Insulated Footings for a Massachusetts Passive House

A view of the perimeter strip footings for the house’s basement foundation walls shows the layer-cake construction of the insulated footing detail: first, a base of compacted “trap rock,” then a layer of compacted crusher run, topped by a layer of sand, and then the insulating layer of Pittsburgh Corning Foamglas. Plastic slip sheeting above the Foamglas keeps the concrete footing isolated from the Foamglas, so that expansion or shrinkage of the concrete won’t damage the relatively brittle insulation material.

Auburndale carpenter Casey installs protective safety caps on the rebar projecting from the building’s perimeter footing.

Unlike many Passive House projects, this building has a relatively complex footprint. Here, we see the perimeter basement wall footings already poured, with forms for the interior pad footings distributed around the gravel base inside the home’s footprint.

Carpenters Casey and Tim locate the positions for the interior pad footing forms.

Carpenters Tim and Matt position another plywood pad footing form.

Left, carpenter Casey marks the position of the plywood form onto the trap rock sub-base using orange spray paint. Right, carpenter Matt pounds a stake to secure the form in place.

Carpenter Matt drives a stake into the trap rock sub-base to secure a plywood pad footing form in position.

Carpenter Tim lays out the planned finish elevation for the concrete pad footing onto the plywood footing form, in preparation for placing base material and fitting insulation into the form.

Carpenter Tim compacts the trap rock sub-base inside the plywood pad footing form.

Carpenter Casey shovels “crusher run” base material into a plywood form. The crusher run contains a graded mix of fine gravel and stone dust, which compacts readily into a strong and smooth surface.

Tim shovels crusher run into a form.

Casey and Tim shovel crusher run into the plywood form.

Tim screeds the crusher run layer smooth and level with a scrap of plywood.

Tim compacts the crusher run layer before adding an additional protective layer of sand.

Compacting the crusher run on top of the trap rock inside the plywood form.

Adding sand to the form. Sand serves to protect the relatively fragile Foamglas and provide a smooth, flat cushion of support for the insulating material.

Adding sand to the form.

Troweling the sand smooth and flat.

Tim places a two-foot-square piece of Foamglas into the prepared form (left), then trims another piece of Foamglas to fit using a trowel. Foamglas cuts easily with a trowel or handsaw, but workers should wear respiratory protection while cutting the material in order to guard against small glass particles or fibers, which carry a risk of lung damage.

Wearing a respirator to protect his lungs, carpenter Tim cuts a piece of Foamglas (left) and places it into the form (right).

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