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.

Building an "air tight living envelope" is one of the factors necessary in order to create a low energy house. It does, however have drawbacks- the main one being that toxic gasses do not dissipate as they do in leaky buildings. Please rethink your idea of locating your garage under the house - while you can install fans to vent the garage, it is nevertheless a very bad, and potentially very dangerous idea for two simple reasons" you are introducing toxic fumes into your living environment through every tiny hole in your envelope, and you are bringing into that envelope huge amounts of cold air, which you should be sealing out.

"cold won't flow upwards". Wrong. Heat flows to cold, whchever direction that may be. Yes, hot air typically rises and cold air sinks, but HEAT always flows to cold. So the heat in your house always will flow to colder areas, whether they be the attic or the basement. Misunderstanding this basic law of physics probably causes more problems in building than anything else.

Instead, embrace the concept of the thermal envelope where all of the useable space in the envelope is insulated and conditioned, including the basement and, if you have one, a storage attic. It makes sense, it creates a more comfortable and safe living environment and will be far easier and less expensive to heat - forever - than the alternative.

Thanks for the insight. My thought of having the basement for a garage/storage area was to drastically reduce the cost of building an above-grade garage (more concrete with a slab, wall framing, roof trusses, roofing, lot space, etc.). With an additional structure, I'm adding more cost that could be used in creating a better thermal envelope.

What are you referring to when you mention "toxic gases"? Radon? That can be taken care of with a radon mitigation system under the concrete am I right?

Could I realistically create a thermal, air tight basement ceiling to create a thermal envelop from 1st floor and up?

Finally, back to the original question, assuming my air tight envelop is effective (spray foam+self-leveling concrete above the floor joists), and knowing that heat flows to cold, is it necessary to load up on the insulation between the basement and 1st floor living space or spend $$$ insulating in other areas (ie. attic space).

thanks for your time.

I'd skip the basement, use a frost protected shallow foundation slab (the design that doesn't make use of building heat) and make the first floor your garage/storage area (aka, a fully exposed basement).

Minimize heat loss by minimizing the heated area (ie, insulate and heat only the upper floors).

Use a HRV and monitor/control pressures to deal with air quality.

Toxic gases include carbon monxide - (you are driving the car into and out of the garage,after all) as well as whatever is offgassing from the engine compartment, the catalytic converter and the other places in the car. If you put the garage under the house you need to have an exhaust fan which comes on when the garage door is opened and stays on for 15 - 30 minutes after the door closes. At least.

There was an interesting article in Building Science Advisor where someone built a room addition on piers, using stress skin (SIPS) as a floor. So it was an R-30, thermally broken, airtight floor system - and it was noticeably colder than the rooms in the house which were above the basement. Your house will be colder and will require more heat - forever - if the garage is under the house.

If you do it, I'd spray 2" closed cell spray foam on the subfloor, insulate the bays with dense pack cellulose, and install 2" or 4" of XPS foam with taped seams under that, then do a blower door and fog test to make sure it is really air tight. It will still be colder, but that will HELP seal out the gases. However, that creates the potential for moisture accumulation and rot in the floor cavities if there are ANY penatrations.

Best way is to bite the bullet and build a separate garage. Being a builder, I understand the cost dilemma very well; having built garages under houses myself. It's quick and quite a bit less expensive, but it does not work with superinsualted airtight houses. Repeat: it does not work with superinsulated airtight houses.

I thought I'd jump in here. If you live in an area where basements are feasible, basements are a great value even if you don't live there - a place to store stuff. Even if not well insulated to the outside, the basement is warmer than the cold weather outside and cooler than hot weather outside. Geothermal systems are starting to show up and one person on our www.NegaWattChallenge.org added one, but we don't have data yet - basements provide some of the same effect.

I like the idea of the basement being insulated on the outside walls and windows well insulated and then I figure any heat in the basement will tend to travel up through the house. In the summer, we sometimes, with the furnace off, open the furnace door in the basement and turn on the fan which then helps circulate cooler air from the basement around the house for a while. On warm days in the winter, like today, which we sometimes get in Denver, I open the basement window and a window upstairs and open the two connecting doors to help air out the house.

I grant that it is helpful to control how ventilation to the outside occurs, but seems to be a good idea to have some.

