How to Insulate Slopped Roof Ceiling Edges From the Attic


Learn how to fix a house with a gently sloping roof and little to no insulation at the eaves. Ceilings anywhere except in the most mild climates should have between 10 and 15 inches of insulation material.

Things You'll Need

  • Hacksaw
  • 1" polyisocyanurate rigid foam insulation
  • 2-1/4" Tyveck house tape
  • 1/4" faced pressboard
  • String
  • Measuring tape
  • Old leaf rake, metal tines preferred
  • Examine attic edge construction and insulation. Observe all necessary precautions anytime you are working in the attic (see Warnings section and other on-line recommendations on how to work safely in the attic). Look for roof slope (a typical gentle slope would be 5 degrees); ceiling / roof intersection details (is there 2 feet or more of space at the edges?); existing insulation type, depth and condition (if any); and weather or not there are soffit vents (holes at the edge of the eave that allow air from outside into the attic for circulation). If there is anything less than R-20 already installed then read on - this eHow is for you. Note that you may seem to have a high degree of insulation but it might be blocking soffit vents in which case you should still seriously consider making this change for the health and welfare of your housing frame.

  • Make an "attic rake". Use an old leaf rake to reach the edge of the attic and pull back any insulation that may already be there. Clip off either side to ensure it fits easily within any joist bay. If it's loose fill insulation (as opposed to batts) then modify a rake that has metal tines so that the end of each tine is even with the next (usually a leaf rake is arced). Trim and file excess. Duct tape over any metal edges otherwise the rake will easily tear duct wrap or snag on various things in the attic.

  • Make an "loading slide". When you encounter obstructions (like bath fans or air ducts) within 4 or 5 feet of the attic edge then you will need a mechanism to help slide the insulation stacks over the obstructions and into place. Even if you don't have any obstructions the slide will make installation easier than crawling down on all fours out at the edge of your attic. Use a piece of ¼ inch thick pressboard making the length 5 feet and the width safely less than the width of any joist bay - again, it needs to fit easily between the joists. Poke two holes into each corner of one of the short sides. Slip a four foot piece of string through either hole and knot it off well. Be sure to position both knots on the same side of the board - this will be the "down" side.

  • Calculate dimensions of the new insulation stack. Width: if joists are spaced 24 inch On Center (as measured between the center of one joist to the center of the next joist) then use 22 ½ inches for the width otherwise measure the distance between joists and subtract ½ inch for the width. If you have soffit vents then the max insulation stack height is the distance between the drywall ceiling and the bottom of the soffit vents (refer to image). If no soffit vents then the max height is the distance between the drywall ceiling and the bottom of the roof plywood. The height of our stacks is 3 inches. Use a depth of 1 foot.

  • Calculate dimensions of required attic vent baffles. If you have soffit vents and loose fill insulation then you'll need to attach an attic baffle to the back of the insulation stack. They should be 7 to 9 inches high and the width computed in the previous step. Attic baffles protect loose fill insulation from the effects of wind entering the soffit vents and clearing it away from the ceiling edges (wind wash) but still allow air into the attic - mandatory to keep the building frame healthy. You'll also need to hinge the baffle to make it install correctly. Attic baffles are also available commercially and you may, in fact, already have some installed but integrating it into the back of the stack makes a more effective solution.

  • Purchase required insulation, pressboard and tape. Brands such as Dow TUFF-R polyisocyanurate (iso) fiberboard insulation comes in 4 x 8 feet sheets at big box stores. It can range from ½ inch up to 3 inches or more in thickness. 1 inch is probably the most versatile but if your "max insulation stack height" (Step 4) is 2 or 3 inches then buying the sheets in this thickness saves on cutting time. Using your measurements calculate how many sheets you need to make the required insulation stacks. Also pick up a roll of Tyveck tape (2 ¼ inch width preferred but is expensive - 1 inch will work fine too) and additional ¼" pressboard to use as attic baffles.

  • Clear area where new insulation will be installed. Again, be mindful of loose wires or other hazards - if you cant actually see what you are trying to move then don't reach in there. Using the rake pull back existing insulation from the soffit about 1-1/2 feet. Don't move it or redistribute it - you will need to push it back in place when you are done with this joist bay.

