Question:
Dear Sir;
In the spring of 2001, I started building a cabin. After the foundation, framing, roofing and the sides (covered with plywood) were finished I had used up the largest lump of money I had available for the initial construction. I had hoped that for each following year I could add a bit more and in the meantime I would be willing to rough it. My question concerns the outside plywood walls. I cannot afford to put in all the windows and the siding at this time. I am wondering how long can I leave the outside walls as they are? How long before the plywood begins to rot? Is there any danger that the whole frame could buckle or be affected? Is there anything I could do in the meantime? How long can Tyvek (or similar material) stay on before the siding is installed? Could a person put strips of plastic or tarpaper where moisture comes in?
My cabin is located in Eastern Canada where they get quite a bit of precipitation. I would really appreciate your views and ideas on this subject.
Thank you.
Answer:
The exterior walls of a modern home are part of a very complex system often referred to as the “building envelope”. This envelope is complex due to the large amount of insulation installed for energy efficiency and comfort. This insulation and corresponding air-vapour barrier make the walls well sealed on the inside of the home. With the plywood on the exterior, this can create a fairly tight area in the wall cavity. If condensation and moisture develop in this cavity, it must have an escape route to the exterior to prevent moisture damage. This damage may occur because the inside of the wall is protected from air and moisture leakage by the polyethylene sheathing, installed behind the interior wall covering.
The only route of moisture escape is through gaps between sheets of the plywood sheathing, and other small spaces or holes, in the outer wall. Tyvec or Housewrap is designed to prevent air movement into the wall cavity due to wind (or other exterior factors) while allowing moisture to escape. This moisture should drain or evaporate in the space between the siding and the housewrap, preventing rot in the wall framing and sheathing.
The exterior siding prevents moisture from entering the wall cavity and protects the wall sheathing (plywood, in this case) from the elements. If the exterior siding and housewrap or building paper is not installed, moisture and wind may be allowed to enter the wall cavity. This may allow the insulation to become damp and less effective. Excessively wet insulation will promote rot in the wooden wall components. Even a properly installed housewrap may not protect the plywood for very long, as it will become deteriorated due to ultraviolet light from the sun as well as rain and snow. The housewrap or building paper on the exterior is designed to be covered by a proper siding, to do its job effectively.
For these reasons it is important to complete the various components of the building envelope as close as possible to each other. Leaving many seasons between installations of the various components may indeed allow the wall sheathing to rot. The housewrap will certainly become deteriorated, and may require complete replacement before the siding is installed.
I am assuming that the cabin is to be fully insulated for year round use, but this may be improper. If the use is summer only, no insulation is installed, and the building not heated, there may not be as much urgency in installing the siding and windows. Excess moisture will not normally develop in the wall cavity if the building is not heated in cold weather. Any condensation developing in between the exterior studs in the winter should easily evaporate into the open cabin, as long as walls are not covered and no vapour barrier installed. Siding should still be installed as early as possible, because the building paper or housewrap on the exterior will not last long exposed to the elements. Rain and snow will eventually rot the plywood sheathing.
I would not recommend installing any plastic sheathing or other band-aid measure on the exterior of the building, as it will easily deteriorate and may cause more problems than it helps prevent. If funding is a major factor, I would recommend putting all resources into finishing the building envelope and exterior before any of the interior components are attempted. If this is not possible, a trip to the local bank or financial institution may be the solution to preventing replacement of damaged wall components before the cabin is complete.