Why should I be concerned with vapor barriers and ventilation?

            For starters, we live obviously live in a heating climate, so insulation is needed.  But the problems start because living in a house creates high humidity from cooking, bathrooms, breathing, house plants etc.  Warmer temperatures obviously can hold more moisture.  Colder temperatures can’t hold as much humidity.  This is the root of the problem.

            If  high indoor humidity is left unchecked during the heating season, it will find a cold spot, hit "dew point", condense and cause rot, mold etc.  Where this condensation occurs depends on the integrity and permeability of the vapor barrier.  The first visible evidence is usually condensation on the window.  Yes you can put an exhaust fan in for most cases to remove the bulk of the humidity from the structure.  But relying on fans alone causes negative pressure inside the "building envelope", and fresh air gets drawn through small cracks and voids everywhere.  Not only does this expel moisture out of the structure, but it also expels heat.  This in turn causes inherently colder spots where the condensation occurs the most.  This process of condensation is typically an invisible problem that occurs inside walls and roofs, where the humidity most often hits dew point. 

            Condensation (which occurs as frost) can get trapped inside a wall and literally soak the insulation and framing.    To combat this problem there are a few different approaches.  The approach usually taken due to cost (and misunderstanding) is to use a polyethylene vapor barrier on the walls and a kraft paper vapor retarder on the underside of the roof.  The thinking behind this is that the permeability rating of the poly is less than the kraft, so moisture will not enter the walls but it will enter the roof.  This is seen as acceptable because the walls are sealed cavities, and little ventilation passes through them.  The roof, on the other hand, has soffit vents and ridge vents that produce a continuous, passive convection current up and out.  So if any moisture enters it will be carried away, or so the thought.    This obviously has a few drawbacks.  1)  It is hard to achieve "equal leakage" as to not have extremely cold spots prone to condensation. 2)  Air infiltration losses take a lot of heat with the humidity.  3)  Diffusion driven air movement intensifies the problem and causes drafts.   An area of high humidity wants to diffuse into an area of lower humidity. 4)  The convection currents created by the soffit vent/ridge vent correlation only works when the ridge vent is open.  When it is needed the most, it is typically covered with snow, hence impeding the air flow.   

            The other approach is to "seal it tight and ventilate it right".  This is obviously a selling slogan.  The approach is to seal the entire structure inside with a continuous poly membrane with taped seams.  Then instead of exhaust fans, an "air-to-air" heat recovery ventilator is installed.  All normal exhaust fan locations will be connected to it, and each location will have a manual switch.  The stale, moist air will be sucked out and it will pass through a heat recovery core where it preheats the incoming cold air without mixing with it.  Thereby all heat just isn't wasted because some is recovered.  The unit, besides being controlled by manual switches, also automatically reads the humidity and turns on accordingly.  And besides that, it will run 20 minutes every hour to insure fresh air.  This too has drawbacks and benefits: 1)  eliminates rot and condensation, a plus. 2)  Installation cost-maybe four thousand for a single family dwelling, a drawback. 3)   Keeps a colder roof, less apt to form ice dams, a major plus.  4)  Does away with the "sick house syndrome" with continual fresh air, another major plus.  5)  Some appliances, such as a clothes dryer and some fireplaces, need free air to operate well because they do not yet use outside air-a drawback.   

            A heat exchanger really only has to be used when you are staying there,  therefore in a seasonal residence it would be shut off when you leave.  If you are not planning on heating the structure during the winter you won’t benefit from it’s features.  But if you ever plan to, it provides many benefits. For more information on sealing and ventilation you can visit Shelter Supply

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