Insulating Sloped Ceilings

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Many homes have sloped ceiling areas—places where plaster or drywall is applied directly to the underside of the roof rafters. Examples include Cape-style houses, which usually have sloped ceilings between the kneewall and the flat ceiling above or rooms with partial or full cathedral ceilings. Some homes just have a narrow sloped area for a few feet near the eaves. .

These enclosed cavities present more of a challenge than an open attic. Some, like the typical Cape, have fairly easy access from an attic space either above or below. Others, like a full cathedral ceiling, are more difficult. The basic techniques shown in this section for dense-packing cellulose are referred to in many other parts of the book. The biggest distinction between dense-packed and open-blown cellulose is that dense-pack is installed in an enclosed cavity. But it’s not enough just to be enclosed—to dense-pack, by definition, means using enough air pressure to compress the material more densely than it could ever compress from settling.


Stuff pieces of fiberglass batts into rafter bays so that cellulose won’t blow through when you fill them from the other end. Stuff the batts into the empty cellulose bags, as shown here, or just stuff the fiberglass alone.

Prepping for the job

When filling in a rafter bay with cellulose, you must prevent the insulation from pouring out of the cavity on the other end. If the rafter bay is accessible from only one end—for example, from the attic above—this may be easy enough, but you will need to plan for extra material because the eaves may become filled. If you can reach both the top and the bottom, choose the end that is more awkward to reach and stuff a piece of fiberglass into each bay; then blow in the insulation from the other end.

Make sure the ceiling is sound before starting. In particular, acoustical tiles pose a real potential for blowout, as well as air leakage. They should be dealt with before dense-packing. And make sure the roof is in decent shape before insulating closed cavities; any water that leaks in after insulating will make a bigger mess—and be more difficult to locate—than it would otherwise.

Another preparation issue, which is the same for flat attic areas, relates to venting a cathedral—ceiling space. Good air-sealing (which dense-packing helps) and indoor humidity control via mechanical ventilation best address most of the concerns around condensation and moisture buildup.

Roof-shingle warranties are another concern. Unvented cathedral ceilings experience hotter roof temperatures—typically by 2% to 4%—than vented roofs. Because those higher temperatures may accelerate degradation (venting is assumed to reduce roof temperatures), many roofing manufacturers won’t honor warranty claims on unvented, or hot, roofs. But research has shown that shingle color actually has a much larger impact (about 10%) on roof temperature than venting does, and several manufacturers do pro vide warranty service for unvented roofs. Choose one of those, or ventilate if shingle-warranty service is important to you.

I’ve insulated many roofs (including my own), and I know a number of contractors who collectively have done hundreds of hot roofs with cellulose—very few have reported problems. But the decision is yours, and it depends on the climate, building inspector, shingle warranty, and condition of the roof, as well as your confidence in the success of air-sealing and indoor humidity control.

Tip: Low coatings on glass do cut heat loss substantially, but Low-e paint is a different matter. Save your money and invest in insulation instead.

Inside Info: Don’t waste your money on “energy saving” Low-e paint. The claims of potential savings at a level comparable to insulating a wall or roof are purely theoretical. It would be nice if an inexpensive, noninvasive product could make such a big impact, but here’s the truth:

Adding insulation is a proven strategy that really makes a difference. And if your house is already insulated, Low-e paint won’t make a significant impact.

The Details: From a building science perspective, venting in sloped ceilings is essentially a backup strategy; indoor humidity control and air-sealing are more important. In general, attic ventilation is a good thing because it helps the roof deck dry out, but you needn’t be fanatic about it. Enclosed cavities can be difficult or impossible to vent properly, and it’s more important to insulate them well.


On this machine, you increase the air-to-cellulose mix for dense-packing by closing a gate in the hopper, which reduces the material feed. Other machines have air-inlet ports that open to increase the air pressure; either type will increase the air-to-material ratio.


