Insulating or NOT insulating the basement slab
I previously had asked about the potential of constructing a rudimentary geothermal basement by purposefully NOT insulating the masonry walls of a basement. Martin pointed out that the ground temps in NY/NJ/CT in winter at the surface are freezing and not until something like 48 inches are they above freezing, say around 40 to 45 degrees? Thus making it clear the walls should be insulated. But what about the the floor slab? Assuming a depth of say 9 feet at the slab’s underside, would this not be a constant 55 degrees or so? Thus be warming in winter and cooling in summer? During one of our first 80 degree days I walked into the basement and was greeted by the free cool (albeit humid) temperature. So by insulating under the slab with an inch or so of EPS or XPS wouldn’t you stand to lose this rudimentary geothermal benefit? I could appreciate a solid waterproofing membrane over the gravel of the slab, more than a flimsy 6mil poly. A real waterproof membrane to create a solid capillary break. But leave the welcome, constant 55 degree slab uninsulated, and install tile, no?
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Sal,
A slab at 55 degrees only provides "warming in winter" if your indoor temperature is at 53 degrees or less.
If this is a basement without living space, it's no big deal if it is cool during the winter. Who cares? You're not hanging out there. (Although your heating bills will be a little higher than otherwise.) However, then you won't enjoy the benefits during the summer, either -- because you won't be hanging out in the basement.
And if you finish off a room of your basement to enjoy the benefits, you'll want insulation under the slab, because you'll be heating the room to 70 degrees during the winter.
One of the major disadvantages of leaving out the horizontal rigid insulation under a basement slab in your climate is that you will find it hard to finish the basement in the future if you ever want to do so. You will certainly never be able to install carpeting on such a slab, because if you do, you'll get mold.
The thing to do is to insulate under the slab now, because you can't retrofit this detail. You won't regret it.
If your deep subsoil temps are between 65F and low 70s there can be an annual net benefit to earth coupling to an uninsulated basement slab, but not at 55F.
With 55F and lower subsoil temps in an mid-Atlantic or southern New England climate an uninsulated slab is a mold hazard for anything you put down on it, since the dew point of the summertime ventilation air will average more than 10F higher than the slab temp. A cardboard box on the floor will end up with moldy bottom, every time, even if you place it on a sheet of poly.
You can mechanically dehumidify the air to bring it's dew point below the slab temp, but you've then blown the energy benefit of the "free" sensible cooling the cool slab provides. (In the current state of my house that's to the tune of something around 400 kwh/year, a high single-digits fraction of the total annual power bill.) In this region the annual latent cooling load will often exceed the sensible load, even for the above-grade rooms, but ALWAYS for basements (even insulted basements with fully insulated slabs.)
By insulating under the slab the average basement temps will rise a few degrees, but more importantly, the slab temp will rise SEVERAL degrees, reducing the mold risk and/or the required dehumidification energy use needed to mitigate that risk.
As a retrofit insulating my central-MA basement slab would not be cost effective. But in new construction there's a pretty good argument for 2" (~R8) or more of Type-II EPS, where basement slab insulation is not (currently) required by code. See Table 2, p.10 of this document:
http://www.buildingscience.com/documents/reports/rr-1005-building-america-high-r-value-high-performance-residential-buildings-all-climate-zones
(NY/NJ/CT locations are all in climate zones 4 through 6, depending on exact ZIP code.)
Note, from a total lifecyle impact point of view EPS is a better choice than XPS for two reasons: The blowing agents used for XPS have ~200 x the global warming impact of those used for EPS, and after 50-100 years of outgassing the R-value of XPS will be identical to that of EPS of equal density. For the loads of residential slabs Type-II EPS (~1.5lbs per cubic foot nominal density) has adequate compressive strength, but in some locations Type-IX (2lbs density) goods are required by code. Both will usually be cheaper than any XPS product.
While the World is covered in mold spores, you don,t see everything covered in mold!
Mold can only grow where there is food and water, and this means in a typical home - wood or paper.
Mold does not grow on clean concrete - no food. Carpet? depends what its made of, mmf probably not.
