Installing rigid foam insulation on inside walls or ceilings.

Old House Remodeling:

Insulating A Sloping Ceiling With
Rigid Foam Insulation

In This Article:

Rigid foam insulation is fastened to an angled ceiling and held in place with 1x3 furring strips.

Related Articles:
Skill Level: 2-3 (Basic to Intermediate) Time Taken: About 8 Hours

By , Editor

Start:

In the second floor of my own 1-story house, I decided to knock out the knee walls and rebuild them farther back. Even though the new space had low headroom, it gave us more area for storage and possibly a large home office.

The upstairs room after relocating the knee walls.

Before installing new drywall, I wanted to improve the energy efficiency of the sloped ceiling. The original insulation was 3" thick R-11 fiberglass. The insulation I added between the 2x6 rafters (above the new section) was R-13 fiberglass. These are not good R-values for a ceiling in a house in Northern Michigan.

Half-story room during remodeling, showing framing and insulation.

 

Chimney chase framing is obstacle when hanging foam insulation on sloped ceiling. Installing foam insulation on my ceiling was simple. There was only one obstacle... this chimney chase.

In my situation, the simplest and cheapest way to increase the R-value was to apply a layer of rigid foam insulation over (below, actually) the rafters before installing new drywall. I chose polyisocyanate foam insulation, which has the greatest R-value per inch of commonly available insulation materials. I used 3/4" thick foam because it was readily available at my local Home Depot. This foam is available in greater thicknesses... I've seen 6-inch sheets of polyisocyanate foam used on the roofs of commercial buildings.

I installed the sheets of foam so the long dimension was running perpendicular to the rafters, just  like drywall is normally hung.

On this sheet I had to cut the length short by a couple of inches, because the right-hand edge did not land on a rafter. (I had installed a 2x4 nailer before installing the fiberglass insulation.)

I also had to cut a large notch to go around the chimney chase.

First sheet of foam installed around small bump-out in framing.

 

Cutting Rigid Foam:

Cutting rigid foam insulation is simple... just use a long sharp knife and a goof straight-edge. I prefer to use one of those knives with the snap-off blades, because they can be extended very long (if the blade segments haven't been broken off). A wide-bladed knife is better than a skinny blade.

I mark the cut line with a permanent marker and a long ruler. A drywall T-square (which is 4 feet long) is the best tool for laying out cuts that are perpendicular to the panel edges. It's common for the ruler to slip while cutting foam, so marking the cut first tells me when the ruler has slipped.

I placed a scrap of plywood beneath the cut so I wouldn't cut into the existing hardwood floor. When cutting on a plywood subfloor, this cutting board prevents the knife from hitting nails or screws in the subfloor, which will dull the tip. Keeping the knife tip sharp is very important, otherwise chunks get torn from the foam.

 

Rigid foam insulation being hung from sloping ceiling, using small blocks of wood to fasten. The next piece I installed was a full sheet.

To hold the foam in place, I screwed a couple of short 1x3 blocks to the rafter, using 2" deck screws. My goal was to get about one inch of screw penetration into the framing.

These blocks of 1x3 were carefully placed along the centerline of the sheet (i.e. 24" from the upper edge). I did this to avoid interference with the 1x3 furring strips that will be installed later.

After the first row of foam was installed, I hung the upper row.

These sheets needed to be cut to a narrower width. I found that two 8-inch long blocks of 1x3 were adequate for temporarily holding the foam up, though some of the edges were not tight to the rafters.

Then I applied housewrap tape to the seams between the panels. This is important to prevent air leaks.

Rigid foam insulation fastened to ceiling with small blocks, joints taped.

 

The "Normal" Method Of Fastening Foam Insulation:

These plastic cap ring-shank nails are the fastener most commonly used for installing foam insulation.

The plastic cap prevents the nail head from pulling through the foam. Since these are ring-shank nails, they hold very well and are quite difficult to remove.

Note that the container says "Round Plastic Cap Roofing Nails". Cap nails were originally used to fasten roofing felt (tar paper) during shingle installation, though I can't imagine why anybody would need 2-inch nails for that job... especially ring-shank nails.

Plastic cap nails typically used for fastening foam insulation.

 

Installing 1x3 furring strips to sloped ceiling to hold foam in place. I fastened 1x3 furring strips over the foam, using 2" deck screws.

I made sure the boards "broke" over a rafter, so the ends could be fastened securely. Pre-drilling the holes at the ends is necessary to prevent splitting the wood.

 

Installation Tips:

To make installation easier, I held the boards against the ceiling and marked the rafter locations on the board. Then I pre-drilled the holes in the middle of the boards, (to make sure the screw heads didn't stick out) and started a deck screw in each hole.

I just held the board in place and drove in one screw to hold the board up. Then I made sure the board was positioned properly (i.e. along the 16" on-center lines I had marked earlier) and I drove in the remaining screws.

Driving screws into furring strips.

Using a few simple tricks, I was able to install these boards without a helper.

Pre-Drilling?: If I didn't pre-drill the holes, I discovered that the foam would get crushed before the screw head became sunk into the wood.

 

Furring Strip Materials:

Instead of buying 1x3's for furring strips, I bought some good-quality 1x6's from Home Depot and ripped them in half on my table saw, so the final width was just under 2-3/4".

I could have bought 1x2 or 1x3 furring strips, but those boards are often very low quality, and they are often much thinner than normal 1x material. I have seen some big variations in thickness in furring strips at Home Depot and their ilk, so I prefer to spend a little more money on the Swedish-made 1x boards that Home Depot has carried for several years now. That wood is excellent... exactly 3/4" thick and exactly the industry standard widths (3", 5", 7", etc.). The knots are almost always very small, so when the boards are ripped narrower the wood doesn't fall apart because some big knot dominates the board. I rue the day when Home Depot discontinues those Swedish boards.

 

Rows of 1x3 furring strips installed over foam insulation. It took me about an hour to install 7 rows of furring strips, spaced at 16 inches on center.

The room is just under 17 feet long, so I had to use 10-foot boards and 8-foot boards to span the entire length.

 

After installing the horizontal furring strips, I installed some small pieces of 1x3 between the horizontal boards. These extra blocks were placed so they would provide backing for the edges of the drywall panels. I did this because it's easy to bump the ceiling, and pushing on a drywall seam might make it crack.

These extra boards are not necessary, but they also should help slow the spread of flames in the event of a fire.

Additional blocking installed between furring strips.

 

Horizontal Or Vertical Furring?

One reason I chose to install foam and horizontal furring strips was to allow me to hang the ceiling drywall vertically instead of horizontally. This way all of the ceiling drywall will be just less than 8 feet long. If installed horizontally, I would need some 8-foot sheets of drywall and some others that would be almost 9 feet long. It was difficult enough to carry 8-foot panels up the stairs and around the corner... it might not be possible to get longer sheets up the stairs and over the pony wall around the stair opening.

It's not necessary to install the furring strips horizontally like I did... it all depends on the desired orientation of the wallboard. I could've just installed furring strips directly over the rafters if I wanted to hang the drywall horizontally.

 

More Info:

Tools Used:
  • Basic Carpentry Tools
  • Cordless Drill/Driver With Countersink Drill Bit
  • Cordless Impact Driver
  • Drywall T-Square
  • 4-Foot Level or Straight-edge
Materials Used:
  • Polyisocyanate Rigid Foam Insulation
  • 1x6's, Ripped In Half
  • Short Pieces Of 1x3
  • Deck Screws, 2" Long
  • Housewrap Seam Tape
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Written June 9, 2009