Stucco - the easy way?

The onset of winter has greatly elevated our desire to seal up the exterior of our house. Most of the house is covered with SIPs (Structural Insulated Panels) - they are airtight and provide great insulation... no drafts there! But for one area of our house, we chose to try and replicate the older (European?) style of timberframing, by exposing timbers on both the inside and outside of the house. Making this section of the house weather tight presented a unique challenge.

Ancient timberframes were infilled with all sorts of creative materials... cow manure, mud, and wooden lathe, to name a few. For energy efficiency, hygiene, and aesthetics we decided to try some more modern materials. For insulation, we used the same poly-iso foam panels that we used in the roof. For siding, we were going to do a traditional 3-step stucco process until I discovered some cement based stucco-textured panels at Lowes. These "HardiePanels" are made by the James Hardie company - the same company that makes "HardieBacker" sheets that are typically used for underlayment beneath tile floors. We reasoned that if we weren't happy with the final look of the panels, a few years down the road they would make a suitable base on which we could apply traditional stucco.

I bought the pre-primed panels in 4x10 sheets. They were $36 apiece, so that worked out to be about $1/square foot (not counting paint, nails, and caulk). We did a final sanding of the exterior white-oak timbers and applied spar-polyurethane to them before installing the panels between them. Should you make a mistake with a paint brush or caulk gun, timbers sealed with polyurethane are much easier to clean than raw or oiled wood. I cut the first panel to fit "loose" within the timbers, so there would be about 3/16" of an inch gap in which I could apply a bead of flexible caulk around the perimeter of the panel. The caulk was expensive (over $5 per tube), but it was labelled "for use with pre-primed cement based siding," and allows 50% movement so I bought it. The consistency of the caulk was somewhere between thick latex paint and silicone and I found it hard to place consitently and neatly, but I started getting the hang of it. Actually I just decided to call my results "good enuff."

I tried attaching the panels with the same screws sold for use with hardie-backer underlayment, but I found it hard to drive them flush without damaging the panel or stripping the heads, especially when working from a ladder. After a dozen frustrating screws, I switched to stainless roofing nails (left over from the slate installation and purchased from (http://www.jenkinsslate.com). They were much easier to drive flush, and don't rely on a coating to keep from rusting. My wife smeared cement based patching material over the heads of the nails. The texture was a good match for the pre-embossed texture of the panels and the cement material bonded tightly to the stucco panels.

Although the panels were preprimed, it was necessary to prime the cementous material that hid the nail heads, so I painted primer over the entire surface. I figured it could not hurt and I was anxious to see what the panels would look like in place and one continuous color. By the way, the primer is specifically labelled for use over cement based materials. For the final coat of paint, we'll probably choose something close to the color of the mortar in our stonework.