March 12, 2010
We have put a lot of thought into space planning for the house- to make it beautiful, practical, and energy efficient. Construction wise we also have a plan. The plan in general is to make the most well insulated, and well air-sealed envelope (basement, walls and roof) that we can. Then to use the most energy efficient heating, cooling and ventilation system that we can. To specify high efficiency windows, lights, appliances and plumbing fixtures and to use only sustainable, healthy, no-voc (or if we must low-voc) materials
We considered a number of wall assemblies that we are thinking of for the house. Often we will frame a house 24” on center instead of 16” on center- thus allowing for more insulation and less thermal bridging. But with this house so much of the existing 16” on center exterior wall framing and floor framing was going to remain and new structure needs to tie into the 16” on center, so it is more efficient -framing wise- to stick to the 16” and not go to 24” on center with the new parts.
The existing house is also 2x4 construction. New houses in our climate are generally built with 2x6 walls to allow for more insulation in the wall cavity. We will build the new exterior walls with 2x6 but we are stuck with the existing 2x4 shell where it occurs (about 1/4 of the final exterior wall area.) We would make these walls thicker to the inside but there is literally no room in most cases as a large area of it borders the L shaped stair case which is pinned on its other side by a massive masonry fireplace and chimney. The stair width barely meets code as it is. But we will thicken the walls to the inside anywhere we can.
We decide right off the bat to wrap the house in a continuous rigid insulation. This will help -significantly -to kill thermal bridging (the place where hot or cold can travel between interior and exterior through the studs in the envelope.) and it will also help make up for the 2x4 'thin' walls where they occur. This rigid foam insulation will add about R7 to the exterior of the house and act as a continuous air barrier as well (when the seams are taped).
(It is sort of 'ironic' that we also cannot go out to far to the exterior- this rigid board insulation cannot be too thick- because we are already on the line for setbacks and coverage with the town and adding an extra inch or 2 around the perimeter of the house will truly anger the zoning board and stop the project. This is also so ridiculous! We are HEMMED IN! Yet another hindrance to building the right way. Especially when we are doing the town a favor. We are saving the framing of the old house! That is not easy or cheap to do. Demo is easy and cheap! Saving stud walls when everything is gutted around them is not. We are saving recourses and retaining the existing footprint of much of the house. We are building a very small, very conservation oriented home! All of this will reduce impact on the town itself. One would think that the town elders would be happy to have us further reduce the carbon footprint of this house by allowing us another inch all the way around for better insulation. Sigh ~)
Next we need to choose an insulation. When faced with 2x4 wall cavities we usually choose a closed cell blow in foam. Basically the insulation options available are fiberglass batt (the pink stuff we mostly all have in our houses) fiberglass blow-in (a blown in version of the batt, better because it gets in nooks and crannies that batt cannot get into.) Blown-in cellulose- which is recycled newspaper, etc in a blown-in form – again better than batt because of the way it fills the cavity plus better for the earth. (There are actually 2 forms of cellulose- wet and dry. We never spec the wet, it has a history with too many problems with developing moisture problems in the walls). There is a denim batt insulation- thick blankets of recycled blue-jeans. And lastly there are open and closed cell foams. Closed cell foam has a higher R-value but also has a higher carbon footprint- basically it is not very good for the earth due to its production, its off-gassing during installation (and after) or… during a fire. Open cell foam has a smaller carbon footprint but also a lower R-value than the closed cell.
We will leave this decision for now as the clients have a lot of things to think about regardign insulation and it gets quite complicated.
We have already got a landscape designer on board who is focused on LEED. We know that both contractors under consideration have either built LEED homes before or have built many super efficient green homes that could have been LEED certified. I am serving as the building envelope and construction efficiency expert until we hire the contractor at which point he and I will collaborate for exact technique and cost efficiency measures.
Ideally the contractor is hired in the design phase and travels through the entire design process with the client- architect team. Then the contractor – or the contractor and architect together- would serve as the building envelope and construction efficiency expert. Having designed and built one LEED home before and many other very efficient homes that could have been LEED certified before that, I have a lot of experience in designing a building envelope and nuancing construction efficiencies. So it works to start without the contractor, it is just not ideal. When we hire a contractor we will meet with them and go through all the details. Fine tuning things here and there to make the contractor happy (some contractors just like to do things one way while another might do things another- the results being the equal. ) Or we may revise to save the owner money or making something work better. 2 heads are always better than one!
What does all of this mean and why does it matter? Basically to build and energy efficient home and to get LEED certification you need to build a home differently than ‘normal’. The walls of the house and the roof and basement (the building envelope) need to be highly insulated and also assembled in such a way to reduce air infiltration and thermal bridging (the ability of heat or cold to pass physically between materials) to as close to zero as possible. Wall assemblies, etc can get highly complicated, I have been attending green building trade conferences for 12 years and have watched the guys who have been doing this diligently for over 30 years still argue vehemently about how to do it best. The biggest dangers of a poorly thought through wall assembly being moisture control and the potential for mold, mildew etc. It is a given that any house built tight will need a method of ventilation and air exchange, but even with great air flow a house can still encounter moisture issues if the wall assembly is wrong. `