By next summer I plan on building a passive solar home in the cold climate of northern Maine (~8,300 heating degree days). Over the past several years I have been sifting through hundreds of blogs, articles, forums, trying to educate myself (and my building contractor) about the nuances of passive house building construction. My question today is basement insulation and the cost:benefit of insulating the walls, ceiling or both. My plan is to have my garage/storage/utilities in an unheated basement, making the ceiling and first floor (which will have a concrete thermal mass) one in the same. As three of the four basement walls will be below grade (the forth being insulated, R-17, 2 garage doors), I expect that the unheated basement will remain ~50 degrees even in the winter. In order to create an air tight living envelope, I plan on using spray foam on the first floor joists. I have come across conflicting opinions on whether I should insulate the ceiling beyond the air sealing of the spray foam (1 inch~R-6.5). My 1st floor plumbing will be within the basement ceiling. If I insulate the ceiling and surrounding plumbing, there is a possibility that the heat from the 1st floor won't be allowed to travel from hot to cold, risking freezing of the pipes. However, by limiting my insulation, will my 1st floor thermal mass be cold from the unheated basement below? I know cold won't flow upwards, but through conduction, will the 1st floor be cold? Conversely, through conduction will the basement ceiling be warm from the warm 1st floor thermal mass above conducting downward to the colder area? If I invest on more basement wall insulation then in the ceiling, do the above concerns not exist as I am now creating a warmer basement in general? Moreover, should my thermal break of the basement walls occur on the outside or the inside? My thought is to insulate the outer concrete wall which is easy during a new construction and eliminates moisture buildup on the inside wall surface. Any thought would be greatly appreciated. Thanks.

Given the garage doors, it would be difficult to heat the basement space efficiently, so I guess unheated space it is. Still, it will pick up some ground heat in winter from the slab and parts of the foundation walls well below frost line. The parts of the walls near and above frost line will tend to make the basement much colder than 50, so insulating the walls is in order; whether you put that insulation on the outside, with protection above grade, or inside, with sheetrock over foam for fire protection, is up to you. Considering that the basement is to be unheated, I'd leave out the insulation under the slab, the idea being to let that small amount of ground heat up.

As to plumbing in the ceiling, if that is above the insulation, then it is protected from any freeze issues. There will be a temperature profile going from hot to cool, top-down. There will be higher temperature drops across the layers with higher R. Thus the concrete floor will be near room temperature, and the insulation layers will have most of the temperature drop toward whatever the basement/garage temperature is. If the piping is below that insulation, it will be near basement temperature. You might consider attaching 5-6" of iso insulation sheets under the floor framing, covered with sheetrock for fire protection. That would place the plumbing totally within conditioned space. It also may give you the opportunity for better air sealing between basement and conditioned space.

I want to create a thermal envelope that is air tight, so I'd need to use spray foam under the 1st floor to seal it off from the basement. The plumbing would be up against the bottom of the first floor, and be covered with the spray foam. Assuming that some small amounts of foam would make it between the pipes and the first floor OSB, would this insulation prevent the warm temps from the concrete mass above to move toward to cold, thus allowing the plumbing to freeze?

Insulation doesn't prevent heat conduction; it just slows it down. The temperature profile across a layer of thermally conductive material of uniform conductivity throughout is linear. This is true for low conductivity materials such as foam and for high conductivity materials like metal. Thus if you had, say, one inch of insulation above the pipe and nine inches below (yeah, I know, the pipe itself has thickness, but we're talking hypothetical here), then the temperature at the pipe would be one tenth of the way from the warm side above the one inch of insulation to the cold side below the nine inches underneath it. For your purposes you simply won't have any freeze issues. I had assumed that at least some of the piping would have to run perpendicular to the floor joists; then the typical installation would be up against the bottoms of the joists. Even if PEX were used, routed through holes in the joists, those holes would be halfway up the height of the joists. If your layout is such that the piping can be routed parallel to the joists so as to get up near the flooring, with most of the insulation below it, then you're fine.

If you did go the route of rigid board foam under the floor joists, that would be cheaper than spray foam. Getting a good air seal should not be difficult, especially if you insulate the walls inside, from floor to ceiling. The foam board joints would be taped, as would the joints between ceiling and walls. SPF may be appropriate in the rim joist area, though, depending on the construction in that area. Either way, you need sheetrock fire protection over any foam, whether SPF or rigid board, as well as over any combustible framing (garage situation). One advantage of SPF is the isolation of cold water piping from summer humidity, to prevent sweating, although the other way is to install tubular insulation around the pipes. SPF against pipes would insulate them from heat loss, but I wonder if thermal expansion of hot water pipes encased in SPF would result in squeaking. Then, too, if you use air-sealed board insulation and sheetrock below the joists, you've prevented humid summer air from getting to the cold piping, so sweating should not be a problem. Just my thoughts.

Posted By Myrtleboone on 03 Dec 2012 12:50 PM
I want to create a thermal envelope that is air tight, so I'd need to use spray foam under the 1st floor to seal it off from the basement. The plumbing would be up against the bottom of the first floor, and be covered with the spray foam. Assuming that some small amounts of foam would make it between the pipes and the first floor OSB, would this insulation prevent the warm temps from the concrete mass above to move toward to cold, thus allowing the plumbing to freeze?

