How To Insulate a Vaulted Ceiling

Insulating a vaulted ceiling requires a more thoughtful approach than insulating attics. Types of insulation materials, gravity, and ventilation need to be considered.

Vaulted and Cathedral Ceilings

Most people use the terms vaulted and cathedral interchangeably to describe ceilings that are not flat. They are not the same architecturally.

Vaulted Ceiling insulation

Vaulted Ceilings

Vaulted ceilings do not follow the roofline. Modern construction makes use of scissor trusses to achieve the desired effect. The highest part of the ceiling is at least 10 feet from the floor. Lower slope scissor-trussed vaults can be insulated on the attic floor with fiberglass blanket insulation or blown-in loose-fill insulation. Loose-fill does not work in steeper slope vaulted ceilings.

It is difficult to properly insulate the underside of a scissor truss-built roof because the web design of the truss makes gap-free attachment almost impossible. Trusses are usually manufactured of 2 x 4 material leaving shallow cavities to insulate.

Cathedral Ceilings

Cathedral ceilings begin from the top of the wall and follow the roofline to a triangular peak. The peak is 13 feet or more from the floor. Insulation is installed between the roof rafters or attached to the underside or both. They can be ventilated or unventilated–affecting how they are insulated.

Insulating a Rafter Sloped Roof

Sloped roofs are insulated from the inside using the rafters and roof deck to attach the products.

Types of Insulation for Vaulted Ceilings

Here are the products used most often to insulate vaulted ceilings. Each has pros and cons. Most are DIY-friendly. Some are best left to professionals.

Regardless of the type and R-value of the insulation you install, consider installing a ceiling fan to circulate the air in living areas. Warm air accumulates near the ceiling. Moving it around keeps the room more comfortable and reduces heating and cooling costs.

Blanket Insulation

Use fiberglass batts or rolls, mineral wool insulation, or denim insulation for the vaulted ceiling. Fiberglass is the most popular. It is available in batts or rolls–faced or unfaced, readily available, and the least expensive option. Faced rolls make the most sense because they are easily stapled onto the bottoms of the rafters to keep them in place and provide a vapor barrier. Rolls reduce the amount of cutting and fitting required.

Mineral wool and denim batts are unfaced and rely on friction to keep them in place when installed in wall cavities where gravity is not a problem. Install 1 x 2 strapping across the rafters to keep the insulation from falling out. (Unfaced fiberglass also requires strapping.)

Rigid Foam Board Insulation

Apply rigid foam board insulation in one of two ways. It can be installed between the rafters directly onto the undersides of the roof deck or across the bottoms of the rafters. Use acoustic caulking or small cans of spray foam insulation to seal the foam to the rafters and fill all cracks and gaps.

The three most popular options are expanded polystyrene (EPS) insulation, extruded polystyrene (XPS) insulation, and polyisocyanurate (ISO) insulation. XPS and ISO provide a vapor barrier when two inches thick or more and all gaps and cracks are sealed.

Cellulose Insulation

Cellulose insulation is applied wet or dry. Wet applications should be left to professionals with the proper equipment. (Not all insulation companies offer wet spray options.) Sprayed directly onto the underside of the roof deck, it sticks in place once it dries. Many flat-roofed commercial buildings are insulated this way and left uncovered.

Dry cellulose installations require a net to be attached to the undersides of the rafters first. The net holds the cellulose in place during and after it is blown in dry. You can rent the blowing machine, buy the material, and do it yourself, or hire an insulation contractor.

Cellulose insulation–both wet and dry–fills all voids when applied correctly. Dry cellulose can also be installed with the drywall already on the ceiling by using a dense-pack application. Small holes are cut into the drywall to allow hose access–then patched. Contractor installation is usually recommended.

Spray Foam Insulation

Spray foam insulation provides the best R-value to your vaulted ceiling. It seals all cracks and gaps and fills voids around and behind wires, pipes, and electrical fixtures. It is also the most expensive insulation product.

Spray your vaulted ceiling with closed-cell foam–not open-cell foam. Open cell absorbs water, has a lower R-value per inch, and does not qualify as a vapor barrier.

Most people hire contractors to foam vaulted ceilings. But DIY kits are available from home improvement outlets or online. The kits are ideal for smaller areas–under 1000 square feet–or houses located in remote areas. When insulating larger areas, contractor costs are usually less than buying multiple kits.