One thing that has surprised me with our prototype Trombe wall, is how much more we open the kitchen windows. 53 degrees outside now at 10:15 AM with 70 degree air coming in the Trombe wall, definitely warmer than our house. Yesterday I tried a window fan, worked for a while, but too much for my prototype Trombe wall: http://negawattchallenge.org/challenger/bean.html However, I've yet to test with a really cold, sunning day - we haven't had one of those yet this year. Also, with all solar, we're somewhat treading water December through January, outside of those two months the solar heat systems are much more. Last year, I took our reflective mirrors down the first part of March because they were frying us.

My neighbor added a window that does not let the sun's heat through - filled with some kind of gas, I think. I hated it, at least outside of the summer, the sun feels like it gives "life" - not sure about the window and plants. I better like windows that let the sun's heat in and then shade the window on the outside when necessary.

Anyway, my point is, give a choice when buying a house where basements are feasible, I'd pick one with a basement to buy, so might also consider for home vale.

What types of gases do I have to deal with assuming I have a radon mitigation unit and my utilities do not include a central boiler, either wood, coal or oil?

If you have an all electric house (easy to do with minisplit heating) and with the exception of a motor vehicle inside your envelope, the main gas you need to be aware of is water vapor. There are also cooking odors etc. You'll need to install an HRV for fresh air ventilation to maintain good fresh air levels and to mitigate other miscellaneous gases and odors.

The thermal envelope will enclose the 1st/2nd floor living space only. The in-ground garage/basement will not be insulated but will take advantage of the warmth of the earth during winter months and cold during summer months. I'm not too worried about the temp in the basement so as long as it doesn't drop below ~45-50.

My basement is not considered part of my thermal/air tight envelope. Yes, aside from a small pellet stove located centrally on the 1st floor, I will be heating with electric. I will have a heat recovery air exchange unit to take care of ventilation. With taped Zip Panel sheathing on the outer shell and spray foam surrounding the entire thermal envelope, I don't think I'll have to worry about water vapor.

Posted By Myrtleboone on 03 Dec 2012 08:22 PM
My basement is not considered part of my thermal/air tight envelope. Yes, aside from a small pellet stove located centrally on the 1st floor, I will be heating with electric. I will have a heat recovery air exchange unit to take care of ventilation. With taped Zip Panel sheathing on the outer shell and spray foam surrounding the entire thermal envelope, I don't think I'll have to worry about water vapor.

In any wood framed/sheathed structure you have to worry about water vapor, and design the stackup of the assemblies to not accumulate or trap moisture in the wood.  In a flash'n'fill on ZIP you still have to provide sufficient outward drying to keep the ZIP dry, and sufficient foam/fiber R-ratio appropriate to the climate to keep the fiber from reaching mold-threshold levels of moisture content.  (Adding an interior vapor retarder makes the assembly less resiliant to wetting events, potentially trapping moisture between the vapor retarder and foam.)

The cost of insulating & air sealing the slab is often less than doing a truly good job of air-sealing and insulating the basement ceiling from the first floor.  When you thermally isolate between the first floor & basement but leave foundation uninsulated letting the basement run cool increases the mold hazard of anything stored in the basement and the accumulation of moisture in the cool edge of the joists.  In the colder parts of Maine leaving the foundation uninsulated and putting a high-R between the basement & first floor even runs a risk of frost-heaving breaking the slab or foundation wall if you let the basement drop below the subsoil temps.

Bringing the basement inside of the pressure & thermal boundary of the structure is always the right thing to do, whether you insulate between the basement & first floor or not, and whether you fully condition the basement or not.

this is a much larger issue than just Myrtleboon's house. In the 70s we added 2" of fiberglass insulation (from 2x4 to 2x6) plus double glazed windows and thought that solved the energy problem. Now we know better and realize that the way we conceptualize and build has been and basically is, wrong for this day and age. The issue of "Bringing the basement (and the attic) inside of the pressure & thermal boundary of the structure" is huge and requires people to rethink the whole concept of building. I run into this issue with almost everyone I talk to - they want to "add" something - a new foundation type, a new insulation type, more of some type of insulation, while the real issue is much more complicated to conceptualize and to build. I've drawn a large diagram showing the numerous things I'm doing differently throughout the house to try to make the point that true efficency must involve a comprehensive solution, but do not yet have a good method to explain this. Bottom line: does all this add costs? Of course, but it is worth it.

