Underhill House is a hybrid of natural and high-tech green building techniques. Most of the walls will be made of straw bales, but much of the south and east walls have so many windows in them to collect solar heat gain in winter that it made no sense to try to tuck a straw bale wall around them. Those walls are being constructed along conventional lines with 2×8 dimensional lumber so they will be well insulated.
The outside of both straw bale and conventional walls are going to be finished in an exterior three-coat lime plaster, and the work began in earnest on Monday. I’ll talk more about the advantages of lime plaster over Portland Cement in a future post.
We are fortunate to be working with Krome Burke-Scoll and the team at Artisan Exteriors and Plastering.The plywood sheathing over the 2×8 framed walls will be plastered first. The plan is that by the time the plywood walls have all been plastered, the straw bale walls will be up and ready for their own stucco coating.
The first step is to make a water-resistant barrier or WRB to control the flow of moisture and keep it out of the places where it could do damage. The entire surface has been covered with tar paper, making sure that the overlapping seams all take potential moisture down and away from the interior of the walls.
It was a bit of a puzzle how to make the kind of tight seal needed around the top of the wall. Bryan wanted Krome to plaster up to the wood. Krome wanted Bryan to add a piece of wood trim that fit tight around each timber.
The compromise is that each timber was taped around and then circled with 66XF Casing Bead, a metal trim familiar to plasterers. A thin piece of oak trim will eventually be added between each rafter, and for now a right-angle metal gasket will keep the plaster from touching the wood.
The entire wall surface was meticulously covered with a metal lathe. It creates a honey-comb matrix that the plaster will get a good grip on and strengthen it.
Krome says anytime you can make a matrix within a substrate – that’s good. Metal is strong enough to bear the weight of the plaster, and it will strengthen the layer the way steel rebar strengthens concrete. The lathe has self-firring bumps on the back side that hold it slightly out from the surface it is attached to so that it becomes an integral component in the middle of the scratch coat.
While three of the plaster team members were working on surface prep, Eric was mixing up a test batch of lime plaster to see how it will react in these high temperature and medium moisture conditions.
He combined local lime that has been slaking (a process where the dry lime is soaked in water to become a putty) in buckets for a week with PowerPozz It’s a substance similar to volcanic ash that improves the strength and durability. It also speeds up the setting time. Then he added sand and water in a careful ratio.
The next step is to spread it out on a sample board and see how it performs. (The same board will be used to test the brown coat, the name for the second of three coats, and the final coat as they are added.)
Tuesday Eric will mix up big batches of the scratch coat plaster, and the beautiful metal mesh over black tar paper will vanish beneath its first coat of lime plaster.
Categories: Underhill House