This post originally appeared on the Green Building Advisor website.
Through the years, I’ve taken a lot of training and attended dozens of conferences about reducing energy consumption in existing homes. Everything from BPI’s Building Analyst certification to Huber’s Building Science Crossroads. Many of these training courses discuss the easiest and most cost-effective areas of concentration for energy reduction, the so-called low hanging fruit. In this shallow energy retrofit blog, we will be discussing the most common location to improve a home’s performance, the attic and/or roof.
My very cold climate is a great place to see underperforming roofs, simply drive through any older neighborhood during the heating season and you will see many homes that have ice dam issues. Providing a solution to these problem roofs will depend on the type of roof or attic. My intent with this post is not to go over how to improve insulation and air sealing for every type of roof, (Martin Holiday and other GBA contributors have detailed posts on the topic) but instead to go through the general process for diagnosing the issue, basic consideration on addressing the issues, and what the side effects of those improvements might be.
One of the first things I look for is heating and cooling equipment or ductwork located in an attic space. Air sealing and insulating the ductwork, if it’s found in the attic space, needs to be a high priority. Even better, the equipment and ductwork should be moved inside the building envelope. Sometimes this one change can make the biggest difference in how the attic and roof perform.
My home state of Minnesota, like many other jurisdictions around the country, amends the International Residential Code (IRC) to better fit the needs of the state. For better or worse, codes are added or deleted. One provision that has been added to the Minnesota code requires that the attic space of all existing structures be air sealed (when possible) before the addition of any insulation. This is one of the best additions to the Minnesota codes, and in my opinion, should be included in the national IRC code language. When improving the performance of an attic or roof, air sealing the space needs to happen first. The reason being is that often air leaks are a bigger problem than lower insulation levels. Remember the order of importance for the control layers, air control is more important than thermal control.
We determine where to concentrate air sealing efforts in attics and roofs by performing a blower door test. The problem areas I often find are interior wall to ceiling intersects, electrical and plumbing penetrations, ceiling light fixtures (especially recessed lights), attic hatches, and chimney chases. There’s a great example of how to diagnose and perform weatherization on a home on the Energy Conservatory’s YouTube channel.
After air sealing, next we can improve the insulation levels. How this is accomplished will depend on the type of roof. Does the roof have an accessible attic space where more insulation can simply be installed? Does the home feature a vaulted or cathedral ceiling? Vaulted and cathedral ceilings are very difficult to improve insulation levels from the interior. I’ve been involved in a couple projects where the roof sheathing was removed from the exterior and insulation work was completed from the topside. Diagnostics for these issues is usually just a simple visual inspection, though thermal imaging can also be helpful.
How insulation levels are improved will depend on the situation. The easiest is to simply add more insulation to an attic that is accessible. It may be a better choice to move the insulation from the attic floor to the roof deck, either above or below (a great option if there is ductwork in the attic). Maybe all the insulation is installed on the exterior of the roof. Additional insulation will need to be determined house by house, again, it depends.
You’ve gone through and improved the air sealing and insulation levels of the attic and/or roof. The home is now easier and less costly to heat and/or cool and is more comfortable. So, what could go wrong? What needs to be considered as a side effect of these improvements?
One of the issues I commonly see is an increase in interior humidity levels due to better air sealing. Depending on the climate, this may not be a factor. My advice in my climate is to monitor indoor humidity levels. (I recommend monitoring even to homes that are not planning energy improvement work.) Another risk is poor indoor air quality. Increased levels of carbon dioxide, volatile organic compounds, and fine particulate matter can accumulate inside a tight- home. Another risk may be an increase in radon levels. Both indoor air quality and humidity levels can be addressed with a sound ventilation strategy, preferably balanced mechanical ventilation. If improved ventilation isn’t included in the scope of work during a shallow energy retrofit, I am recommending at least monitoring indoor air quality. Personally, I use both the Airthings and Awair monitors in my home.
Another problem I’ve encountered is a shifting of the pressure plane. I’ve seen on a few occasions where the attic and upper level of a two-story home was air sealed during a weatherization project. Air leaking through the attic and roof was reduced, causing the air to leak out of a different location, between the first and second story. This problem presented as moisture and in some cases, icicles forming on the siding between the upper and lower-level floors. This can be a difficult side effect to predict. One contractor I’ve worked with in the past has included a disclaimer about unintended side effects of a shallow energy retrofit in his contract to help minimize his risk.
There are a lot of existing homes that are soundly built but were constructed in a time when energy was cheap, and we didn’t understand or didn’t include good building science principles during the original construction. Deep energy retrofits often are too expensive for many homeowners to consider. A shallow energy retrofit can be an option, just be sure to understand there can be unintended side effects with some being very difficult to predict, even for an experienced building science practitioner. That being said, I still have the opinion that we need to make our existing buildings more energy efficient, comfortable, and healthier to live in. Addressing these concerns smartly may even allow these structures to last many more decades.