You can dehumidify a sealed basement or you can dehumidify and heat it - the former is less $.

In the unlikely event that the basement floor approaches freezing, you can add a little bit of insulation on the upper walls (near ground level).

In the late '70s we also added poly vapor barriers, creating as many or more moisture & mold problems as it was intended to solve.

Prior to the 1970s houses survived by purging moisture by higher heat flux. Things got wet, dried quickly, problems were minimal, but energy use was high. When you lower the heat flux through assemblies things run colder, dry more slowly and humidity issues crop up. Just as 6mil poly and R19 batts didn't pan out as a universal panacea, the flash-foam of closed cell polyurethane isn't either.

Used judciously and appropriately closed cell foam can do a world of good- enough to offset it's the rather high environmental cost of the HFC245fa blowing agent. (There are a couple of water-blown closed cell foams out there- Icynene MD-R-200 and the 1.8lb Aloha Energy goods, but the vast majority of cc foam is blown with HFCs with greenhouse gas potential >1000x that of CO2.) But one always has to pay attention to both the R-value and vapor permeance of foam layers of a stackup and how that is going to play out in the overall moisture accumulation/purging performance of the susceptible layers.

Summertime dew points in ME are high, and the subsoil temps low- if you don't air seal the basement from the exterior and vapor-seal it from the soil, the cold basement has huge mold potential for anything stored down there. Sure, you could spend 500-1000kwh/year with a dehumidifier to remove that moisture, but that's a pretty crummy & less reliable band-aid approach. Insulating & air sealing the foundation dramatically reduces the summertime mold problem, fixes the pipe-freeze & frost-heave potential, and puts the thermal mass of the slab within the thermal boundary of the house for better moderation of temperature swings in a passive-solar house.

and puts the thermal mass of the slab within the thermal boundary of the house for better moderation of temperature swings in a passive-solar house.

Only if one gets the basement slab all the way up to warmer than normal room temperatures. Regularly heating the basement to that extent would be very wasteful (although even without basement insulation you could still do it on the occasions when you have excess solar heat).

Increasing your conditioned volume in an attempt to reduce energy use is truly a band-aid for something else done wrong.

The basement slab never sources stored heat, but it can soak up some of the overheating.

But clearly that's not a primary reason for bringing the basement inside the thermal boundary.

To the contrary; it takes very little energy to bring a fully conditioned and vapor sealed basement up to "room" temperature.
"Increasing your conditioned volume in an attempt to reduce energy use is truly a band-aid for something else done wrong." Meaning no disrespect, I strongly disagree; conditioning a basement is a requirement for a truly energy efficient house, but the thought is typical of the misunderstanding of how such a house is built, as I said above.

Posted By Bob I on 05 Dec 2012 01:28 PM
To the contrary; it takes very little energy to bring a fully conditioned and vapor sealed basement up to "room" temperature.
"Increasing your conditioned volume in an attempt to reduce energy use is truly a band-aid for something else done wrong." Meaning no disrespect, I strongly disagree; conditioning a basement is a requirement for a truly energy efficient house, but the thought is typical of the misunderstanding of how such a house is built, as I said above.

That's right- it's not about volume, it's about the heat loss rates through the expanded exposed exterior wall area, which is minimal in a sealed & insulated, thus conditioned but unheated basement.  Most of the increased wall area is sub-grade and subject to less than half the heat loss/unit area.  Without actively heating the basement the heat loss from the fully conditioned space through the floor will barely budge and may even be less than through the joists to an uninsulated & quite cool basement.   But with an insulated foundation the basement temps will be warmer, and basement conditions much drier, and the house will usually leak less air.

If it's a space of ANY use to the homeowner,  it's worth bringing inside the pressure & thermal boundary.

You mean if it's not heated it's of no use? My experience has been the opposite - if it's not fully heated, it's has very little more use than when it is fully isolated from the heated area (ie, unheated except for ground heat and inadvertent heat from an fully insulated ceiling above). If expanding your fully heated volume and surface area reduces heat loss, you are doing something wrong (misplaced ducts, poor air sealing, etc) somewhere.

Some people believe in the "dirt is free insulation" theory, and if you believe that, you can be consistent by berming it up against above grade walls (or build completely underground).

Search for "decouple" or "basement" here: http://www.architects.org/category/keywords/passive-house. Or "Rainbow passive duplex" for an effective garage under, unheated basement design.