Tuesday, November 17, 2009

Progress Report: Spray Foam Insulation

I'm happy to report that things are moving along quite nicely! Presby sent us more pictures last week of our home’s development.

In the picture below you can see our foundation is now standing without help from the preset forms. Click it to make it larger, and you'll see our view to the Southeast: "downtown," Cannon Ski Mountain, and Mt. Lincoln and Mt. Lafayette in Franconia Notch State Park.

Look beneath this paragraph to see our foundation coated with two inches of spray foam insulation. Spray foam is able to fill the tiniest cavities—giving it twice the R-value per inch than traditional batt insulation. Our two-inch insulation will provide us with an additional R14 value. Our interior basement walls will be framed and insulated as well. (For those who don’t know, the higher the R-value, the greater the insulating effectiveness.)


  1. Hi! I love what you're doing with spray foam! I'm one of the editors of SprayFoam Magazine, and I'm always looking for interesting projects to write about. I'd love it if you sent me more information about your house. You can find my email address on our website, www.sprayfoam-mag.com.

  2. Since you are building a passive solar home, why add insulation on the interior of those walls instead of putting it all on the outside? You're going to loose that thermal mass when those concrete walls are insulated on the inside. I'm currently struggling with my below grade walls and how to get sufficient thermal mass in the passive solar home I plan to build in the Spring. I'm leaning towards ICFs, but really hate using a high embodied energy material like concrete and then not getting the mass benefits.

  3. Hey Dan,
    Those below grade walls are for our walkout basement, not our main living area, and due to the nature of our site they will not receive enough solar gain to heat the thermal mass effectively. The foundation floor will have radiant heat, in addition to the main floor and loft, so we will be utilizing that thermal mass to maintain the warmth. We are also planning on slate flooring for the open living area on the south side of the home. This will give us a decent area of thermal mass to retain the heat from the passive solar gain. In our research we found that limiting the thermal mass to a certain extent is more effective due to the limited duration and strength of the sun during the northern New England winter. We too originally looked to ICFs, but as you can see that was not in the cards. Where are you building in the spring?

  4. I'll be building in Waterbury VT.

    Thermal mass seems to me to still be a lot of art with a little bit of science. I've not come across a source indicating that you can have too much thermal mass. I'd be very interested in reading up on that if you happen to have a readily available source.

    The idea of too much mass seems counter-intuitive to me. Yes, the sun won't bring the temp up as much, but it also won't drop as far when the sun is down. I think of it as a sine wave with a period of 24 hours. More mass means the temp peak to trough will be less. That all seems like a good thing to me. Unless there's a way that more mass results in more heat loss out of the building, I'm not sure how more mass would be a bad thing. That doesn't mean it couldn't be that way, but I'm not sure how at this point.

  5. Hey Dan,
    Waterbury….very nice and not too far from us!
    Yes, like you said there is still not a lot of science behind calculating the perfect thermal mass equation. Right now the one school of thought that is commonly practiced is the 7 percent rule, a home with less than 7% of the total square footage made up of south facing glazing can obtain sufficient mass from a traditional interior. For each square foot of glass above 7 percent, add 5.5 sq. ft. of mass in floors that receive direct sunlight, 8.3 sq. ft. of mass in walls and ceilings in the same room, 40 sq. ft. of mass in floors that don´t receive direct solar gain. I think limiting thermal mass is certainly more critical in Canada, northern New England, and similar cold climates. It is tough to get any amount of mass up to an acceptable temperature on those all too frequent gray, sub zero January days. We are certainly building the thermal mass into our open living area, but knowing the climate/solar gain for our area, and taking into account the limited basement windows and deck above, the concrete slab should provide plenty of thermal mass. And hopefully that extra heat will be retained in the basement with the spray foamed exterior walls! ;)

  6. Thanks for your reply. I've read Daniel Chiras's "The solar house" book which I'm guessing is the source for the rules of thumb you mentioned. More recently I've come across another set of rules in "Green from the Ground Up". On page 52 there is a more extensive set of guidelines quoted from "The Thermal Mass Pattern Book". The quidelines are more detailed than can be posted here. The amount of South glass supported is broken down by thickness of material for concrete, brick, gypsum, oak and pine. I have a spreadsheet with all of it included if you'd like a copy. Send me an email and I'll forward it to you. My address is my first name dot last name at vtmednet dot org.

    Neither of the two books mention that you can have too much mass, at least that I'm aware of. I don't have the orginal source quoted in "Green from the Ground Up", however, and it could be in there??? If you have a source that says there can be too much mass, please let me know. I don't want to argue the point, but would like to know if my thinking about this is off somehow. Thanks.

  7. Thanks Dan.
    I will send you an email later today. Like you mentioned spreadsheets and more precise studies & formulas are certainly more useful than the vague comments found on the net. Our architects (www.revisionarch.com) were the ones that initially brought the mass debate to our attention. With the software they had they were able to calculate our heating requirements based upon the January climate and making adjustments to our sq. footage, thermal mass, etc. Not sure what software they had, but Energy- 10 seems to be popular in the realm of thermal mass calculation.
    Getting back to our basement, The logic of using only the floor as thermal mass and insulating the perimeter walls is based upon this general premise from the Sustainable Energy Authority,
    “In situations where solar access is poor, thermal mass could increase winter heating requirements. Where there is little possibility of solar gain, because windows are overshadowed (in this case by our Deck), the benefits provided by the use of thermal mass will be minimal.”

  8. I would tend to agree with the second part of the quote, i.e. benefits are minimal if there isn't solar gain. However, the first part of the quote doesn't make sense to me. Maybe they say something elsewhere, but it doesn't say HOW additional mass would increase the heating requirements. To do this, the thermal mass would somehow have to be responsible for an increase in the transfer of heat from the interior of the house to the outside. I'm skeptical that that's the case.