Why Don't They Build Houses Like They  Used To?

Notes from the 2005 NAHB Green Building Conference

They don’t build them like they used to because energy codes, materials, construction methods, and design have changed.

Building requires careful management of the way water, vapor, air, and heat move on or through homes. The rules of physics govern the movement of these elements.

Ignoring the physics of water, vapor, air, and heat as they relate to home construction often leads to damage:  material defects, wood rot, mold, uneven heating and cooling, and poor indoor air quality.

“Builders who follow these rules can create homes that are safe, healthy, durable, comfortable, and economical to operate.”

An understanding of these rules on the part of designers, builders, inspectors and homeowners is critical in preventing defects in new construction.

The National Association of Homebuilders 2005 National Green Building Conference,  “Greening the American Dream” was held at Atlanta’s Westin Peachtree Hotel March 13-15.

The following ten principles from Houses That Work (U.S. Dept of Energy NREL/SR-550-37664 Introduction to Building Systems Performance:  Houses that Work II. Revised February 2005) address performance issues related to moisture.

  1. Our efforts to save energy and reduce the flow of heat through building assemblies have reduced drying potentials and, therefore, increased the importance of controlling moisture flow through building assemblies.
  2. Ideally, building assemblies should be designed to dry to both the interior and exterior.  In heating climates, the primary drying potential is to the exterior (but not necessarily exclusively so): in cooling climates, the primary drying potential is to the interior (but not necessarily exclusively so); and in climates with both heating and cooling (Atlanta), some drying potential in both directions is typically a good idea (but not necessarily exclusively so).
  3. Building materials last longer when their faces are exposed to similar or equal temperature and humidity.  This is why the ventilation of claddings (siding), particularly those that store moisture (reservoir claddings-brick, stone, stucco, fiber-cement) can be important.
  4.   air barriers, and thermal barriers must be continuous to be truly effective.  Being able to trace each of these on a full elevation drawing without lifting your finger from the elevation is a good test of continuity.
  5. In moisture control, the priority is liquid water first, particularly when it comes in the forms of rain and groundwater.  In these forms it is referred to as “bulk” water.  Following are air-transported vapor and then diffusive vapor, all other things being equal.  It’s a question of quantities and rates, of wetting and drying, and the tolerance of materials (individually and in combination) for each and all of the above.
  6. Three things destroy materials in general and wood in particular:  water, heat and ultra-violet radiation.  Of these three, water is the most important by an order of magnitude.
  7. When the rate of wetting exceeds the rate of drying, accumulation occurs.
  8. When the quantity of accumulated moisture exceeds the storage capacity of the material or assembly, problems occur.
  9. The storage capacity of a material or assembly depends on time, temperature, and the material itself.
  10. The drying potential of an assembly decreases with the level of insulation and increases with the rate of air flow (except in the case of air flow in severe cold climates during cold periods where interior moisture levels are high).

A lot of time was spent discussing exterior wall assemblies. Here’s why.

Before the code was changed energy was allowed to flow through exterior assemblies: this allowed a drying out of the wall, especially in winter.

As the temperature drops relative humidity rises.

The threshold for mold growth is 70% RH.

Energy code required sealed and insulated wall assemblies use less energy to heat the home. The drying effect of furnace heat no longer provides a dry wall assembly.

In the book “Architecture of Country Homes” (1832) Andrew Jackson Downing reiterated common knowledge that a good fire dried exterior walls.

With less energy to dry out walls and a higher differential of heat and moisture between the exterior and interior condensation will occur at unsealed openings.

New methods were outlined to isolate the exterior cladding from the insulated interior portions of the wall.

Other sources of wall moisture are due to improper flashing at openings, particularly windows.

Creating a drainage plane, a vertical path for moisture to move behind the cladding, is critical.  Housewrap and felt are used to shield wall assemblies from exterior moisture.  This single layer of protection has proved in many cases to be insufficient. Furring strips or some other secondary protection is considered critical, especially where masonry products are used as cladding. Code requirements for a secondary moisture barrier are anticipated.

Brick, stone, and fiber-cement claddings are reservoirs:  they store moisture.  As they dry there is a potential for large amounts of moisture to enter the wall assembly.
Some brickmasons don’t “clean” the backside of the brick as before:  the drainage plane between the brick and exterior sheathing may become fouled with spilled mortar.  Some masons don’t butter head joints:  thinner mortar allows easier passage of water through the brick.

