Increase Seismic Resistance and Thermal Performance of Basement, Crawl Space, and Crawl Space with Cripple Wall Foundations

This guide describes options for providing seismic and thermal resistance for homes with basement foundations, crawlspace foundations, or crawlspace foundations with a “cripple” wall.

Seismic Design Approach

Most of the damage to a building during an earthquake is caused by lateral movements that disconnect the house from its foundation. Uplift forces can also occur. These lateral loads (shear) and uplift forces due to ground movement need to be transferred to the ground to minimize the likelihood of the building being pulled apart. New homes and existing homes need to be connected to their foundations and the structural elements should be connected to each other to withstand seismic activity.

Although unrelated to earthquake resistance, when retrofitting a foundation to make it more earthquake resistant, the foundation should also be assessed to see if the insulation, air sealing, and moisture protection could be improved to make the home more thermally efficient and durable.

Building-to-Foundation Connection

Figure 1 illustrates the three foundation types covered in this guide: a basement foundation, crawlspace foundation, and crawlspace foundation with a cripple wall. Note in all three foundations the connection of the continuous exterior structural wood sheathing (plywood or oriented strand board “OSB”) to the foundation sill plate. Also, note that the foundation sill plate is connected to the concrete foundation with a foundation anchor bolt. In this manner, the wall and -floor diaphragms are connected to each other and to the foundation.

Building bracing is provided by the use of continuous exterior structural wood sheathing (plywood or OSB). Note that the crawlspace foundation with a cripple wall has structural wood sheathing on both sides. Main-floor walls are internally reinforced with gypsum sheathing.

Additional bracing is provided in both crawlspace assemblies by installing cross bracing at interior posts and piers (Figure 2). 

All three foundation types should also have solid blocking of dimensional lumber installed between the floor joists. (The blocking is shown in Figure 2 as the white boxes with black outlines between the floor joists.) The solid wood blocking prevents the floor joists from racking or rolling sideways.

Thermal Efficiency

New homes and existing homes should be thermally efficient. The basis of all thermal efficiency is to provide environmental separation. Environmental separation requires a continuous water control layer (rain and groundwater), a continuous air control layer (air barrier), a vapor control layer, and a thermal control layer (insulation).

Basement Foundation

Figure 3 illustrates one option for installing environmental separation in a basement foundation. In some cases, one product can serve as multiple control layers.

A continuous water control layer for controlling rainwater is installed on the exterior of the above-grade walls’ structural sheathing; this is connected to the continuous water control layer on the exterior of the basement foundation. This below-grade water control layer, not shown in the figure, could be a fluid-applied waterproofing membrane or dimpled plastic drain mat. A continuous air control layer is installed on the exterior of the above-grade walls’ structural sheathing and is connected to the concrete foundation wall, which is acting as the foundation air control layer. In Figure 3, one product serves as both the water and air control layer. This product could be a fully adhered membrane, a fluid-applied membrane, or house wrap with seams overlapped and taped. The sealed rigid insulation serves as the air control layer on the basement foundation.

Vapor control is provided by controlling the temperature of the wall assembly condensing surface by installing continuous exterior insulation. Vapor control of the concrete foundation is provided by installing damp proofing on the exterior of the foundation wall and a sheet of polyethylene under the foundation slab.

Thermal control is provided by installing continuous rigid insulation exterior of the framing and cavity insulation within the wall cavities. Rigid insulation is installed on the interior of the basement foundation wall.

This approach works in all climates.

Crawlspace Foundation

Figure 4 illustrates one option for insulating vented crawlspaces.   

A continuous water control layer for controlling rainwater is installed on the exterior of the wall structural sheathing and is connected to the continuous water control layer on the exterior of the concrete foundation stem wall. This water control layer for the above-grade walls could be the rigid foam shown in Figure 4 if the seams are taped and the edges are sealed; it could also be a fully adhered membrane, a fluid-applied membrane, or house wrap with seams overlapped and taped. These products could also serve as the above-grade air barrier. A fluid-applied waterproofing membrane or dimpled plastic drain mat could serve as the water control layer for the below-grade walls.

The above-grade air control layer is installed on the exterior of the wall structural sheathing and is connected to the top of the concrete foundation stem and in turn to the continuous rigid insulation installed under the floor joists. The continuous rigid insulation under the floor joists is sealed at all seams to act as the air control layer of the insulated floor assembly.

Vapor control is provided by controlling the temperature of the wall assembly condensing surface by installing continuous exterior insulation. Vapor control of the crawlspace foundation is provided by installing impermeable continuous rigid insulation under the floor joists. Additionally, a continuous polyethylene ground cover is provided.

Thermal control is provided by installing continuous rigid insulation exterior of the sheathing and cavity insulation within the wall cavities. Additionally, continuous rigid insulation is installed under the floor joists.

This approach works in all climates.

Crawlspace Foundation with a Cripple Wall

Figure 5 illustrates one option for insulating an unvented crawlspace foundation with cripple walls. A cripple wall is a short wall that rests on the foundation and supports the floor and exterior walls (Figure 5).

A continuous water control layer for controlling rainwater is installed on the exterior of the wall structural sheathing and is connected to the continuous water control layer on the exterior of the concrete foundation stem wall. This water control layer for the above-grade walls could be the rigid foam shown in Figure 6 if the seams are taped and the edges are sealed; it could also be a fully adhered membrane, a fluid-applied membrane, or house wrap with seams overlapped and taped. These products could also serve as the above-grade air barrier. A fluid-applied waterproofing membrane or dimpled plastic drain mat could serve as the water control layer for the below-grade walls.

The above-grade air control layer is installed on the exterior of the wall structural sheathing and is connected to the top of the concrete foundation stem wall and in turn to a continuous polyethylene ground cover that is extended up the interior of the concrete foundation stem wall.

Vapor control is provided by controlling the temperature of the wall assembly condensing surface by installing continuous exterior insulation. Vapor control of the crawl space foundation is provided by installing a continuous polyethylene ground cover that is extended up the interior of the concrete foundation stem wall. Figure 7 illustrates continuity of the air and vapor control layers at the interior posts and piers.

Thermal control is provided by installing continuous rigid insulation exterior of the sheathing and cavity insulation in the wall cavities. Additionally, foil-faced or plastic-faced batt insulation is installed on the interior of the concrete foundation stem wall and extends horizontally over the polyethylene ground cover at the perimeter of the crawl space.

This approach works in all climates with the following limitation – in Climate Zones 5 and higher, the foil-faced or plastic-faced batt or roll insulation must be replaced with impermeable rigid insulation or closed cell spray polyurethane foam.