It is nearly impossible to adequately air-seal between the basement & first floor, even with copious quantities of foam + taped-seam rigid board. It's better to insulate at the walls and slab, making the basement conditioned space. See the discussion starting on p17 about the retrofit in Arlington MA that failed to meet the air-tightness goals:

http://apps1.eere.energy.gov/buildings/publications/pdfs/building_america/measure_guide_hybrid_found.pdf

For a passive solar houses in an 8000+ HDD climate, seriously consider pouring the foundation with insulated concrete forms, and putting at least 2" of Type-II EPS (3" or even 4" is better) under the slab. The cost adder of going with an ICF and an insulated slab are likely less than a joist-fill (or even a flash-inch) of spray foam, and it's a lot easier to achieve reliable air tightness that way. The tougher part will be air-sealing between the rest of the place and the garage.

Dana, if I understand Myrtle's opening post, she plans to have the basement level be a garage and storage area, with garage doors. That complicates things. Such doors leak air badly. If the basement level ceiling can't be air-sealed well, perhaps she's better off with a design that has the garage off to the side or even detached, rather than as part of the basement space.

Edit: I looked at the article you referenced, and the two situations were retrofits. It would seem that a new build, with tightness between basement and living space above in mind, could be designed to be fairly tight, although admittedly more difficult than when just making the basement (no garage) part of conditioned space.

So from what I'm reading, should I scrap the garage in the basement, focus on either an attached or detached garage on a slab that is separate from the conditioned space, and place the house on a slab on-grade? I'm obviously concerned about the cost of building the above. Although, I'm assuming that a slab on-grade (even with EPS underneath) for the house and an attached garage (on an uninsulated slab) may balance out an ICF basement foundation that requires spray foam on the ceiling. I am correct in my assumption?

It is nearly impossible to adequately air-seal between the basement & first floor... See the discussion starting on p17 about the retrofit ...

Their reasons consist of basically, "it wasn't done right" (see page 3) and data from retrofits. Seal the penetrations, detail an air barrier properly and don't locate the heating system in the basement and it becomes just as possible as the upper floor/attic layer.

It would seem that a new build, with tightness between basement and living space above in mind, could be designed to be fairly tight

Yes.

Posted By Dana1 on 03 Dec 2012 05:52 PM
For a passive solar houses in an 8000+ HDD climate, seriously consider pouring the foundation with insulated concrete forms, and putting at least 2" of Type-II EPS (3" or even 4" is better) under the slab.

What use Type II EPS? Do you know of any dealers in Maine?

Would there not be a wall between basement area and garage area?

Drive-under garages are still used, I have one.

ChrisJ

And I have an attached garage now and previously lived above a drive-under garage and infiltration and fumes weren't a problem- a whiff of fumes if the wind is just right and you open the house door, but nothing compared to sitting in rush hour traffic. Also note that spray foam is not a requirement to get a very well sealed house.

"infiltration and fumes weren't a problem"
carbon monoxide is odorless

Fortunately, all the other gases that a car puts out at the same time aren't. Plus I have an up-to-date CO detector. If you want to worry about low levels of CO, make sure you don't have a gas stove. And of course you don't leave a car engine (or portable generator) running with any garage door closed.

Posted By Myrtleboone on 03 Dec 2012 09:57 PM

Posted By Dana1 on 03 Dec 2012 05:52 PM
For a passive solar houses in an 8000+ HDD climate, seriously consider pouring the foundation with insulated concrete forms, and putting at least 2" of Type-II EPS (3" or even 4" is better) under the slab.

What use Type II EPS? Do you know of any dealers in Maine?

Type-II EPS is 1.5lbs per cubic foot nominal density, which is one of the most commonly sold forms of EPS. Some EPS used in commercial roofing applications is often Type-I (1.0lbs density), and sometimes has issues due to it's lower compressive strength, and it's more easily damaged in handling. It can be used on foundation walls, but it's not the best choice for sub-slab apps.

Some local codes require 2lb density goods (aka Type IX ) compression-rated at >25psi when EPS used under slabs, but there's not a good rationale for that in a residential application. Type-II EPS is good for 15psi, which under a code-min slab thickness can handle parking your F350 with a full load of firewood on without permanent deformation of the foam or cracking the slab.

You could also use 1.5lb XPS, which is more resistant to dings when handling, but it's blown with HCF134fa, which has about 200x the greenhouse gas potential of the pentane used for blowing EPS. In a sub-slab application the "environmental payback" on XPS in reduced energy use is several decades, whereas with EPS it's measured in months.

Branch River in Rhode Island is a manufacturer; you can also call Maine Green Buidling Supply in Portland.

FWIW I wouldn't want a detached garage in a 8000 HDD climate
In fact we quite enjoy our drive under garage during the "winter" months of our 1350 HDD climate

Posted By engineer on 06 Dec 2012 09:11 PM
FWIW I wouldn't want a detached garage in a 8000 HDD climate
In fact we quite enjoy our drive under garage during the "winter" months of our 1350 HDD climate

Yet a wall-attached garage it's much easier to air seal and insulate reasonably from the house than with a drive-under.

One has vertical separation (a wall) and the other has horizontal (a ceiling/floor). I suggest that the latter is slightly easier since you don't have to worry about insulation settling and causing gaps.