Sealing Air Channels in Your Attic

Anytime there is a significant temperature difference between the inside and outside of your home, good air-sealing and insulation will pay dividends in energy savings, air quality and avoiding damage to your home.  If you don’t understand what an attic bypass is, this video gives a number of great examples.  The host discourages homeowners from doing their own air sealing, but the fact is that not all states require attics to be air sealed before adding insulation to the attic (unlike Minnesota).  Therefore, you may be hard-pressed to find a contractor in your area who is knowledgeable about doing this.  However, if you are physically agile and willing to put in some time and effort, it’s achievable!  

The best times to get up there and tackle air sealing in your attic is during a swing season when you’re not using your heater or air conditioner (it’s a comfortable temperature in the attic).  Also, do it BEFORE adding any more insulation (you don’t want to be digging through your new insulation to find these gaps!)

Don’t forget to dress appropriately!  You’ll need:

  • Old clothes that can be laundered or thrown away, and a hat
  • Vinyl gloves to keep the sprayfoam off your hands (it’s sticky and stains!)
  • A headlamp if there’s not adequate lighting everywhere 
  • A respirator to prevent inhalation of fiberglass and dust (it’s good to keep extra cartridges on hand)
  • Safety glasses (because spray foam gets everywhere, trust me!) 
  • Knee pads (because low attics require a lot of crawling!)
  • Seal your cellphone in a ziploc plastic bag if you bring it with you (because spray foam gets everywhere, trust me!)

Here are some typical areas you will want to address:

Sealing Duct Shafts

According to Building America, a government program for testing and education about energy-efficient homebuilding, the proper way to seal duct shafts involves cutting a piece of plywood, rigid foam, or drywall to fit around the ducts, applying a bead of sealant like caulk to the supporting surface, fitting and securing the board with screws or nails, and applying more sealant around the inner and outer edges.  This is a great idea, except that caulk doesn’t usually cut it.  With the advent of spray foam in a can, it’s easier to get a good seal with spray foam than with caulk because the foam continues to expand for some time and will fill any voids.  

To seal gaps in and around ductwork, this video is great.  It shows several different types of repairs.  If you’re a novice, it might seem like you could just use “duct tape” on ducts, right?  Wrong!  Regular duct tape does not work well long-term on HVAC ducts.  It does not form an air-tight seal and over time, dries out and disintegrates.  That’s why you’ll want to use the following sealants instead:

  • Using Air Duct Sealant (paste, also called mastic) and fiberglass tape or fiberglass cloth to patch small holes and larger gaps in ductwork (for a small kit, check here)
  • Using spray foam to seal around ducts where they penetrate walls or unconditioned space like the attic or crawlspace.  There are different formulations in spray foam:
  • Use foil tape to seal around loose ends of insulation.  If there are any leaks in the ductwork, this will minimize air leakage in/out of the duct.  
  • Check out our article on sealing your registers: you’ll want to do this to prevent condensation on the grilles and registers in your rooms!

I had an idea that my attic was leaky; I just didn’t know the extent of it until renovating for a new HVAC system.  I found a shaft used to hold ductwork that was open to the attic, but plunged down one floor and shared a wall with my bedroom.  This air space channeled hot/cold dusty air from my unconditioned attic right down the wall and it was NOT insulated.  

Because the shaft was rectangular and housed 2 round ducts in it, I decided to stuff some loose fiberglass around the ducts at the top and spray foam over them; the fiberglass only acted to plug large gaps and keep the foam from falling down the shaft before it hardened.  The only thing to remember is that the foam must contact solid surfaces, like foil-covered insulation, wood, or foam board, in order to make a continuous seal.  If I had left loose fiberglass sticking up out of the shaft, the air could pass right through it. 

Insulating Electrical Boxes and Can Lights

Here is something I’ve been wondering about for quite a while.  I have a number of electrical boxes for ceiling fans, lights and even the bathroom vent which are not sealed.  What kind of sealant should I use?  According to this short video (with happy music) by the EPA, spray foam comes to the rescue again!  As long as your fixture box doesn’t generate heat, you are fine with spray foam.  This brings us to can lights.  Foam and insulation must not come into contact with older style can lights (which use incandescent, halogen or another heat-generating bulb) because of fire risk.  In this case, you’ll want to purchase a can light cover with fire rating specs in order to safely insulate the area around the light.  It can be sealed with spray foam around the bottom perimeter of the cover to the sheetrock.  Again, you do not want to let the spray foam contact any part of the can light, which can get very hot, so be sure to hold it down to the sheetrock when spraying, and minimize any holes necessary to accomodate electrical wires going to the light.  

It’s also recommended to plug holes drilled for electrical wires through the top plates of your walls (typically these are wood 2x4’s or 2x6’s).  If your local code requires that these penetrations be sealed with a fire retardant sprayfoam, then Great Stuff Fireblock with smart dispenser ($10) will work. 

If you want to keep your heating and air conditioning system working well, prevent mold and moisture damage, lower humidity in your home as well as lower energy costs, tackling air sealing in your attic is a must!

Photo by Will Francis on Unsplash