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Building Envelope

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The building envelope is the boundary zone between the conditioned air in the home (cooled or heated) and the outside elements. One can think of the building envelope as the skin of the home, much akin to the skin on the human body.  It completely covers the body from head to toe and serves as the interface of the body to the outside.  Like our body, the home's skin (building envelope) needs to encompass the entire home and that includes wall systems, roof, and floors.

 One often sees house wrap hanging from the sides of the house during construction, but it is almost never taped or sealed properly.  Floors, ceilings and attics are commonly not addressed at all as being part of the envelope and thus no attention is paid to the ceiling/roof system as well as the floors/foundation.    Loose walls, windows, doors, floors, and ceilings allow for uncontrolled ventilation and an impossible situation when it comes to managing temperature and humidity within the home.


In order for a home to really do it’s job when it comes to managing the heat losses and gains, it is of utmost importance that you deal with the entire envelope so as to control and manage water, air, and humidity infiltraton into and out of a home. One often hears that you shouldn’t make a house too tight as it needs to breath. Just as you need to breath, you do it through a specific orfice in your body, your nose. A house, too, needs to breath, but it needs to be through an orfice that you design into the home. This way, you can control when, and how much breathing needs to take place. We call it “ventilation”.   You are actually better off to have a home that is “tight” with no insulation, that one that “leaks” and has insulation.

Most older homes have ventilated attics as do a considerable number of new homes.  But current building practices have switched to sealing and insulating the underside of the roof and thus making the attic part of the conditioned space of the house.  The illustrations shown here show the two types of conditioned space concepts:

 

 

For our habitat homes, we insulate the roof and thus have a conditioned attic.  The major reason for this is to have the HVAC system 100% within the insulated envelope of the home.  This way any conditioned air (heating or cooling) is all inside the insulated home and none of its energy is lost into a hot attic thru the duct insulation or any potential air leaks.  Other reasons is that you now have a clean attic which makes it much easier to run additional wiring (TV, internet, etc) in the attic after the home has been completed.  And, if you store items such as holiday decorations in the attic, they don't melt and deteriorate as they would in a hot attic.

 




MCHFH, takes special care to manage the air and moisture infiltration thru the building envelope:

  • We build on a slab, and provide a plastic barrier under the slab, and wall beams to mimimize moisture infiltration up through the slab.

  • The exterior walls are 100% covered with house wrap (no voids, tears, etc are permitted). It is sealed at the bottom plate, taped around all windows and doors, and sealed all the way to the roof deck.  Our roof deck stops at the exterior walls, so it makes it easy to bring housewrap all the way to the roof deck where it can be sealed (see the section "Floating Roof" for more informaiton and illustrations).

  • 100% seal the roof deck. We insulate the underside of the roof deck with spray foam insulation. I have to admit we have used open cell spray foam and made the “incorrect” assumption that we have an air barrier. It slows air flow, but you need at least a layer of closed cell foam first to seal the surface, the fill the rafter cavities with open cell foam to properly seal it.  We now seal the roof deck against air and water infiltration then use spray foam in the open attic area and Blown-in-Batts in cathedral ceiling areas.


I tell our volunteers to seal the envelope up so the house would float. And they do!
We do make penetrations in the envelope as needed for dryer vents, HVAC tubing, bath and kitchen vents, electrical wiring, but we carefully seal around each opening to prevent any air and/or moisture penetration. We also provide an opening for our fresh air ventilation. This is the “controlled” point for makeup air for the home (see the "Ventilation" section for details of ventilation options and a description of how we do it).