The object is to create a wall with the air, water, and vapor control layers on the exterior of the polyisocyanurate insulation. The weak points are at each joint in the insulation, the transition between the above ground frame wall and the basement wall, where the wall control layers meet the roof or ceiling control layers, and the rough openings for the windows and doors.Using two layers of insulation provides greater durability than a single layer. The reason being that the outer layer will expand and contract some as the outdoor temperature changes. The inner layer is protected from the weather by the outer layer and will not expand and contract nearly as much. The seams of the outer layer need to be offset from the seams of the inner layer by at least six inches. This will increase the integrity of the insulation and the wall assembly. The joints of the first layer do not have to align with the wall studs, but on the second layer they do. The joints on both layers must be sealed. Dow Weathermate construction tape works well for this because it has been specifically engineered to chemically bond with the wax layer on the aluminum face. The tape must be pressure set using a roller or plastic scraper to develop its full bond. Using two layers of two inch thick Dow Thermax provides all of the insulation the walls require. The stud spaces could be filled with even more insulation, and any insulation that is in sound condition should be left in place. Insulation that is polluted with mold, animal feces, or other contaminates, should be removed. There needs to be a flat and sound surface on which to mount the insulation, if there is sheathing, strip the walls down to it. Where there is no sheathing the walls should generally be stripped down to the stud walls. On projects where there is no need to access the wall cavities, and the siding is in good repair, it might be possible to leave the siding in place, which is what we did with the Cottekill House. The novelty siding provided a sound, even surface we could use as a base for the insulation.
If the electrical or other mechanical systems in the exterior walls need repairing or updating, be sure to do this prior to implementing the DER wall insulation process.If present, remove attached porches. The porch roof will be rebuilt after the walls have been insulated and the furring strips have been attached. The drainage plane will continue down behind the porch header. The header will be bolted through the wall insulation and into the wall studs. Be sure the bolts go into the wall studs at least two and one half inches. The bolts will put the assembly under tension and it will not sag. There is no advisable way to leave the porch roof on and work the wall’s control layers around it. Once the walls have been stripped in preparation for the insulation, any needed repair should be completed. Replace rotten timbers. Make any desired changes to the home now, such as adding or relocating windows and doors. The exterior will last for decades and making these changes later will be more expensive and more difficult to properly integrate into the assembly after the fact.
The transition zone between the masonry wall and the wooden structure needs close attention. In a typical NYS home, the basement wall is stacked dry stone, CMUs, or poured concrete. Dry stone was the most common construction method used until early last century. While the stone is a vapor barrier, the lime mortar, dirt and debris are not. Cleaning the space between the beam and the top stones down to the stone is required in preparation for the spray foam, and this is all the vapor proofing needed. With poured concrete and CMU walls, the house will have to be jacked up enough to get a piece of neoprene rubber between the wood and concrete. Sometimes the building is so close to the ground that extreme measures are required.The Rhinebeck House had to have the corner of an addition, which was resting on ledge, raised up and rubber roofing placed under the sill.
The exterior insulation should come down past the neoprene extending the drainage plane over it.
Creating an air seal between the exterior basement wall and the exterior wall insulation is mechanically difficult. Air will infiltrate under the bottom of the insulation and up the backside until an opening into the living space is found. Air will also go through any crack left between the basement wall and the base of the wood wall. The best solution will vary depending upon the structure. The important thing to remember is that the air seal needs to last for the life of the wall. A combination of spray foam, caulk, and flashing will be used.We used bent metal flashing to create a manageable transition zone where the bottom of the insulation is exposed. The flashing is bent into a J channel with the long side nailed to the wood. Two beads of caulk or adhesive foam was applied to the wall behind the metal.
The bottom of the J flashing is wide enough to hold the two layers of insulation. The short side is taped to the front of the insulation, to complete the drainage plane. This joint is reversed flash, and thus likely to develop leaks. In order to provide a drainage path for the water that does get between the insulation and the flashing, 1/8” holes are drilled every two feet. This metal flashing provides a long lasting transition point that protects the insulation from insect, weather and impact damage.
Once the walls are insulated, furring strips need to be applied over the insulation. Screws will need to penetrate the wood studs at least 1 ½”. The NYS Energy Conservation Construction Code of 2010, table 402.1.5.2 on page 15, specifies type, spacing and size of the fasteners needed, based on the weight of the sheathing to be installed onto the furring.The furring serves to anchor the insulation to the walls, and provides functional support for the cladding. It also creates a vented drainage plane. We used rough cut pine from local mills for the furring.
We found that the heads on Philips head screws would strip due to the high torque required to set them. We settled on using Headlock Screws, which have a straight sided spine drive and heads designed to secure the furring solidly.