New York can be uncomfortably hot in summer, especially on those days when the air feels so thick you can walk on it.
Yes you can use your basement to find relief from the heat - blowing air down there will cool it.
If you want to try it, keep in mind that you can always cover the concrete with sheets of polystyrene and a fully floating OSB floor. This will give you a much warmer basement than one where the concrete floor in not totally isolated from the gravel under and the walls are in contact with the outside soil. Concrete is one of the best absorbers of heat.
I was thinking about your idea yesterday, when I remembered that the Iranians had beat you to the idea, (with a twist), over a thousand years ago. And their idea spread thoughout the Middle East and as far as the visitor center at Zion National Park, Utah.
Basically what you have is a cool basement, like yours, and a tower that has holes in the sides, these capture the wind and funnel it down into the basement where it is cooled, filtered through various rooms which it cools on its way up and is then dragged back up the tower by the low pressure area to the lee of the tower, it works really well - and of course it has the maximum Green Passive house benefit. Cooling without using energy.
Where so many people are cursed by having vents in their roofs that pull hot air into their roofs and homes from outside and pull their expensive warm air from inside their comfort zone out into the sky, during the winter, the same idea provides a cooling air movement for the Iranians, lots of the Middle East and the visitors the the Zion National Park in Utah.
The Houses of Parliament in London, England incorporate a similar idea. As London is in a valley surrounded by chalk hills and it suffers from chronic pollution and heat due to the heat island effect. The designer built a tower at each end, these towers rise above the pollution into fresh cool air. As London is in the confluence of seven weather systems and the wind blows from 360 degrees there is a large fan at the bottom of each tower one pulls the fresh air down the other the waste air up and out.
With so many people, in so many countries using this system, I am sure you can find a way to make it work for you.
Earth tempered cooling using solar chimneys or other stack effect approaches works in climates where the summertime outdoor dew points are reasonably low like Seattle, the UK, Persia or Utah, but not so much in sticky places like NY/NJ , Bangladesh, or Suriname, even if you take great pains to remove the mold-food from the interior environment. In more humid climates with 70F+ outdoor dew points, so long as you build with stone and don't use carpeting or drapes or upholstered furniture it can sort of work, but saturated 75F air isn't exactly comfortable or healthy for the humans (who become food for fungi themselves as the humidity rises.)
Roger in regards to the Iranian/Utah system, I recall reading in one of the ph/net zero resources a system, which I think is commonly used, where a central house fan/HVAC system is used to create a low pressue in the house venting to the exterior, while low placed windows are opened in the evening/night. The fan vents out the hot air while drawing in cool air in the evening/night. Agree with Dana this does not negate the high humidity, which despite cooler air, can still feel quite uncomfortable. Although once the air is exchanged perhaps a simple dehumidier may now serve to dry it out mitigating, to some degree, the expense of full AC. Makes me think also of tunneled tubing in the ground that is used to draw air through into the living environment (forget the name) that was flawed due to poor quality of air (mold, etc.. that accumulated in the tubing). Thanks for the input and stimulating discussion.
Earth tubes for ventilation air tempering/cooling is expensive to install, and works only so-so and only appropriate for relatively dry climates. The were a popular disaster to install back in the 1970, but created more indoor air quality issues than they solved in most of the eastern US. The concept was revived in the 1990s by the PassivHaus Institut in Germany, with mixed but sometimes decent results, but there are many ways to screw it up even in the drier climates of Europe.
Some US PassiveHouse builders have installed them, but even where they're done right and work, it's hard to rationalize the complexity & expense (both initial, and maintenance) even where EXTREMELY low site energy use is the goal. Even Katrin Klingenberg (founder of the US PassiveHouse) doesn't recommend them, after monitoring performance in her own Urbana IL PassiveHouse. According to her sometimes the earth tubes are heating the air when you want cooling & conversely. Constant air flow turbulence sufficient to prevent mold colonies from getting established are critical to keeping them from becoming a major indoor air pollution source, as well as periodically inspected & cleaned. In short, in an eastern US climate they'd be a real PITA, even if implemented well enough to work.