Extra-Passive House

On a heavily wooded dead-end street, the Orr Ln. site offered many conifers to log for finish materials, which allowed the mature oaks to create a full western flank of deciduous shade for the new house.

The logs were slabbed and dried on site, limbs chipped for pathways, and stumps removed for the new house

Construction began late June 2022, with the delivery of pumice and perlite for the first foam-free Passive House slab-on-grade foundation in a cold climate.

A mountain of pumice was trucked in from Chemult, with bagged perlite from Supreme Perlite in Portland for under the slab and in perlite-crete support beams.

The natural materials felt right to work with, pointing us in the right direction for the aesthetic goals of this house

3/8 minus pumice

1-1/4" slabs of douglas fir and cedar

The house pad was cut at the same time as the 16" deep footing trenches for the pumice around the perimeter

Excavation was at times rough going with rocky areas, but thankfully full depth was accomplished and tidied up nicely.

An approximately 4" layer of drain-rock was placed and compacted both on the pad and in the perimeter trench to provide a drainage plane below the insulating materials. The drain-rock was sloped 2.5" from the far corner of the site down to a sump, so the vapor barrier above could convey any water used in compaction to an outlet. This outlet at the sump created a place for future monitoring and water mitigation if needed.

Pumice being placed over the 15-mil vapor barrier

The pumice had to be significantly wetted down to achieve compaction, and fine-grading was performed a few times to accomplish a level perimeter for footings.

Next up was underground infrastructure, then building the lightweight formwork required for perlite-crete support beams under load bearing walls, and as slab supports between bagged perlite insulation areas. These forms were also lined with 15-mil vapor barrier to prevent moisture migration up into them.

To mix perlite-crete, we put water in a portable mixer, then added perlite from the four cubic foot bags Supreme Perlite in Portland supplied. Thankfully it was summer, and we did not have to worry about the paper bags getting wet and tearing as they can do. The unavoidable perlite and cement dust next to the mixer was easy enough to keep under control with an N95 dust mask during the batching.

After perlite and water came the cement, more of everything, air entrainment admixture, and prodding to verify a complete mix:

And finally into the forms for where it could be spread and leveled off. There is significant water in the mix, and it was important to keep the mixes from being too wet as the cement could migrate with the water to the bottom of the slurry otherwise.

Stripping these a couple days after pouring allowed for adequate cure time, revealed some nice crisp insulative material, about R1.85/in. in this mix

The vapor barrier under the perlite-crete beams is laid flat to connect to the rest of the vapor barrier under the bagged perlite

Beams were completed prior to placing the additional vapor barrier between, and then the bagged perlite

To place the perlite, some bags had to be cut down in size, and corners were chinked throughout to minimize any future settling from consolidation after the bags deteriorate with time.

alas, all was ready for vapor barrier to cap the insulative materials

and compacted crushed rock to prepare the thickened edges over perlite-crete supports below