Q: The foundation on my 1980s-era house is insulated on the exterior with sheets of foam board. Several inches of foundation are exposed above grade, where the foam is protected by aluminum flashing. The foam and flashing are unsightly.
Would it risk termites or other problems to raise up the ground level with soil or river rock to cover up the foam? Are there better materials to cover and protect the foam? Thanks. – Robert T.
A: While widely used at one time, exterior foam insulation on basement walls is less common today. I’m not a big fan of exterior foam insulation on foundations for some of the reasons you mention. It is difficult to protect during construction and after the home is completed. Most coverings deteriorate over time. And it also provides easy access to the house for subterranean termites as well as ideal nesting material for carpenter ants.
On the plus side, it allows for continuous insulation for homes with exterior foam on the walls. It also provides a small thermal mass benefit, although this is mainly true in climates with wide daily temperature swings above and below the comfort zone. An example would be the high deserts of the Southwest. And even in these climates, the benefit would be primarily for above-grade walls, not foundation walls.
Clearance to Grade
Since you already have foam insulation in place, however, you will need to find a suitable retrofit product to protect it or remove the above-grade foam (and add interior foundation insulation).
Most of the materials used to protect the foam eventually get damaged by lawn mowers, baseballs, or just normal wear and tear at the ground level.
Raising the grade level to bury the foam is not a good approach. The minimum distance from grade to untreated wood is 6 in. in the International Residential Code (IRC). Some state codes require 8 in. of clearance. You can view the detail for ground clearance at this link.
I recommend a minimum of 8 in. from grade to any non-treated wood (framing, sheathing, or siding). This is a safer target since people often add mulch, pavers, or plantings that raises the grade.
The goal is to keep water away from the wood. The water can come from soil, wet leaf litter, plantings, splashback, and snow melt. In termite-prone areas, the gap creates an inspection area for termite mud tubes leading from the soil to wood framing.
So, yes, you can raise the soil level on the exterior, but should still leave at least 6 inches of exposed foundation. If the aluminum flashing is unsightly, you can consider replacing it with a more durable material.
There is no perfect material for covering and protecting exterior foam on foundations.
The most common approach is to apply a stucco-like “parged” finish with a surface-bonding cement. These finishes range from a thin paint-on coating to a thick acrylic-modified mortar applied over hardware cloth or metal or fiberglass lath.
The thin paint-on coatings are easily chipped and prone to peeling. Mortar with acrylic additives has the strength and flexibility needed for this application and the metal or plastic lath provides additional strength and a bonding surface with “teeth”. One benefit of mortar is that it is relatively easy to patch if it is damaged.
Others prefer panel products such as pressure-treated plywood or cement board (such as James Hardie HardieBacker). However, most cement-board manufacturers do not recommend direct contact with soil, so check first before taking this approach. Panel products can be finished with a stucco coating or exterior paint, depending on the appearance you want. Another durable, but high-cost, option is cultured stone veneer.
There are also a number of manufactured systems using panels made from fiberglass, pvc, or other materials. The main problem with these systems is that they come and go from the market. So you may end up with an orphaned system in 5 to 10 years when you need to replace a damaged panel.
Termites and Ants
Whatever system you use to cover and protect the insulation, you still have a direct conduit for termites to move from the soil to the wood structure. The termites can easily move behind the foam and escape detection.
In some termite-prone areas, codes do not allow exterior foam on the above-grade portion of the foundation, or require an uninsulated inspection gap. The gap allows inspectors to find the telltale mud tubes that subterranean termites build to travel from the soil to the wood portions of your house – their food source.
Carpenter ants also like to excavate exterior insulation to create a warm living space. While they do not eat the foam for nourishment, they can still do considerable damage to the foam.
So if you are in a termite- or ant-prone area, you may want to consider moving the insulation to the interior side of the foundation wall. In any event, you should check first with a termite expert for an effective treatment strategy.
Interior vs. Exterior Insulation
With interior foundation insulation, you can add whatever thickness you want and use any type of foam. By insulating the band joists, you can get continuity of insulation from foundation to above-grade wall.
You avoid all the issues with termites, carpenter ants, and finding an insulation covering that’s attractive and durable. Seems like a no-brainer to me. – Steve Bliss, Editor, BuildingAdvisor.com
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