I'm in the planning stages of building a house in Western North Carolina and trying to figure out if I should do a slab or unfinished basement. I'm lucky enough to have a really flat lot that would be great for a slab on grade, but my wife and I really like the idea of having the extra storage as well as the option to finish the basement in the future and add to our square footage. But my main concern is water or moisture infiltration, being that the basement would be completely below grade. Should I really be this concerned or am I overreacting. I grew up in a house that took on water in the basement all the time and I cringe at the thought of water or moisture in a newly constructed house. Please give me your opinions so that I can make an informed decision.

Why not just use a "Frost Protected Shallow Foundation" with a monolithic slab and then add some unfinished space to the house? The old thinking was that one had to dig down and build footings anyway, so a basement didn't cost that much more. But now foam is standard, so why dig down? Utility room space is also more valuable above ground - you can have windows and wheeling things in and out is easier.

I tend to agree that staying above grade would be a better option. One other factor to consider: How prevalent are slab on grade foundations in your area? In some areas, slabs are not normally associated with single family construction and can lead to lower appraisals and resale value. Your geograpihic location may be such an area. I would consult with local builders and real estate people before making a decision on slab vs. crawl space vs basement. Also, the above grade living space will carry a much higher appraisal value than basement space, and I suspect you are correct that a basement in your area would be subject to flooding, requiring a full time sump system to remain usable.

If you are concerned about protection from storms, then consider building the master bedroom closet as a safe room.  That has worked well for my clients when they did not want a basement.

It would be interesting to see how a 2000 sqft ranch house with a basement appraises as compared to a "4000" sqft, two story slab house with 1/2 of it unfinished (perhaps with the lower floor unheated, uninsulated and with minimal windows). Building cost is probably similar and I would much prefer the "4000" sq ft house.

Or perhaps another way to describe it would be a 2000 sqft house with no basement and a 2000 sqft garage as the first floor.

Thats funny because I'm a real estate appraiser by trade and I agree with you in the fact that above grade square footage is a lot more valuable than below grade square footage.  It also seems from the water and moisture aspects its much safer to build on a slab rather than an unfinished basement.  Thank you again for your input as it is very helpful.

Jonathan

Posted By hunterjonathan on 07 Jul 2010 10:20 PM
I'm in the planning stages of building a house in Western North Carolina and trying to figure out if I should do a slab or unfinished basement. I'm lucky enough to have a really flat lot that would be great for a slab on grade, but my wife and I really like the idea of having the extra storage as well as the option to finish the basement in the future and add to our square footage. But my main concern is water or moisture infiltration, being that the basement would be completely below grade. Should I really be this concerned or am I overreacting. I grew up in a house that took on water in the basement all the time and I cringe at the thought of water or moisture in a newly constructed house. Please give me your opinions so that I can make an informed decision.

I think it is a regional thing, but I couldn't imagine not having a basement, for the reasons you mentioned - storage, flexibility in finishing it later, a safe place to go in case of tornado, etc.  I don't know this as fact, but I would assume that having that basement space should act as a buffer zone in terms of your inside temps, too.  It would be interesting to do a comparison of heating/cooling costs of identical homes, one with a basement & the other on a slab.

One other thing that just popped into my head..  concrete floors can be less than comfortable if you spend a lot of time on your feet.  On slab you'll be walking on concrete all the time - with a basement you'll have floor trusses, which have more give.  Just a thought..

Using a crawlspace to get a wood beam floor or using concrete directly is a choice to be made. I think concrete is fine, especially for utility space (ie, replacing a basement).

I've lived on concrete for the last 30 years, and hope I never have to again. It is harder on the back and feet, unless you have carpet and padding. About half of our house is tile, and it is hard! To me, wood is homier and warmer. I guess you could put in a suspended wood floor pretty easily, though. I also don't like having my plumbing under the slab. When there is a problem, or if you want to change something, it's jackhammer time, instead of going into the basement or crawl space. I would be worried about a full basement on flat ground in western NC, though- they get a lot of rain. I'd go crawl space.

