Gulf-Island Super-Insulated Home: Part II- Foundation and Framing
When building an ultra-high performance, super-insulated home, there is a vast amount of planning, design and construction detailing to be worked through before the first shovel hits the ground. More so than any other custom home build, a house that is to be built as closely as possible to the Passive House standard (recognized as the world’s most stringent energy efficient building standard) requires an even greater lever of planning. Assemblies meeting very low u-values must be designed; structural elements must be created with no thermal bridging; window and door installations maintaining envelope protection while still minimizing isotherm deformation need to be developed; air sealing strategies must be determined to ensure that every three dimensional corner between every surface of the house and all penetrations are accounted and prepared for. In short, it is an incredible amount of thought to ensure that when construction begins, critical opportunities are not missed or overlooked to ensure that the final product resembles a top of the line luxury car (or Tesla, as would be more appropriate!) in the form of home efficiency when it is all done.
A super-insulated house is akin to a person wearing a down-filled one-piece snowsuit with tight-fitting snowcuffs and collars. While this may seem counter-intuitive in our relatively warm climate, from an energy standpoint a well-insulated house keeps occupants as comfortable as possible in all seasons with a minimal use of energy to heat or cool the interior space. Key to this concept are three things:
- The foundation (slab, crawlspace, or full basement), walls and roof must all have exceptionally high insulating values, often twice that of code minimums
- The windows and doors must be very high performance, allowing a minimum of heat loss, air leakage and still allowing the sun’s heat to enter the home where desired
- The entire outside shell of the home (“all six sides” of the envelope) must be constructed in an airtight fashion to minimize the leakage of conditioned air to the exterior.
The focus with this blog will be on the first item in the list, in specific the foundation insulation and the wall and roof systems.
Looking at this Gabriola residence, images of the foundation reveal how much we have used rigid foam insulation inside the perimeter foundation walls and under the slab to keep heat loss through the concrete slab of the house to a minimum. By using Type II EPS, we have an insulation which has a reasonably high R-value (about R3.5 per inch), is not affected by any moisture which may be in the ground, and which easily supports the loads imposed on it from above (Type II EPS can support 15 lbs per square inch). EPS is also the most environmentally benign of all the rigid foam products in terms of its green-house gas production during manufacturing and chemicals used to make it. While it is important to consider the complete life-cycles and environmental effects of all insulation products, rigid foams do have a place in construction because of their critical ability and handle moisture and physical loads.
Equally important are the walls and roof insulation levels. BC Passive House supplied the complete wall and roof panel package for the house. They are a full capability custom wall panel manufacturer in Pemberton BC specializing in super-insulated wall assemblies. They use sound building science, top-quality products and great precision and engineering to deliver a terrific product for homes of this calibre.
The walls are essentially a 2×8 exterior panel wall with 5/8″ OSB on the interior as the air barrier, vapour retarder and structural sheathing, and a European fibreboard sheathing on the exterior to allow for maximum drying potential of the assembly to the outside. The walls arrive pre-insulated with dense-pack cellulose, which is regarded by most in the green building realm as the insulation product with by far the least detrimental environmental impact of anything on the market. It is essentially like a combination of finely shredded newspaper and dryer lint that has been made fire and insect resistant. The roof panels are almost identical, but utilize 12″ TJI joists as their framing members.
The wall and roof panels are erected relatively quickly on site with the aid of a large crane (90-ton in this case, with a horizontal reach of 180′). The largest panels measured about 10′ by 38′, so parts of the house went up very quickly. Key to the assembly of the panels is the air sealing detail. Where each panel connected to the next, a double row of neoprene rubber gasket was used to create a more airtight seal between the members. At the critical roof panel connections, two rows of acoustical caulking were used in addition to the gaskets, because the ceiling/roof plane is the location where air pressures are consistently higher, and the potential risk from failures due to air leakage moisture damage are the most dangerous. The house went up beautifully, and we immediately wrapped the house in a waterproof, vapour permeable membrane to prevent any moisture damage. Building any house in the a coastal BC winter is less than ideal, but when your walls and roof arrive pre-insulated, it is absolutely critical to ensure that bulk water is never permitted to enter the assembly.
The incredible SIGA Majcoat roof membrane unerringly protected the house during several torrential downpours. I am convinced it is a higher quality fabric than was used in my expensive 3-ply gore-tex snowboarding jacket! For the walls, Typar strikes a great balance between waterproofness, being vapour diffuse and having great tear resistance. SIGA makes amazing products for the walls as well, but Typar met the panel manufacturer’s and envelope engineer’s requirements, and we used it as part of our responsibility to try and realize cost savings for the client wherever we could. Sometimes you can leave the Lamborghini at home and just take the Honda to get the groceries.
Architecturally, the home also has overhangs which are created by cedar “purlins” which cantilever out from on top of the panels. On top of them is clear, Alaskan yellow cedar (supplied by Canadian Bavarian Millwork and Lumber in Chemainus) for the soffits/exposed areas, and inboard of that is standard fir plywood sheathing. A standing seam metal roof will cover all when the home is complete. Originally the home had this purlin roof system, as well as a dropped ceiling on the interior of the house which held additional insulation and wiring for ceiling lights, etc. However, recognizing that the home had almost no conventional ceiling fixtures, we proposed to the architect that we eliminate the insulated interior ceiling drop and instead place appropriate insulation in the already existing purlin roof assembly. This allowed us the same levels of insulation, but created great material and labour savings by essentially removing an entire ceiling drop from the home. We made the necessary purlin roof do double duty, achieving a construction equivalent of car-pooling. It was one example of the way a collaborative architect can benefit from the on-site experience of a contractor that understands and believes in the philosophy of a Passive House or super-insulated project.
The walls were a different story, however. Because the home has no crawlspace or attic, much of the electrical wiring and plumbing was going to have to run around the perimeter of the home. Pheasant Hill Homes built what are known as “service walls” inside the panelized exterior walls to contain these components of the home. These are, essentially, completely normal 2×4 walls which are also insulated with conventional batt insulation. The sub-contractors are able to run their services through these exterior walls in a usual fashion, and because the OSB on the panel wall is our air barrier, we do not have to concern ourselves with air sealing every time a wire runs from a partition wall into this “service wall.” Here we made another change to the architectural detailing and chose to place EPS insulation between the bottom plate of this service wall and the panelized exterior wall. This reduced any thermal bridging at the critical junction of the concrete slab, the exterior wall and the concrete foundation. It may seem like a minor detail change, but in considering the roughly 400 lineal feet of perimeter wall, the potential effect of this small thermal bridge becomes greatly magnified.
That takes us through the first two major stages of construction on this home. Follow this blog in the coming weeks to see posts tackling the next two items on our envelope efficiency checklist: windows and doors, and air-tightness. Or click here to see some of Pheasant Hill’s other posts and projects covered in the blog.