Deep Energy Retrofitting an Existing House
Passivhaus practice essentially treats the existing house as if it were the inner core of a new Passivhaus – “wrapping it” from the outside in a superinsulated, airtight “comforter”. The first step is system optimization: how best to thermally isolate the heated envelope from unheated spaces. Therefore, unheated basement ceilings are sealed and insulated. For walls, the process begins with the removal of the existing siding, typar, tyvek or other such material. Once the existing outside boarding is exposed, a non-permeable building fabric is applied to the exterior, with all seams taped. This fabric extends to attic or roof and is taped to the concrete slab or the air barrier of the or basement, forming a continuous air/moisture barrier around the heated envelope.
Outside of this new airtight surface a superinsulated wall segment is added. It is critically important to add enough insulation to prevent the temperature of the former outer boarding from ever dropping below the dew point, causing condensation within the wall structure. This means one cannot arbitrarily add a thinner layer of insulation because there is no room under the eves for a thicker wall, or in order to save money. It is better not to add any insulation to the outside, than to have the building destroyed by condensation inside its walls. Accordingly, I-joists may be bolted, through the air barrier and existing outer boarding, to the existing studs, as the framing of a wall system designed to dry toward the outdoors. A covering of a heavy duty building fabric is applied to the outside of this I-joist framing, taped together to form a continuous film. This fabric sheds rain, but allows moisture to permeate from inside the wall to the outside. Next follow horizontal wooden battens, tying together the outer edges of the I-joists. Vertical battens to hold the siding follow. The vertical battens also provide room for drainage as the outer wall dries out behind the siding. The pre-existing inner stud-wall dries toward the indoors.
Windows and doors must be framed out to accommodate the new, thicker walls. Windows should be passivhaus rated and taped to the continuous air/vapor barrier of the walls. A typical existing double glass window has an R value of about 2, while Passivhaus windows hover around R-10.
Attics and roofs are dealt with according to local conditions. If there is sufficient room inside an attic to accommodate substantial amounts (2' or more) of cellulose insulation and a vapor barrier can be added and sealed to the walls, the attic floor may be superinsulated. Otherwise, especially when re-roofing is contemplated, all roof coverings are removed to bare wood and the impermeable air/vapor barrier of the walls is continued across the roof, along with I-beams and rain shedding, inside-to-out permeable building fabric and battens, like on the walls.
Once the building is ready for insulation, a blower door test is conducted so any air leaks may be detected and fixed. The idea is for the building’s air tightness to meet the Passivhaus standard or come as close to it as humanly possible. Once the building passes the air tightness test, all cavities outside the original structure are blown full of dense-packed cellulose insulation through carefully cut blow holes, which are then hermetically sealed. Then new siding is installed. In our Maine climate, a superinsulated Passivhaus walls have an R-value of 55 to 60.
As an alternative to the I-beam/cellulose insulation approach, a continuous layer of sufficiently thick rigid foam insulation may be attached to the pre-existing outer boarding, sealed together to function both as insulation and air/vapor barrier. In Maine this foam layer must be at least 4" thick, to provide the minimum insulation necessary to prevent condensation inside the structure.
Even when deep energy retrofitting is not feasible, it is best to follow Passivhaus principles (maximum insulation, continuous air/vapor barrier on the warm side, ventilation) or moisture problems will occur. Existing walls may be caulked, electrical outlets sealed and the inside given a shellac-based prime-coat of paint as a vapor barrier before being blown full of dense packed cellulose or a suitable insulating spray foam.
Existing walls may also be augmented with additional insulation (and a new finished surface) applied to the inside. Note, however, that when insulation is applied to the inside of existing walls, unsealed and uninsulated cavities between floors and within partition walls will remain, in effect thermally connected to the outdoors. In any event, beyond minor caulking, sealing electrical outlets and switches, adding weather-stripping to doors, and the like, the proper approach to retrofitting an existing house is determined by an independent professional energy audit. Information gained from this audit should be the basis of coherent, system-level planning.