Evaluating the Best Insulation for Your Attic Retrofit
Originally published by: Washington Post — January 17, 2017
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There are many building components in a home that are hidden behind drywall or just out of sight.
As an architect in the Washington area, I’ve worked with numerous homeowners to help them understand what’s hidden behind their walls and ceilings and evaluate the right home improvement options. With colder temperatures upon us, it’s important to be prepared for the winter and the additional energy costs you’ll pay to stay warm.
Sure, you could just dial down the thermostat and wear yet another sweater to cut costs, but investing in the right home improvements can go a lot further to cut expenses and to improve your day-to-day comfort.
One of the places in your home where a little bit of attention can make a big difference in your comfort — and heating bill — is the attic.
According to the Department of Energy, properly insulating the attic can save 10 to 50 percent on a typical heating bill. In the Washington area, there are many styles of homes and roofs, but typically most have some degree of attic space, either walk-in or crawl space. For many homeowners, insulating the attic is the most effective method to save on energy costs.
Here’s how to get started:
Evaluate the current condition
Before adding insulation, evaluate the condition of the attic, which may require the assistance of an architect or builder. Wet or damp insulation and moldy or rotted rafters may be indicative of moisture problems, potentially stemming from a roof leak, which should be addressed before installing insulation.
Additionally, having a structural engineer evaluate the size of the rafters is also important. Many older homes in the D.C. area have rafters that may be undersized compared with current code requirements. If the rafters are split or appear to be sagging, they may need to be strengthened before proceeding further with insulation.
This is because without attic insulation, heat typically rises in the house and warms the roof. After a snowstorm, this heat aids in melting the snow more quickly, thus minimizing the snow load on the roof. But a fully insulated attic prevents the roof from warming, and the snow can continue to accumulate as it melts at a much slower rate. If the existing rafters are undersized, there is more risk of roof failure with the increased snow load.
During the area’s major snow storms, there have been the occasional roof collapses, resulting in a lot of expense and inconvenience for property owners. Understanding the conditions of your existing roof structure and attic space are crucial before starting any related improvement project.
Seal the space properly
After the evaluation, make sure your attic is air sealed — meaning eliminating any holes, cracks or other gaps that can allow air transfer between the inside and the outside. This will help maintain the desired temperature.
Common methods for air sealing include filling holes with expanding foam and caulk. Also keep in mind that even with air sealing, attics typically require some degree of ventilation. A professional can assist with evaluating your roof ventilation system and determining how it needs to be maintained.
Choose the right insulation solution
After confirming that your attic is air sealed, you need to select insulation. There are many types to choose from, including blown-in cellulose, batt (such as fiberglass, mineral wool or cotton) and spray foam.
When selecting insulation, there are three main factors to consider: desired R-value, available space and cost of insulation. The R-value is the rating for the insulating material’s thermal resistance, usually per inch of material. Thus, the higher the R-value, the better the insulation. In the metro area, the required R-value for attic insulation varies between each jurisdiction. Using Washington as an example, the energy code requires a minimum R-value of 49 for attic insulation, but more can be added if desired.
Different types of insulation take up different amounts of space to insulate effectively. Typical fiberglass batt insulation has an R-value between 2.9 and 3.8 per inch of thickness. So to achieve an R-49, you will need between 13 and 17 inches thick of batt insulation. Depending on the conditions of your attic, that may be difficult to achieve.
In contrast, spray-foam insulation has an R-value of 3.7 per inch for open cell and 6.2 per inch for closed cell. The closed cell type can easily achieve an R-49 at only 8 inches thick, which is a common rafter height for many older homes.
However, spray foam insulation is more expensive to install than batt insulation. A good median option between the two — both in cost and R-value — is blown-in cellulose. A cellulose installation over the 2014-2015 winter for a 385-square-foot attic in a two-story, 850-square-foot rowhouse cost about $900 to achieve R-49. In this example, the homeowner was also installing a skylight simultaneously, so the cellulose was blown-in through the new skylight opening. Additionally, blown-in insulation is a great solution for attics that are not accessible.
Installing attic insulation this winter is a worthwhile long-term investment because it can significantly decrease your heating bills. As an added benefit, when the hot D.C. temperatures come back, the attic insulation will also minimize your air conditioning bills.
Consider reaching out to an architect, structural engineer or builder to evaluate the conditions of your attic and to provide recommendations for selecting the appropriate insulation that meets your budget.