Design issues discussed include the orientation of the home, roof drainage and the lack of adequate roof extensions.

Moisture enters the building more readily if there is no shelter provided by the roof extending beyond exterior walls.  

West-facing windows can increase the cooling load up to 40%.

Multiple roof configurations and vertical wall intersections combined with poor gutter design (upper roof downspouts dumped onto vertical sidewalls, etc.) increase the potential for moisture.

Roughly  $13,000 of a new home price is related to insurance costs and class action suits:  22 billion dollars a year in the U.S.

Building a “green” house adds roughly 2-4% to the cost of construction.

Homebuyers associate energy efficient homes with quality construction.
A common misconception about energy efficient homes is that they are “too tight” and will aggravate poor air quality.  Providing adequate ventilation and make-up air is the solution. New products allow controlled intake of fresh air and pressure balancing of the house.

Home maintenance is important.
If 8% of the surface of an A/C coil is fouled with dirt there is a 21% drop in cooling efficiency.

Filter ratings are for coarse particles only, 99% of all particle pollution are fine particles.  Fiberglass filters are designed to protect the equipment:  the use of “people filters”, pleated, electrostatic or electronic, was recommended.

Insulation, especially in vaulted ceilings, is often improperly installed.

The face of the insulation must contact the drywall to be effective.

Misaligned insulation leads to loss of energy through convection.
Drywall installers do not like this method because it may distort the surface of the finished wall.

“Cold spots” caused by misaligned insulation can be detected using an infared camera.
Infared cameras can also detect the movement of cool air as it leaks through holes in the ceiling and then flows out of soffit vents.  Discolored paint at the soffit may indicate a poorly sealed home.

Conditioned crawl spaces
Data shows that ventilated crawl spaces contribute to moisture problems in the home. The rationale for crawl space ventilation is moisture removal. Assuming a reasonably dry crawl space (yes, they do exist), open vents allow unconditioned exterior air within the underfloor space. Hot, humid air in the summer condenses on pipes and ducting promoting formation of mold.  In the winter condensation occurs at the framing perimeter and on subfloor.

Sealing the vents, installing a thick vapor barrier on the ground, insulating exterior walls, and providing 1-2cfm/ft3 supply airflow to the space has proven effective in conditioning the space and controlling moisture and temperature.

Bonus Rooms
Insulating and sealing knee walls is key to providing comfort in the spaces above garages.  Insulation is effective when it has six sides of rigid enclosure: Top plate, bottom plate, drywall to the finished space, studs at either side and rigid product at the back.  Trussed knee walls often omit top and bottom plates. The back of the wall is rarely sealed with rigid board. Unblocked floor joists over the unconditioned garage ceiling permit convective energy loss.

Flexible ducting has replaced sheet metal (hardpipe) as the primary duct material in residential construction. Because of its’ ribbed design there is a significant reduction of air flow due to friction loss. CFM:  cubic foot of air flow per minute.

Airflow            6” flexduct:  50-75 cfm            6” hardpipe:  66-110 cfm

Design and installation is key.  Ducting should be installed according to the manufacturers printed instructions.  Excess lengths should be removed.  Ducting should not be bent or otherwise restricted.

These websites provide information about efficient construction and related topics

www.buildingamerica.gov  U.S. Dept of Energy building technologies programs

www.buildingscience.com  Building Science Corp:  industry leader in building science and construction

www.ashrae.org  search the latest ventilation standard:   62.2  2004

www.eflbuilder.com  building science, performance standards for builders

www.eflhome.com  the same for homeowners

www.nahbrc.org    National Association of Homebuilders research site

www.betterhomesandgardens.com  good all-around site, practical advice

www.cphb.org  Atlanta area members of the certified builder program

www.crawlspaces.org  Advanced Energy Corp. studies of crawl spaces

www.freefrommold.com  American Plywood Association site about how to keep homes mold free

www.jacksonemc.com  Jackson EMC Right Choice Homes program is worth a look.

Notes from the 2005 National Association of Homebuilders Green Building Conference
Articles About Home Inspections
Dan Curl - Comprehensive Home Inspections Atlanta

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