A 1¼-in.-dia. vinyl fill tube attached to the end of a larger-diameter hose makes it easy to snake into confined spaces for dense-packing cellulose. The marker line near the end warns you that the end is near as you pull the tube out
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Filling rafter bays — dense-packing cellulose

To dense-pack cellulose, you’ll first need to set up the blowing machine. Depending on the machine, this means setting it for either less material or more air. If you have a large, open ceiling cavity, use 3-in, flex hose. For cavities less than 5 in. deep, or for those that already contain some insulation, use a smaller vinyl fill tube. Starting at one end of the house, insert the hose into the first cavity. Try to insert it so that the end is within 1 ft. to 2 ft. of the far end of the cavity, and cover the end of the bay with a loose piece of fiberglass to keep the cellulose from blowing back out. It helps if you keep the fill tube or hose in one corner of the opening. Make sure the switch is handy, turn on the machine, and let the fun begin!

The tricky part of any dense-pack operation is making sure that you pack in as much material as possible, without clogging the hose or blowing out the drywall or plaster. As the material fills near the end of the tube, the flow through the hose will start to back up. When it does, pull back the tube 6 in. to 8 in. to relieve the pressure and allow more packing of the cavity. It may take a while before you have to back up the first time, but then you may have to pull back every 5 to 10 seconds until the cavity is full (depending on the size of the fill tube and the pressure of the machine). When the cavity is full, turn off the blower and let the pressure drop before pulling out the hose. You will know that you have reached the proper density when you remove the covering batt and press the insulation, The material should be firm, and it should be difficult to force your hand through the insulation.

If your sloped or cathedral ceilings aren’t accessible from an attic space, you have several options for dense-packing cellulose. You can get at the rafter bays by opening up the soffit and/or fascia, or by removing the cap layer of shingles and cut ting back the sheathing a few inches. Either method allows you to run a fill tube into the rafter cavity to fill it with insulation. Another option is to drill and patch cathedral ceilings from the inside, similar to filling walls from indoors. Of course, be extra careful whenever you work on a roof or on a ladder.


Dense-Packing Cellulose in a Sloped Ceiling Air pressure in the enclosed cavity pushes the loose fibers tightly against the sides, packing them into place. Once installed, the high-density material slows air movement through the cavity, gives the insulation a higher R-value, and prevents future settling.


When insulating a closed cavity, such as this rafter bay, hold a piece of fiberglass batt across the opening. Wear a dust mask and safety glasses in case the cellulose blows back at your face.

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Dense-Packing

It’s not difficult to notice the changes that happen just before your blowing machine starts to bog down. The sound changes, the material feed slows, and the added pressure may make the hose wiggle. If you wait too long to pull the tube back, the cellulose can back up and dense-pack the hose instead of the wall. On the other hand, pull the tube out too quickly, and the density will suffer.

Clogs are most likely to occur at a reducer or other restriction. Usually, opening up the hose at the restriction will do the trick.

If not, a long stick or a heavy-duty tape measure and a lot of thwacking on the side of the hose can loosen it. If you find that you are getting a lot of clogs, try reducing on the material feed or increasing the amount of air. If you are lucky enough to have a machine with separate switches for the agitator and the blower, you can maintain the air pressure and reduce clogging by turning off the agitator for short periods of time. That allows the air pressure to build without feeding any material, and it is a handy technique to use when you are almost finished with each cavity or bay.


Clogs occur where hoses reduce in size. To clear the blockage disassemble the joint
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Tip: Whether you drill from inside or outside to blow in cellulose, it’s important to make a hole (or a pair of holes) in each stud bay.

If you’re insulating a cathedral ceiling, stay away from non-IC-rated recessed light fixtures and chimneys. Blowing cellulose in these areas could create a fire hazard.


I make a probe out of a piece of insulated wire stripping the jacket off a piece of electric cable and pulling out one wire with the insulation still on it. I use it to explore stud bays after drilling them, probing sideways to find the next stud, as well as up and down to find any blocking or diagonal bracing that may be in the wall. Now I can aim for the center of the next stud bay to drill the next hole.


Before you try insulating blind roof cavities, make sure the rafters aren’t already full of insulation. If so, you’ll have trouble getting a fill tube—or much cellulose—into the space.

Next: Insulating Walls

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