Unless you have a walk-in attic, your attic access is most likely a ceiling trap door with a set of fold-down stairs. Even if your attic is insulated (which it should be), the trap door is unlikely to be. That is an issue.
Fiberglass Insulation
Covering the backside of the trap door with fibreglass insulation is the simplest way to prevent a thermal leak.
Steps:
- Cut the batts so that they have a slight overhang on the sides. (To cover the entire door, you may need two pieces.)
- Place the insulation against the door, the fibreglass side down, and staple it in place.
- Although it may be tempting, don’t apply more than one layer to the door—fiberglass works best when its fibres aren’t compressed.
Applying adhesive-backed weatherstripping to the perimeter of the door side that closes on the framed opening is also a good idea. This will assist in preventing conditioned air from leaking into the attic.
Rigid Insulation
Fiberglass will help, but a more long-term solution is to build a box out of rigid insulation that fits over the framed opening of the door (and the folded stairs, too, if you have them). Rigid insulation is more efficient than fibreglass insulation, and the box will be more airtight. Win-win.
You’ll want the R-value of the insulation to be as high as possible (the capacity of insulation to resist the conductive flow of heat). To make the box, you’ll only need one sheet.
Rigid insulation comes in three varieties:
They’re all available in 4×8-foot sheets; buy the 2-inch-thick stock if possible.
- EPS (expanded polystyrene) with an R-3.8 rating per inch of thickness
- XPS (extruded polystyrene) with an R-5 per inch rating
- Polyisocyanurate (Polyiso) has an R-6.5 per inch rating.
You’ll need the following items to make the box:
- Measuring tape
- Knife or saw to cut the insulation
- Packing or duct tape
- Caulk gun
- Construction adhesive
Steps:
- Go into the attic first. Pull the fold-down stairs up into the attic if you have them. Measure the framed opening of the door, as well as the height of the folded stairs above the framed opening if necessary.
- Transfer the measurements to the rigid insulation sheet now.
- Mark out the box’s sides that will correspond to the framed opening (two long, two short), and if there are stairs, make the pieces’ width a couple of inches wider than the height of the folded stairs, so that the stairs don’t hit the box when closed.
- Form a box by applying construction adhesive to the ends of the four pieces. Check to see if the box and the framed opening are the same size. Tape the corners together until the adhesive dries.
- Cut a rectangle the length and width of the box from the remaining insulation. Using adhesive and tape, adhere it to the box.
- The box is now ready to be installed in the attic once the adhesive has dried. Close the door and place it on top of the framed opening.
- You can also add a strip of adhesive-backed weatherstripping to the bottom edges of the box to seal any potential leaks if you prefer the belt-and-suspenders method.