Tag Archives: Thermal Envelope

Elements, Items and Practices involving the Thermal Envelope of a Building.

Tight Houses cause Water and HVAC design concerns.

Two real life, on the ground,  issues have me concerned both as a builder and purported Hi-Performance, Energy Efficient design consultant.

1.  In pre-planning a Structurally Insulated Panel(SIP) condominium quadruplex there are  practical heating and air conditioning problems to be solved.  Do you use time tested conventional approaches or install evolving HVAC equipment?

This building’s thermal envelope(exterior shell) is so energy efficient and tight that the HVAC load requirements for each unit(950 SF) are below that of the smallest sized, readily available, cost effective, conventionally ducted HVAC equipment.  This means the conventional unit is oversized, a no-no in High Performance housing.

A readily available, 16 SEER, 1.5 ton Heat Pump is almost double the size necessary to heat and cool these units.  The problem isn’t about being able to heat and cool the unit. It is how do you remove the interior moisture(humidity) consistently?  An oversized unit runs in short spurts and every time it stops, dehumidification stops. The solution is either utilizing the new mini-split, inverter technology units or adding a dehumidification system to the conventional unit.  The decision has to be based on performance and cost.  Stay tuned.

2.  A group of affordable, energy efficient houses were built with conditioned crawl spaces and a super tight building envelope. The houses have developed condensation/moisture issues on the interior of the homes.  The homes were built with no design input from experienced Green Builders or real world Hi-Performance home consultants.  They were not Energy Star Certified, so there were no third party Hers Raters involved.  This type of problem is caused by the theory of, “Make the house tight, and forget about evaluating other issues that have to be solved in designing a tight, Green Home.”   Stay tuned for solutions.

The theory to reality application is going to present issues now and into the future, as more and more builders, designers, etc. jump on this believed economic windfall Green job wagon. Many so called experts are Tin Men, just selling stuff, not economic, practical solutions.

Conditioned Crawl Spaces Misnomer!

A Conditioned Crawl Space is one that has one or more heating/ac supply ducts in it.  The underlying idea is to keep the crawl space at a slightly positive pressure.  This means that the crawl space, which by nature of its position at the bottom of a building or house, in its natural state, is usually under a negative pressure.  What does that mean?  It means in common terms– it is always sucking in moisture and air.  Crawl spaces have been verified to have sucked in moisture from 40′ away.  A crawl space under a negative pressure, naturally sucks in outside air and moisture, and this is what we want to prevent, the sucking, along with keeping the temperature of the crawl space somewhat moderate in temperature and relative humidity.  Now to do this we put in a one or more HVAC supply vents, to keep the positive pressure and prevent the sucking in of contaminants, etc.

Conditioned Crawl Pre-Floor Joists
Conditioned Crawl-Foundation and Rim

Well now some group of highly intelligent people of a nether world have made it code to now include a return duct in this crawl space.  So now we are back to the negative pressure crawl space.  We put air in, we take it out and we suck more in from outside.  This means radon, other contaminants, moisture, is now brought into our duct system purposely.

This is a prime example of Green Stupid.  Wait until someone proves that the return, because a leak occurred in a gas furnace, put in a conditioned crawl space, caused death by spreading carbon monoxide gas or bringing excessive radon into a home, there will be hell to pay.  Dumb, dumb, and dumber.