The first substantial structure we built was a straw bale cabin and we were immediately hooked on the beauty of the bale walls and the low environmental impact of the construction. We were already experienced with conventional stick framing construction and were impressed with its efficiencies, but we had a gripping fascination with timber framing that wouldn't let us rest. After moving to Central New York, with its cold wet climate, we put straw bale and timber framing together and found it to be a very high performing and aesthetically unmatched construction method. Along the way we experimented with other construction techniques, such as, double stud construction, earth bags, living roofs, light clay straw, tree houses, cob, rammed earth tires, and earthen floors. Below we highlight three methods that we use the most and have found to work best in our climate.
We join posts and beams with traditional joinery methods which are completely exposed to the interior space. The visible mass of the surrounding structure and carefully crafted mortise and tenon joinery can be stunning even in it's most basic form. The durability of timber frames is evident from neglected old barns in the countryside that refuse to give up easily. Minimal processing along with durability make timber framing a good environmental choice, since timbers are sawn green and little cutting is necessary.
This sturdy, time-proven method can be adapted to almost any structure - from a garden pergola or simple cabin to a full-size house. Straw bale walls work well with timber framing to enclose a house because they wrap around the outside of the frame, making studding unnecessary. This eliminates thermal bridging and increases the performance of the building envelope. Structural stick framing can also be integrated with timber framing. Some areas of a house can be timber framed and others stick framed for various reasons. It is also possible to fully insulate a building with cellulose insulation instead of straw bale, which is discussed in the cellulose section below.
We prefer to use locally available white pine timber which is easy to work with, has minimal shrinkage when drying, and is both economical and beautiful. We have built with many other wood species as well, including dramatic woods like black walnut, but be aware that hardwoods add to the material and labor costs. We like to incorporate natural tree forms and enjoy adding curved timbers to bracing and truss work.
Timber frame structures do require some extra design considerations to create an affordable and workable home. Maximum floor spans and available timber sizes should guide decision making of the size and shape of a building. Foundations need to be adapted to accept the point loads of posts. Plumbing and electrical lines need to be mapped out ahead of time and routed around timbers if necessary. Bracing and their relation to window locations and chimney routing should all be thought out ahead of time to avoid unsightly or unseemly solutions.
STRAW BALE WALLS
The thickness of straw bale walls and their curvaceous and undulating form compare favorably to the beauty of adobe buildings of the Southwest and the old cob buildings of Europe. Bale walls have the advantage of being much faster to build with the added bonus of excellent insulation values. Straw is a readily renewable resource and the bales possess very little embodied energy or toxicity – making the environmental impact of straw bale construction very low compared to more conventional wall building methods.
In our wet climate we find it easier to build bale walls after the roof is complete. The bale walls usually wrap around an interior timber frame but we also build infill bale walls where bales are fit between unseen stick framing within the wall. Bale walls can be built on any type of foundation including concrete slabs (with a stem wall to raise their elevation) or standard wood floor decks over a basement or crawl space.
Plaster is an essential part of bale walls, protecting them from weather, pests and fire damage and operating as an air barrier while allowing potentially damaging water vapor to escape. We use clay plaster base coats and lime plaster fill and finish coats instead of cement stucco. Cement stucco permeates water vapor more slowly increasing the potential for moisture damage. Cement production is also a very energy intensive and emits a high level of CO2 to the atmosphere. Lime produces CO2 in its production but reabsorbs it during carbonation in its application.
Moisture damage is the greatest concern when designing and building any type of structure in our climate. Properly designed and built bale walls have the same general degree of risk to moisture damage as well crafted conventional stick built construction. However, their moisture management strategies are very different and it is essential to understand how they work and design appropriately. Building a bale wall well takes care, attention and experience.
CELLULOSE INSULATED BUILDING ENVELOPES
While straw bales are our preferred insulation choice, they aren't ideal for every situation. We've found cellulose insulation to be an excellent alternative and use it in floors and roofs, and between floors if sound deadening is desired. It is also common for us to build partial and whole cellulose-insulated wall sections within a bale wall system for a variety of reasons. For example, we routinely build double studded service cavities under bale walls to allow easier installation of pipes, wires, and other mechanicals. Additionally, if straw bales are not wanted, non-structural lightly framed strap and stud walls (of any thickness) can be built onto an inner structural timber frame and insulated with cellulose as a whole wall system.
Cellulose insulation is made from recycled newspaper that has been treated with a non-toxic borate compound to resist mold, insects and fire. It has very good insulation values and can be blown into closed cavities or in open attic spaces where, unlike batt insulation, it thoroughly fills all nooks and crannies. Cavities can be built to a custom thicknesses to achieve desired insulation values. Using recycled cellulose insulation removes newsprint from the waste stream and requires approximately eight times less energy to produce than fiber glass insulation, and significantly less than foam insulation products.
Closed insulation cavities need to be carefully constructed with a continuous air barrier and be able to resist the force of a dense pack insulation blower. We feel that it is best to have an experienced contractor fill these cavities and check the results with infrared scanning upon completion.