Monday, March 28th, 2022 by Erin Dougherty
It’s a great question and one we get all the time. Shouldn’t insulation keep heat out during the summer and in during winter? Well, sort of. Insulation is only half of the solution to keeping your home comfortable and reducing your energy bills when you’re insulating your attic.
When homes are built dozens of holes are created between the conditioned living space and the unconditioned attic. These holes occur in the form of plumbing, electrical, HVAC penetrations. Bath fans, chimney chases, top plates, and recessed can lights, to name a few.
In the summer, the attic gets extremely hot. The sun heats the roof and that heat turns the attic into an oven, exceeding outdoor temperatures by 15 degrees or more, upwards of 130 degrees Fahrenheit making the attic blazing hot. This heats the ceiling shared with your living space, and the heat radiates down into the rooms below. If you have ductwork in your attic that’s trying to push cool air to your living space, the ductwork and the stagnant air in it is just as hot as the attic, warming up the air you just paid to cool down and making it more difficult to cool down your living space. Now, your HVAC is working harder and running more and you’re still not comfortable.
In the winter, the warm air in your home that you paid to heat is rising through your living space and escaping through all the holes between your conditioned living space and the unconditioned attic. Heat also moves from more to less, with the attic being as cold as the outside and the ceiling being a similar temperature, it’s sucking up that heat. The cold air then falls, and you start to notice drafts in your home known as convective looping. If you have ductwork in your attic, it and the air in it is cold, and it’s cooling down the air you paid to heat up, making it more difficult to warm up the rooms in your home. Your HVAC is working harder and running more often to try and warm up your home.
The Stack Effect is the driving force behind what’s happening between your conditioned living space and your unconditioned attic. As air rises it creates a positive pressure at the top of your home which pushes conditioned air from the living space into the unconditioned attic space. This creates a negative pressure at the bottom of the home, which sucks in outside unconditioned air.
As new unconditioned air is brought in, it pulls in dust and allergens. (About 50% of the air you breathe in your home comes from under your home, the basement, or crawl space). This air needs to be conditioned, so your HVAC works to heat or cool this air, then redistribute it throughout your home, only to be lost again.
Every home is different and can experience air loss at varying rates, this is the amount of air exchanged in a home in an hour. The more times the air in your home is replaced, the more your HVAC is running, the higher your energy bills. You are also likely experiencing drafts, cold floors, rooms that are too hot or too cold, and in some cases, high humidity and even musty odors throughout the home.
Air sealing all these holes between the living space and the unconditioned attic greatly reduces air loss. Meaning, the air you paid to heat and cool is staying in your home longer and your HVAC will run less. Insulating and air sealing the attic access is key to helping reduce air loss and heat transfer.
Insulating the attic ceiling with at least an R-38 will slow heat transfer between the living space and attic, keeping you more comfortable. (In our area, Energy Star recommends insulating with an R-38 to R-60 in an uninsulated attic, or adding an R-38 to an existing 3-4 inches of insulation).
You will notice that rooms that were once too hot or too cold are now comfortable and in colder months, you’ll notice fewer drafts throughout the home. Your energy bills will begin to decrease and over time (usually 6-10 years depending on the home) you will gain the cost of your project in energy savings.