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The Building Envelope: Winnipeg

The first article in this series identified the importance of a tight building envelope in creating green homes.  Structural Insulated Panels and Insulated Concrete forms create excellent air tight wall Assemblies because they do no breathe, have a high resistance to heat loss and create healthy indoor living environments in Winnipeg by controlling the flow of moisture and eliminating condensing surfaces on the interior wall surfaces.

However 99% of all homes are built with conventional 2x6 framing in Winnipeg.  This article looks at the design of conventional building envelopes, the advantages of sealing homes with spray foam insulation and the importance of effective air barrier and vapor barrier control layers.


The building envelope is made up of the roof, above grade walls, below grade walls, and the basement floor.  These envelope components should be thought of as systems for each is made of an assembly of individual products, working together.  In Winnipeg, code requires that these products include exterior cladding, control layers (moisture barrier, air barrier, thermal barrier) and the structural members (framing).  Each of these components plays an integral role in how our house performs - the failure of any of them can result in the failure of the wall assembly.

Are conventional homes built in Winnipeg satisfactory?  Modern homes are code required to be built with 2x6 structural framing, a minimum of R-20 insulation (thermal barrier) and a layer of 6 mil polyethylene sheet (vapor barrier & air barrier).  There are additional layers to the wall assembly including gypsum wall board, and paint on the interior and OSB, house wrap and cladding on the exterior.
The problem with the above wall assembly is this:  The entire system relies on the 6 mil polyethylene membrane to stop air movement, vapor movement and to permit the fiberglass batt to perform as designed!  In Winnipeg, the polyethylene layer goes on the warm side of the wall because we are in a climate which predominantly requires heating.  This means it is subject to dozens of penetrations at the time of installation (electrical outlets, light switches, window frames and doors, recessed lighting enclosures, seams - top, bottom and where the layer must be installed around framing members.)  This says nothing of the penetrations added with decorative wall hangings, nail holes and modifications done after the fact.  Unless these seams and penetrations are sealed in such a way as they never release, plenty of opportunity for air movement and with it moisture movement is created.  Worse, because fiberglass batt insulation allows air and moisture to move through it, penetrations and deficiencies in the 6 mil poly layer allow for air pressure differences across the wall assembly to degrade the performance of the insulating layer over time resulting in effective R-Values of half (or less) there advertised value.  This is a significant issue in Winnipeg - with its wide ranging climatic extremes.

If this same wall assembly were insulated and sealed with a  single application of spray foam insulation, the result in Winnipeg would be significantly different.  Because spray foam insulation is manufactured on site, it conforms to every minute variance, nook and cranny in the building envelope.  It is a perfect, custom made fit.  Made up of billions of tiny, microscopic bubbles, it will never settle, sag, or move, it absolutely and completely seals a wall without any penetrations.  It conforms to electrical boxes, outlets, and recessed light cans, wrapping itself around and behind them to completely enclose them.  It is inches thick and not subject to any of the potential problems of a layer only 6 mil thick.  Spray Foam insulation is not dependent on caulking, glue or sealants to act as an air barrier or vapor barrier and results in a wall assembly that is air tight, impenetrable to moisture, and has a stable R-Value much higher than fiberglass.  At an R value of 6 per inch, there is potentially R-33 available in a 2x6 cavity wall if super insulating is required.

Conclusion:

A robust building envelope is designed to endure and separate two different environments.  Modern, conventional building envelopes do not do this adequately because they are constructed using an air barrier and vapor barrier subject to degradation over time.  The system created to accomplish this is too heavily dependent on a single layer of thin plastic (6 mil polyethylene), and any penetrations or deficiencies in its installation, or subsequent use degrades wall performance allowing moisture into the wall assembly and heat loss resulting in a poor envelope.  In Winnipeg, spray foam.

1 comment:

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