moderates temperature extremes, and wind is not an added burden. Because both are installed as the outermost layer of the building envelope, either could be reduced without significantly affecting the design of the house. And since heat rises and cold falls, common sense suggests that any reduction in insulation happen below ground.

The 12 inches of EPS foam was to be installed in 3 layers of 4-inch-thick foam, with taped seams. The foam comes in 4 foot by 8 foot sheets. The EPS costs about $60 per sheet plus tax, tape, and labor. There are some 70 sheets per layer.

Our energy analyst Marc Rosenbaum calculated that going from 12 inches of EPS foam down to 8 inches would raise our AHD from 3.97 to 4.35 kBTU/(ft2•yr), still comfortably below the 4.75 kBTU/(ft2•yr) ceiling.

So, let’s eliminate one of the three layers of EPS.

Oops! The EPS foam has already been ordered

And then a complication. Four-inch-thick EPS foam is not a commonly stocked item. To prepare for the start of construction, our builder Caleb Johnson Architects had already ordered the material and its delivery was imminent; according to the supplier, it was too late to modify the order. They would not accept a return. We owned the EPS.

How disappointing: after over 1 1/2 years in the design phase of our project, we still could not make all the necessary material decisions in time!

Perhaps we could use the 4-inch-thick EPS to substitute for some of the Roxul. From a thermal perspective, that would work. Marc cautioned us to keep the EPS well below grade, because it tends to harbor ants. (Our EPS destined for use well below grade is not treated with an insecticide.)

If you want that type of Roxul, you have to buy a pallet

Another complication: the planned 2 3/8-inch-thick Roxul Drainboard is also not commonly stocked, and unlike the 1-inch-thick version, only comes in pallet-sized quantities. We would also have to match exterior material thicknesses.

Then our architect Chris Briley learned that we may be able to purchase some surplus 2 3/8 inch Roxul retained by another nearby contractor — price to be determined. Chris has drafted a revised drawing showing the reduced amount of Roxul, the EPS topped with flashing to prevent water from the above drainboard to flow between the EPS and the ICF, and the change to only 8 inches of EPS beneath the slab.

Before we implement this design change, Chris has submitted the revised drawing to PHA. We want PHA to confirm that we still meet the Passivhaus annual heat demand limit, and still retain the Passive House Design Stage Assurance. We expect a quick response from PHA, as excavation is imminent.

The first article in this series was Kicking the Tires on a Passivhaus Project. Roger Normand's construction blog is called EdgewaterHaus.


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Image Credits:

  1. Matthew Bradburn - Brooklyn House blog