Moisture Control And The Building Envelope In Winnipeg, Mb

Moisture is responsible for the majority of structural damage to our homes in Winnipeg

. Moisture ingress into the building envelope and its subsequent accumulation must be kept to a minimum for a buildings long term performance. A building envelope is designed to separate two unlike air masses - the internal from the external. With careful planning, this can be accomplished but the majority of buildings, especially our homes, leak - outdoor air seeps into wall cavities or even interior conditioned space, and conditioned interior air escapes and with it the heat, humidity control, and air purifying dollars spent to make our environment ideal.

Air movement through a building envelope happens because typical envelopes seen in residential buildings are full of penetrations, use fibrous insulating materials, and are inadequately crafted. This is accelerated where pressure is applied in the form of wind washing, large temperature differences across wall assemblies or as a result of vapor pressure differences.

A typical wall assembly is created (in Winnipeg) with cladding on the outside of 2x6 structural framing with house wrap (Tyvek), and OSB in between. The cavity is filled with fibrous insulation and the interior side of the framing is covered with 6 mil polyethylene layer and gypsum wallboard. There are a number of potential problems with this construction. First off, with the vapor barrier on the internal side of the wall there is very little protection afforded to the fibrous insulation from exterior pressures from wind washing and convective currents created due to temperature differences across the wall assembly. This means that exterior air has free access to the insulation in the wall cavity which itself allows air to move through it (fiberglass also doubles as a furnace filter - something you WANT air to move through)! The result - a wall assembly that performs at vastly degraded real world R-Values than advertised. The same thing happens from the interior side in a home that is pressurized. Indoor, moisture laden air is forced through gaps, transitions and inadequately sealed junctions in the wall assembly into the cavity insulation and with it the energy used to condition it. The main problem with this, aside from the immediate energy loss, is the moisture that this air carries with it. When warm, moist air from either side of the wall reaches a sufficiently cool surface (from the cold outside in winter or through the use of air conditioners in summer), a condensing surface is formed allowing moisture to condense, accumulate, cause water damage, permit mold growth and lower the air quality of the home. This is where a 6 mil polyethylene vapor barrier actually works against the building envelope by preventing the wall assembly from drying. Because fiberglass allows air to move through it, and air moves moisture, water is going to get into a fiberglass insulated wall - what is then needed is a way for the water to get out - this cannot readily happen when vapor retarders such as polyethylene are used.

A better wall assembly is built in a continental climate like Winnipeg, MB, when using medium density spray foam insulation such as EcoLogicFoam. By forming billions of tiny bubbles made of thin plastic walls, it is impervious to moisture flow (recognized by the Canadian Construction materials Center as an air & vapor Barrier) and does not allow water to accumulate inside of the material. Further because it is a continuous insulator, the surface of the foam never becomes a condensing surface on either the interior or the exterior surface meaning that any moisture laden air that comes into contact with it cannot deposit or accumulate that moisture on the surface of the foam. This alleviates the pressure driven movement of both air and moisture through the building envelope and results in a tight wall assembly and permits the conditioning of the interior environment with a high degree of efficiency through conservation.

In our next installment we will examine the cost of using spray foam insulation verses typical fiberglass batts.

by: Mark Nelson

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