The R-value of loose fill cellulose is R-3.2 to 3.8 per inch2. Loose fill fiberglass has an R-value of R-2.2 to 2.7 per inch3. Achieving the desired R-value depends on both the depth of the insulation and its density.
Fiberglass: Fiberglass loose-fill insulation has an approximate R-value of R-2.5 to R-3.5 per inch, depending on the density of the material. Cellulose: Cellulose loose-fill insulation typically has an approximate R-value of R-3.5 to R-3.8 per inch.
Pros of Loose-fill Fiberglass
This type of fiberglass insulation is better for attics with limited headroom, irregular joist spacing or many obstructions. It offers seamless coverage and can fill any ceiling cavity and the spaces around piping and ductwork.
Loose fill fiberglass has an R-value of R-2.2 to 2.7 per inch3. Achieving the desired R-value depends on both the depth of the insulation and its density. Depending on your climate, energy codes require varying R-values for ceiling insulation.
When fiberglass batts are compressed, both the thickness and the R-value go down. The deeper the cavity, the more the R-value is reduced. 14 inch R-49 insulation is reduced to R-44 when compressed to fit an 11 ⅞ inch space. Further compression of 5/8 of an inch (11 ¼ inches) reduces the R-value to R-42.
An R-38 fiberglass batt should be approximately 12 inches thick, while the thickness of cellulose insulation of a finished, fitted product will larger.
While both types provide insulation, R30 is the better option for maximizing energy savings in areas where heat loss or gain is a major concern. For example, R19 may suffice for interior walls, but R30 is more appropriate for exterior-facing attics or floors.
In summary, we do not recommend the use of vapor barriers with cellulose insulation, except in circumstances of exceptionally high moisture levels, such as an indoor pool facility, and we warrant our cellulose insulation for the life of the building when installed by an approved GreenFiber insulation contractor.
Air Leakage
Fiberglass insulation is less dense than other types of insulation and does not create an airtight seal, which means air can travel freely through the insulation. This is called air leakage and it's one of the most common sources of energy loss in a house.
It's important to note that some insulation materials, such as spray foam, do not require an air gap. However, if you're using traditional insulation materials such as fibreglass or mineral wool, it's important to follow the manufacturer's guidelines for air gap size and placement.
Rigid foam board insulation is available in many materials, including polystyrene (R-3.8 to R-5.0), polyurethane (R-5.5 to R-6.5) and polyisocyanurate (R-5.6 to R-8.0). Since it has the highest R-value, rigid foam is typically reserved for foundations and basement walls.
Mineral Wool Insulation
In many ways, mineral wool is a superior insulation product. It has a higher R-value per inch compared to fiberglass, about a 22-37% higher R-value.
Key Differences: Thermal Resistance: R-21 polyiso boards offer higher thermal resistance compared to R-19, which makes them more effective at reducing heat transfer in colder climates. Thickness: Both R-19 and R-21 polyiso boards are compact by offering high R-values per inch.
In general, either batt or rolled insulation or blown loose-fill insulation (fiberglass or cellulose) can be installed on top of old insulation.
The dip from R10 to R4 seems odd, as one would expect more R as you travel further north through the zones. Thicker is not better! Doubling the amount of insulation will double the R-Value, but so what? Doubling the insulation from R-19 to R-38 does NOT cut the heat loss in half (50%) it cuts it by around 3%!
Given a benchmark thickness of 3.5 inches, a batt of Rockwool achieves an R-15 insulation value while fiberglass achieves an R-13 value, giving Rockwool about 15% better insulating ability over fiberglass at the same thickness.
Both cellulose and fiberglass insulation have their merits and drawbacks. However, for those prioritizing eco-friendliness, superior thermal performance, and excellent sound insulation, blown-in cellulose insulation is a compelling choice.
The rafters in cathedral ceilings are usually made of 2x12s or 2x10s, and Owens Corning makes two special high-density insulation products specifically for these rafters -- R-38C for 2x12s and R-30C for 2x10s.
Blown-in fiberglass has an R-value of 2.2 to 2.7 per inch.
Fiberglass batt insulation does have some drawbacks. They can be itchy and irritating to handle, and inhaling the fibers can cause respiratory problems. It's important to wear protective clothing and a mask when handling and installing fiberglass batts.
Fiberglass and rock wool batts—2x4 walls can hold R-13 or R-15 batts; 2x6 walls can have R-19 or R-21 products.