Hunterjonathan;

I lived with full basements in the north for 32 years, one thing for sure anything stored in them is usually musty and mildewed, for the last 26 years I have lived in the south with slab on grade, I much prefer planning conditioned storage spaces that are easily accessable, mold and mildew free

Mold and mildew in New England basements that are otherwise reasonably well drained from bulk water issues is primarily a summertime air-infiltration problem.  Sealing & insulating the band joists with foam, and using masonry sealers (not waterproofers) on the interior masonry & slab to minimize capillary draw (but still allow some vapor drying of the walls), and weatherstripping any windows doors, etc.  Dryer vent backdraft preventers and using only direct-vented/sealed combustion gas appliances like water heaters/boilers/furnaces etc and closing off any abandoned flues also count big-time. The key is to make the basement as air tight as possible to the outdoors, and only an exfiltration point from active venting like clothes dryers, if you can.

Case in point: My basement slab is ~12" from the water table most of the summer, but a treatment of acrylic based concrete sealers + air-sealing the basement to modest levels of infiltration was enough to reduce the duty cycle of the dehumidifier (set to 60% RH) by about 60-70%.  The duty cycle is still highest when the outdoor dew points are in the 70sF, and falls to near-zero when the outdoor dew points drop below 60F.  Cardboard boxes placed directly on the slab, (or even on a sheet of poly) will eventually get a slight dusting of mold on the bottom, but it'll take months.  This is likely due to the insulating effect of the cardboard itself lowering the temp of the slab locally under the box, since that spot is now coupled to the ~48-50F subsoil. This makes for a localized spot that is below the dew point of the room air, even if the air movement between the box & slab is very very slight indeed.  With any sort of clearance to the floor or any insulation (like 1/4" of closed cell foam) under boxes on the slab they remain mold-free, despite being mostly mold-food. 

I have 4 sump pumps to keep different corners from flooding when the water table rises due to seasonal rains, etc, but I don't have mold issues with stored items there despite occasional minor flooding when a sump pump fails (which is usually when somebody unplugs one to use the outlet for some other temporary purpose and neglects to re-connect it.) I keep water-susceptible items at least a half-inch off the floor due to the history of the place though.

With windows & exterior doors to the house closed the dehumidifier in the basement is sufficient to keep the RH in the rest of the house under 60% even when the dew points outdoors are well into the 70s. (That small dehumidifier is effectively my primary air-conditioner for the entire house, keeping the latent load under control, which is MOST of the load in my ~350CDD climate.) The basement runs slightly warmer in mid-summer now than 4-5 years ago, after insulating the basement walls to R20, which also has a slight impact on the duty-cycle of the dehumidifier. It now reaches the high-60s sometimes even 70F during extended hot spells, when the floor above is in the high 70s, 2-3 degrees warmer in summers prior to the insulating project.  The warmer temp is good for a few percentage points lower RH for the same air a few degrees cooler, but this effect is secondary to the reduction in outdoor air infiltration.

This sort of treatment should work fine for the humid southern US as well, since the subsoil temps are higher, resulting in a warmer basement than it would be up north at the same insulation levels.  As long as there is some (even incidental) movement of air-conditioned air into the basement it's humidity would be the same as the rest of the house.  Air-sealing crawlspaces and providing ventilation only from conditioned space is now considered best-practice for the most hot-humid regions of the US, which would apply to semi-conditioned basements as well. Ventilating basements & crawlspaces with outdoor air in much of the eastern seaboard or gulf coast is a recipe for mold problems.

I should have mentioned- a perimeter drain and full poly vapor retarder under the slab, and mixing sealers like Xypex into the concrete during footing & wall pours of new construction goes a long way toward halting ALL capillary draw into the masonry for new construction, after which it's primarily a matter of doing a decent job of air-sealing the basement, and doing reasonable grading/drainage at the surface on the exterior to keep water from pooling up against the foundation walls after rain events. Dry basements CAN happen by accident, but they're clearly designable/buildable these days.

In N. Carolina the subsoil temps are warm enough that there's little benefit to insulating under the slab (unless you're going all PassiveHouse standard on it, in which case you might need some), but using insulating concrete forms for the basement walls is still a good idea. If you're concerned about termite tunneling through the EPS concrete forms (it can happen in termite-prone areas), using copper flashing or copper foil/poly lamintes (eg: http://homes.h-b.com/coppertermiteshield.php ) as a capillary break and termite-stop for the foundation sill should suffice. (Foam sill gaskets aren't really great capillary breaks anyway, compared to metal, and copper is toxic to termites. You'd still need to spray foam-seal the sill/foundation interface to reliably stop air infiltration, even where a foam sill gasket is used.)