Insulating a Basement (or Not)
Hi,
We are building a home in Santa Clara, CA (Climate Zone 3C). The home will be made of concrete SIPs. We are planning to build an unconditioned basement to make most efficient use of our available floor area. The basement will be made of solid concrete walls. Currently the architect has specified an uninsulated basement slab floor and only a partial area of the basement walls to have insulation using either rigid or closed cell spray foam. My main floor above will have hydronic heating.
To start with I plan to use the basement as an office space. Eventually maybe a bedroom.
My questions
- Is leaving the basement uninsulated a bad idea if I plan to leave it unconditioned? Does this affect the energy use for the conditioning of the main floor above?
- At some point in the future, if I decide to condition the basement, am I better off insulating under the slab with maybe a couple of inches of XPS/EPS (R-15 per passive house requirements) ?
Thank you for taking the time to read and contribute.
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Replies
The basement needs to be insulated. Even if it is not itself conditioned, the floor above is essentially un-insulated if the basement is uninsulated. What is particularly important is air sealing and moisture sealing. Have your plans been approved? I would think in CA you'd be subject to the energy code which I understand is pretty strict there. R13 has been required in basements in Zone 3 since 2009.
In Zone 3 you may not need to insulate the basement floor, but check your local code. Until 2018 the model code required no insulation on the floor in Zone 3 unless the floor was heated. In warmer climates the basement floor helps to cool the building in summer and that can save more energy than is used to heat it in winter.
The only real way to answer your basement insulation questions is to build a computer model of your home and run the numbers.
My gut says the heat escaping thru the basement floor in the cooling season is greater than the amount of heat escaping in the heating season.
Free BEopt modeling software note about 20 hours the learn and input the data.
https://www.energy.gov/eere/buildings/building-energy-optimization-beopt-software
Training videos
https://www.youtube.com/playlist?list=PLHC0xDtkdjgec8QhVt7exJY3tpSLEFk-d
SIPs were the hot idea 20 years ago but it turned out not to be the most cost effective way to build a wall of a given R value. They did have some moisture problem but that is unlikely to affect you if it is as dry is I think it is in your location.
My guess is your contractor could build a wall of the same R value for about 30% less money. Yes, it would thicker but we are talking about what ten maybe twenty thousand dollars.
Did you see this thread and article about just how unrepairable a zip wall is?
https://www.greenbuildingadvisor.com/article/rotting-walls-in-a-sip-house
https://www.greenbuildingadvisor.com/question/sip-wall-rot-from-window-flashing
Foam maybe the least green product in any home factory made foam is better than spray foam made in the field. I say avoid foam as much as possible when you see it in the plans it is a red flag for a lazy design with a side order contempt for your budget.
Hydronic heating is also a needless 20K + hit to you budget. It looks like you will build a tight well insulated house in a moderate climate. What that means is the heated floors will almost never get warmer than your feet before the room is to hot and the thermostat turns off the heat. I have fond memories of heated floors but that house had no insulation and bad windows and it was very cold outside also I am glad I was not paying that gas bill. Undoubtedly you will install an AC system to cool your home that could also be the heat system home for another $500.
Please make a plan that keeps your HVAC equipment and ductwork out of your attic and inside your conditioned space.
I understand you are emotionally invested in this plan but remember that at this time it is just ideas on paper now is the time find any short comings. Did you ask the architect what is the best way to build your walls or did you tell them to make a plan using SIPs?
Walta
Thank you for taking the time to read and reply to my question. I am looking to build a home with concrete SIPs, which are a bit different from the SIPs that you are referring to. One of my goals is to build a home without any structural wood. Here is a link to the system if you are interested.
https://www.youtube.com/watch?v=dMyg42rK17w
The BEOpt software does not work on my computer. I have to figure out, how to make it work. Thank you for the tip.
If that's what you want, I'm not going to try and talk you out of it...
Why not just make the basement walls out of the same material though?
Is the desire to eliminate wood due to fire risk? If so, maybe consider rock wool insulation instead of foam? You could compare the cost of a regular concrete wall/rockwool insulation to the concrete SIPS that you are planning. Rockwool is expensive compared to fiberglass but I have a feeling that those concrete SIPS are pricey too.
https://youtu.be/T2QUTOoAzjU