  • Prepare the insulation stack "slabs" and optional attic baffle. Using a hacksaw cut off some rigid foam insulation and size it to match the desired dimensions of your stack (Step 4). This part kicks up a lot of fine particles so use a dust mask and cut it outside where ventilation is good but the particles can be collected before they blow all over everything. Professionals use a straight razor to heavily score one or both sides of the sheet and then snap it off. This is a preferred technique that leaves far less particles behind but is very difficult to master and get straight cuts with. Also, if you need an attic baffle (Step 5), then cut this to size now and score the length of the pressboard on one side only, 2/3's from the bottom. Directly behind the score line, on the other side, tape a strip of Tyvek from end-to-end. Snap the pressboard along the score line being careful not to break it in half - this is the hinge and it should fold easily to the tape side.

  • Assemble and tape the stack with optional attic baffle. You may have purchased rigid foam insulation of the precise thickness required. If not, you should have separate pieces that you have just have to stack and tape together. You will need to tape and cover all exposed edges in any case - iso foam degrades after prolonged exposure to the sun and direct elements. Use Tyvek tape, it's a construction grade tape for sealing house wrap to exterior home sheathing so it is meant to last a long time. Try to use one continuous piece of tape to wrap around the edges to prevent fraying and if the insulation is foil faced try to keep the foil up (pointed towards the sky). Also, if you need an attic baffle (Step 5), then tape this securely to the back of the stack, hinge up and flapped back.

  • Slide new insulation stack into joist bay. First consider removing existing attic baffles if you already have them and have integrated new ones into your insulation stack. Then set the loading slide between the joists with one end touching the soffit and the other end (with strings) nearest you, knot side down. Place the newly made insulation stack onto the slide, flap down with the baffle nearest you. Make sure that the width is squarely between the joists. Let go of the stack and gently coax it into place by wiggling the slide. Do not force it - if it becomes stuck or isn't orienting itself correctly as it slides down then use the rake to fish it back and start over. It will usually work on the first try. Once you are sure that it has slid all the way down to where you want it (between the joists and up against the soffit) then you can remove the ramp but be sure - it is not easy to backup after this step. Finally, turn the rake around and use the long handle to nudge the stack firmly into place.

  • Flap up. This part can be a bit awkward. Again with the rake turned around, use the long handle to get under the flap and lift it all the way up. It wont want to stay there so you need to hold it in place with one hand and with the other hand push some insulation into place behind the flap. This holds the flap in the open position completing the baffle effect. It also makes the thermal transition from attic stack to attic insulation as seamless as possible. For example, if your attic stack is 3 inches and your baffle is 7 inches then it would be a transition of roughly R-18 to R-25. That's as good as you can get without redesigning the house! Repeat steps 7 through 11 for each individual joist bay adjusting measurements as required until the job is completed. Although it's tempting to make all of the insulation stacks in advance it is best to do them and install them one at a time until you get the design of your particular stacks perfected.

Tips & Warnings

  • For an extremely tight fit, drill several holes in each joist bay from the outside before installing the insulation stack. Once the stack is installed spray in some expanding foam insulation through the holes (makes there is one at each edge of the stack at least) and then block and paint the holes.
  • If you are already replacing your roof consider removing some of the sheathing, if it isn't being removed or replaced anyway, and accessing the edge of the joist bays this way. It makes the whole operation easier and avoids the need to crawl around in your attic.
  • Look on line for information regarding types of insulation and house construction.
  • Observe all necessary precautions for working in an attic including the use of respirator, long pants and long-sleeved shirt, gloves, eye protection and head protection against exposed nails. If in doubt search on line for guidance on how to safely work in your attic.
  • Be alert for asbestos insulation and asbestos wrapped air-ducts (especially if worn and damaged) in any house but especially in older homes (pre-1980).
  • Be alert for other materials that are potentially harmful to breath or touch such as rodent poisons, traps, droppings or remains.
  • Be careful not to fall through the ceiling between joists - although the area may look thick with insulation the only structural element typically there is 3/8" to ½" drywall which is essentially chalk.
  • Polyisocyanurate (iso) fiberboard insulation is not code approved to be exposed in to a living area. It needs to be faced with gypsum board (drywall), plywood or other sheathing to be approved. This how-to is in line with those recommendations.

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