As building codes around the country tighten in the push for greater energy efficiency, specifying insulation and vapor barriers for homes and buildings requires a greater level of conscientiousness than in years past. Thicker insulation with higher thermal resistivity is a necessity in passive and net zero home design. In buildings with metal exteriors, insulation is even more crucial due to metal being naturally conductive. In humid areas of the country that experience significant temperature swings, properly insulating metal buildings will stabilize the interior temperatures and prevent condensation from collecting in the wall cavity.
Image courtesy of CertainTeed
With thicker insulation, the ability to release moisture from the wall cavity becomes even more critical. Airtight structures are great for limiting air exchange (which is critical for limiting demand on air conditioning systems), but run a considerable risk of trapping moisture behind walls when there are spikes in humidity or rapid drops in temperature. That is where vapor barriers come into play. When moisture has no to go, not having the right type of barrier (i.e., one that responds to humidity) can trap moisture that damages important structures and renders insulation less effective.
Here are some tips to pay close attention to when specifying insulation for any structure.
Location and Assembly Design Go Hand-in-Hand
It’s important to understand how location and assembly construction affect the performance of insulation. The general rule is that more insulation is better, as it limits the ability of cold air to enter a building or heated air to escape. Air exchange, however, shouldn’t be your only consideration. The risk of condensation in the summer and winter, as well as how the drying potential of the wall cavity (i.e., how the wall cavity performance when moisture infiltration occurs) should also be taken into account.
For example, where and how insulation is located in an assembly can affect things like moisture flow. Heat, air and moisture work in concert with one another. If you tighten a wall assembly, it also won’t dry out as easily when it ultimately fails. Keeping the durability (i.e., the drying potential versus the wetting potential) of insulation materials in mind will minimize future failures and ensure buildings stay comfortable and energy efficient over time.
Understand How Components of Assembly Affect Temperature
Temperature profiles across an assembly vary in proportion to the R-values of the individual components. Understanding that concept is useful in predicting temperature changes that can make HVAC systems work harder than they have to.
In buildings with an insulated steel deck roof, suspended ceilings, and ducted HVAC returns, the specifier needs to consider both the cost of energy saved as well as the impact on the temperature of the air in the ceiling cavity. For example, if fiberglass batts were used to insulate the ceiling (effectively lowering the ceiling cavity temperature), the HVAC ducts would still sit outside of the building insulation envelope. Heat loss from supply ducts to the colder ceiling cavity above would reduce heating supply air temperature to the occupied areas, negatively impacting the operating costs of heating the rooms below.
There are several tools and resources that are publically available to do basic evaluations, and consultants and manufacturers can provide support as well to determine where insulation will be most effective and how much should be used.
Embrace Failure to Learn the Path Forward
Assume that products, materials and other elements will fail and design pathways for things like moisture that infiltrates an assembly to find a way out. This thinking should happen early in the design phase and not after the fact because even if you specify all the right materials, there will always be factors outside of your control that negatively impact the wall assembly. A poorly installed house wrap, exterior siding with inadequate drainage, improperly installed flashing around the windows, or improperly installed foam sheathing outside of walls are all ways moisture (often inevitably) finds a way in. Once you accept failure, you can better anticipate it and design with that in mind.
Specifiers should develop and design secondary layers and components to correct issues that may negatively impact insulation. Typically specifiers rely on products like kraft-faced batts, polyethylene vapor retarders, and a number of sealant products. These products are generally good solutions, but aren’t resilient in the face of moisture. Utilizing a smart vapor retarder that becomes more vapor-permeable as humidity increases will allow moisture trapped in the wall to escape, while serving as continuous, energy-efficient air barrier to prevent air infiltration and exfiltration.
Don’t Be Afraid to Seek Support
Major building product manufacturers can offer standard technical support, as well as consult on how to use their products through deeper analysis of projects and applications (e.g., hygrothermal analysis). They can also advise on where and when to install various types of insulation and vapor retarders. Sometimes, avoiding a reputation-damaging wall cavity failure down the road can be solved with a phone call to the manufacturer. Finding the best solution is often a matter of reaching out to the right person.
Ted Winslow is product manager, building science, systems and technical marketing at CertainTeed Corp., Malvern, Pa. For more information, visit www.certainteed.com.