Reflective Insulation

Reflective or radiant barrier insulation has no R-value. Staple it across the bottom of the rafters to reflect heat away from the building. It can reflect up to 97% of the sun’s heat. Reflective insulation is only effective in hot climates. It provides little value in cold climates and can be counter-productive in winter–preventing any solar gain from adding warmth to the house.

Ventilation in Vaulted Ceilings

Spray foam insulation and rigid foam board insulation installed against the roof deck–sealed properly–do not require ventilation. They make the ceiling cavity between rafters airtight–preventing moisture build-up on the underside of the roof deck. Rigid foam installed on the bottom of the rafters should allow ventilation.

Reflective insulation can have ventilation between it and the roof deck because it does not absorb moisture. The foil works best if it is kept clean. Airflow brings dust with it which will adhere to the foil face of the barrier–reducing its effectiveness. If possible, make the rafter cavities ventilation-free.

Fiberglass and cellulose absorb moisture and do not dry out easily. The wetter they get, the less insulation value they have. Soaked fiberglass has no insulation value. In colder climates, a vapor barrier should be installed on the warm side to prevent moisture absorption from inside the house. Most building codes in the northern states and Canada require at least a 6 mil poly vapor barrier.

Vented soffit and a ridge vent are designed to pull cooler air in at the bottom and expel warm air through the ridge. Install fiberglass insulation to leave a 1.5” – 2” gap between the top of the batt and the bottom of the roof deck to allow for airflow. If there is a chance that the insulation will plug the soffit vents, install baffles in the lower 4’ of the cavities. Purpose-made foam baffles are inexpensive and available from your insulation supplier.

Some asphalt and fiberglass shingle manufacturers will void their product warranties if there is no ventilation. Make sure you know the status of your roofing before deciding on how to insulate your ceiling.

Vaulted Ceiling Insulation Safety

Never install any insulation tight to heat sources. Pot lights, in particular, can generate a lot of heat. Wear the appropriate clothing, respirator, and eye protection. Some products like cellulose are fairly benign–but dusty. Others like spray foam can be toxic. Follow all manufacturer’s safety precautions.

Exterior Roof Insulation

One of the best ways to insulate a vaulted ceiling is from the outside. This only makes sense during new construction or if the shingles are being replaced because of the cost. Applying rigid foam board insulation directly onto the roof deck eliminates ventilation concerns.

Extruded polystyrene or polyisocyanurate insulation boards are the best choices. They provide a high R-value per inch and do not absorb water. Apply them directly onto the roof deck, install sleepers, cover with a minimum 7/16’ thick OSB sheathing, and install the roofing material of your choice.

How Much Insulation to Use

Ninety percent of US homes do not have enough insulation. The map and tables below show the recommended amount of attic insulation areas in the United States. The R-values for attics should be used for vaulted ceilings.

Alaska climate zones:

7 – Aleutians East
7 – Aleutians West
7 – Anchorage
7 – Bethel
7 – Bristol Bay
8 – Denali
7 – Dillingham
8 – Fairbanks North Star
6 – Haines
6 – Juneau
7 – Kenai Peninsula
5 – Ketchikan Gateway
6 – Kodiak Island
7 – Lake and Peninsula
7 – Matanuska-Susitna
8 – Nome
8 – North Slope
8 – Northwest Arctic
5 – Prince of Wales-Outer Ketchikan
5 – Sitka
6 – Skagway-Hoonah-Angoon
8 – Southeast Fairbanks
7 – Valdez-Cordova
8 – Wade Hampton
6 – Wrangell-Petersburg
7 – Yakutat
8 – Yukon-Koyukuk

Zone 1 includes Hawaii, Guam, Puerto Rico, and the Virgin Islands.

CLIMATE ZONE	UNINSULATED ATTIC	3-4 INCHES OF EXISTING ATTIC INSULATION	UNINSULATED FLOOR	UNINSULATED WOOD-FRAME WALL
1	R30–R49	R19–R38	R13	R13 or R0 + R10 CI*
2	R49–R60	R38–R49	R13	R13 or R0 + R10 CI
3	R49–R60	R38–R49	R19	R20 or R13 + R5 CI or R0 + R15 CI
4 EXCEPT MARINE	R60	R49	R19	R20 + R5 CI or R13 + R10 CI or R0 + R15 CI
4 MARINE AND 5	R60	R49	R30	R20 + R5 CI or R13 + R10 CI or R0 + R15 CI
6	R60	R49	R30	R20 + R5 CI or R13 + R10 CI or R0 + R20 CI
7 AND 8	R60	R49	R38	R20 + R5 CI or R13 + R10 CI or R0 + R20 CI