Foundations. They’re not sexy. You generally don’t brag to all of your mates about them. Heck, most of the time you don’t even realize they’re there! Ahhh, the foundation, often underappreciated, but they sure are helpful. The foundation of your new house is the thing that your pretty structure sits on and allows it to – hopefully – stand for time immemorial. While the primary purpose is to keep that new home standing tall and proud, your foundation can actually be a key factor in your house’s performance – both in terms of energy as well as functionality. In addition to structural support, the key functions of your foundations are:
- Groundwater management
- Soil gas mitigation
- Water vapor control
- Thermal control (insulation)
These functions are have different implementations depending on whether you choose to have a basement, crawl space, or slab on grade for your foundation. Messing up any one of these key features can spell disaster for your new home!
The most common foundation in Central New York are basements. In Leadville, Colorado, it is common practice to build over a crawl space. Ideally, there aren’t going to be a lot of differences in these two types of foundations – in other words, its best to view and construct a crawl space like a mini basement.
The first step to ensuring your basement or crawl space will perform optimally, is to ensure that you enact a solid water management system. Unless you’re building in the desert, groundwater is going to be something you have to contend with to varying degrees. The first form of water is rain water and runoff. This can be dealt with by properly sizing your overhangs (see previous blog post). The right sized overhang will propel water running off from your roof away from your foundation, thus limiting the amount that will be flowing freely down along your basement/crawl space walls. Couple this with a sloping ground away from your foundation walls at a minimum of 5% grade (or 6 inches over 10 feet).
Moving onward and downward, your footings should be accompanied by a footing drain. This drain should consist of perforated PVC piping running the perimeter of your foundation and covered with gravel. Above and below this gravel/drain assembly should be a filter fabric which inhibits fines from clogging your drainage pipe. Your PVC footing drain will either run to a sump pit or drain to daylight. Above this assembly there will be free draining back-fill to allow water to pass through with relative ease to the drain, therefore inhibiting large amounts of water from sitting against the foundation wall.
Concrete acts very much like a sponge in that it can readily absorb and wick water (through a process called capillarity), therefore, a good practice for the exterior side of your foundation walls is to apply a damp proofing layer. This is commonly a tar/bituminous paint or coating. Damp proofing isn’t water proofing (no such thing exists in the world of basements). The goal of a damp proofing coating is to protect foundation materials from absorbing ground moisture. Another key, and often overlooked, area to include a capillary break is between the footing and your perimeter foundation wall. This can be achieved either by that same damp proofing coating or by the installation of a membrane.
Soil Gas Mitigation
The most commonly known form of soil gas is radon. Radon is present all around the country including both New York and Colorado. If you’re building a new home, it’s absolutely critical to include a radon mitigation system. They are cost effective and can protect you and your family from the disastrous effects of radon.
Beneath your basement slab or crawl space should be a granular drainage pad (commonly consists of gravel). Integrated into this pad is a ventilation system consisting of perforated drain pipe that is able to communicate with all sub-slab areas – this means if there is a thicker section for an interior load bearing wall, piping connections through this thickened section is necessary. This pipe connects to a T-fitting, then is vented from there through the roof (or above the roof according to your local building codes). A negative pressure is created under the slab which draws the gas out either via a passive system (stack effect and natural pressure gradients) or an active system that utilizes an in-line fan.
As you can imagine, the ground surrounding your basement/crawl space is generally wet. Not only that, but the concrete of your foundation can have thousands of gallons of entrained water. Left to its own devices, this moisture would readily find its way into your home. But when do we ever leave anything to its own devices? The best way to combat moisture, is to do it in conjunction with your insulation layer.
You have two choices: insulate the exterior or the interior of your basement/crawl space walls. Each have their benefits and drawbacks. Both of these are achieved with the use of rigid foam insulation (such as expanded polystyrene [EPS], extruded polystyrene [XPS], or polyisocyanurate).
Note: Polyisocyanurate isn’t a good option for exterior foam due to its sponge-like nature and ability to absorb water.
- The insulation protects your damp proofing coating from damage during back-filling
- You can have an uninterrupted insulation layer from your footings all the way to your rafters (if using an exterior foam insulation system – more on that in a later post)
- More interior space in your basement/crawl space
- Avoid expenses of interior studs and drywall
- If you leave the interior of your basement unfinished you can easily check for cracks
- Easier to have an uninterrupted connection between sub-slab insulation and basement walls
- You don’t need to figure out how to protect above grade exterior insulation
The key to insulating, whether you choose interior or exterior foam, is to be sure and tape or foam all joints in the rigid foam. This allows for an airtight and vapor tight assembly that will best combat moisture related problems, as well as perform better for you for years to come.
Another option, that tackles all of these in one seamless step, is to use ICFs (Insulated concrete forms) for your foundation walls. While they do cost more (in general), they can be a simple solution to both your water vapor barrier as well as thermal barrier. Regardless of the path you choose, rest assured that insulating your basement/crawl space is a worthwhile endeavor both in terms of saving money on energy bills as well as mitigating the chances of having moisture related problems.
Note: DO NOT INSTALL A VAPOR BARRIER ON THE INTERIOR SIDE OF AN INTERNALLY INSULATED BASEMENT ASSEMBLY! Moisture will not be able to dry inside (due to barrier) or outside due to moist ground. Therefore, moisture problems will occur. In fact, it’s a good idea to not include polyethylene anywhere in your basement walls.
Slab On Grade
Many of the principles espoused in the above writing are very similar to how they would be implemented in a slab on grade foundation. All of the key functions must still be performed: water management, soil gas mitigation, vapor control, and thermal resistance.
Water management techniques for slab on grade foundations are more or less the same as those listed above and include perimeter drains, properly sized overhangs, appropriate amount of slope away from the foundation, etc. Depending on the style of slab on grade you choose, any exposed concrete should be coated with a latex paint (vapor permeable, but water-repellent).
Soil Gas Mitigation
While it is not necessarily mandated by the building codes to insulate beneath a slab, it is often a good idea, especially for the climate zones of CNY (zone 6) and Central Colorado (zone 8). What IS mandated, is at least R-10 of perimeter insulation that extends down into the earth at least 4 feet. Best practices, however, include sub-slab foam (this is especially a good idea if you have radiant heat in your slab!).
Including sub-slab foam (either EPS or XPS) allows you to get two birds stoned at once – that is, establish a vapor control layer as well as thermal control layer. Much like with basements and crawl spaces, you need to establish a capillary break to avoid the wicking nature of concrete from allowing moisture into your new home.
Note: if you forego sub-slab insulation, you must include a polyethylene vapor barrier. If you go with a monolithic slab, the vapor barrier must extend underneath the thicker footing sections.
While you may think that the foundation for your new home is just something you bury and forget about, it can in fact have profound impacts on not only the energy performance of your new green building, but also health impacts for you and your family if the proper attention isn’t paid during this phase of construction.
Proper drainage details and appropriately sized overhangs will allow water to be whisked away from your foundation, while your use of rigid foam insulation (EPS or XPS, if used on the exterior) will prevent vapor from entering the foundation through capillary drive. Your sub-slab will at minimum include a radon mitigation system, and ideally sub slab insulation to further increase the comfort and reduce energy costs.
No matter what you choose for a foundation – whether it be a full basement, crawl space, or slab on grade – there are several methods that can give you a rock solid base to your house and meet your energy and comfort needs at the same time!