I recently participated in a forum about how to achieve adequate attic ventilation/insulation to allow your attic to expel excessive heat buildup in the summer and I thought, with summer right around the corner, this would be a good time to share this information.
New energy codes require attic insulation to be min R-38 which is approx. 12” of batted or blown insulation. Most older homes built prior to 1980 have R-19, or 6” of insulation above the ceiling. In areas where temperatures reach above 90 degrees in the summer more insulation is definitely going to bring a return on the investment. Insulation is relatively inexpensive and fairly easy for most homeowners to install. If you apply another layer of R-19 over the existing layer I recommend running the batts perpendicular to the existing batts to ensure a better seal.
Most building codes require a ratio of 1/150 of ventilation or 1 square foot of ventilation for every 150 sq. feet of attic space.
In climates like here in Sacramento where I live it’s not at all uncommon for attic temps to reach 150 degrees in the summer.
Improperly vented attics in these conditions can bring on a host of problems including-
- Drastically shortening the lifespan of your roofing.
- Compromising the structural integrity of the roof framing system.
- Allow ice dam buildup in colder climates.
- Shorten the life of your HVAC unit if it is located in the attic.
- Add a significant increase to your energy bills.
- Insulation can trap moisture which will reduce the R-value of the insulation and create a nice environment for the propagation of certain molds, spores, and fungi which will also cause problems.
- In colder climates condensation forms on any metals present in the attic which can lead to rust and corrosion of HVAC and duct strapping.
There are several options for achieving adequate ventilation including gable vents, eve vents, dormer and turbine vents. There are also solar powered attic fans which are thermostatically controlled and do a great job of purging attic air. One of my favorites is a continuous ridge vent which is usually installed with the roofing and is placed at the highest point of the roof where the heat tends be the most prevalent. Applying a radiant barrier can lower your attic temps by up to 20 degrees in extreme heat days. There are several ways to do this including installing radiant barrier plywood roof sheathing before installing a new roof.
Finally, a relatively new application involves closing off all roof ventilation and applying the insulation to the rafters instead of the ceiling joists. This actually lets the conditioned air rise to the attic space thereby drastically lowering attic temps. One argument is that you are paying to condition attic space but the trade-off is that you can greatly increase the life span of your HVAC unit by not placing as much demand by operating it in such extreme temps.
This type of application usually requires some strategic planning to make sure your HVAC is properly ventilated and is able to get a good fresh air supply from out doors.
I highly recommend addressing any attic modifications early in the spring as attic temps are dangerously high in the summer months.