Ceiling Insulation Zone 5, 6, and 7
There are two types of roofs in residential construction – cold and hot. A cold roof ventilates while a hot roof does not.
Roof ventilation is a system that helps the design of a roof; it allows air to naturally enter a house over the top of the thermal insulation. To ensure that a roof will achieve proper ventilation, we rely on a combination of gable vents, soffit vents, ridge vents, baffle vents, roof vents and a build-up of wood furring strips and roof vents. Heel cuts need to stay 1.5″ below the roof deck to allow room for baffle vents if required. Heel cuts need to be a minimum of 17″ for fiberglass systems and 8″ for foam systems. Most cut framed roofs will only partially ventilate leaving a space for warm air to create condensation on the roof deck. If a roof design does not allow for proper ventilation we recommend a hot roof.
A hot roof is a roof system intentionally designed not to ventilate. This is achieved by installing high R values of insulation either in the framing cavity or a combination of continuous roof insulation on the roof deck and cavity insulation. There is no special design criteria for a hot roof except framing material choice.
The more space that is allowed for insulation within the joist cavity greatly increases the options you have for insulation. As codes become more stringent, it will become increasingly more expensive to achieve code required R values with 2×12 rafters or 11 7/8″ I joists. We strongly encourage framing with 14″ I joists to allow plenty of room for any design considerations you may have.
The basic understanding of vapor barriers as it relates to insulation, is fiberglass and cellulose insulation do not provide enough R value to keep the dew point (the temperature at which humid air condenses) out of the insulation. To minimize the amount of moisture allowed to pass through the wall, vapor barriers are often specified. The lower the vapor permeance transmission rate, the greater amount of vapor it will retard.
For vapor barriers to perform to the best of their ability they should be taped and sealed per the manufacturers recommendations. If there are no installation instructions for the specified vapor barrier, the following should be considered: overlap seams 24″, tape seal penetrations, tape seal seams, tape seal edges to plates, tape seal outside corners, and wrap inside corners with enough allowance to accommodate drywall.
Vapor barrier should be a minimum 6 mil polyethylene. Other choices include, but are not limited to, Certainteed Membrain, Raven Industries Griffolyn, and Compacs Foil-Scrim-Kraft (FSK) paper.
Ceiling insulation product choices are fiberglass batts, fiberglass blow, fiberglass blown in blanket, polyurethane foam, and fiberglass/foam hybrid. The systems listed below represent what we would recommend by framing.
R49 Ceiling insulation-Zone 7, 2006 and 2009 IECC code requirement
- System #1 Loose fill fiberglass blowing wool
- System #2 Fiberglass batt insulation
- System #3 Fiberglass Blown-in-Blanket insulation
- System #4 R30 Closed cell rigid polyurethane Spray foam with R19 unfaced fiberglass batt insulation
- System #5 R49 Rigid polyurethane Spray Foam
|Flat truss||yes||yes||no||yes||yes||Sys#1 and #2 only||Sys#1 and #2 only|
|Scissor truss||yes||yes||no||yes||yes||Sys#1 and #2 only||Sys#1 and #2 only|
|11 7/8 TJI||no||no||yes||yes||yes||optional||Sys#2 only|
|14″ BCI||no||yes*||yes||yes||yes||Sys#2 only||Sys#2 and #3 only|
|16″ Parallel Chord Truss||no||yes||yes||yes||yes||Sys#2 and #3 only||Sys#2 and #3 only|
*Knauf Ecobatt 13 ¾” R49 High Density Batt
|Insulation Cost per square foot*||Add for ventilation per square foot*||Add for 6 mil Vapor barrier per square foot*||Total Cost per square foot*|
|System #1||$0.93||perimeter vents only add $.11||$.20||$1.24|
|System #2||$1.49||perimeter vents only add $.11||$.20||$1.80|
|System #3||$2.88-$3.43||Continuous vents only add $.28||$.20||$3.36-3.91|
|System #4||$5.19||Not applicable||Not applicable||$5.19|
|System #5||$7.20||Not applicable||Not applicable||$7.20|
*list pricing as of 2/15/12, actual pricing may vary
Always consult with your architect or design professional when selecting roof framing and insulation systems.
Summary of roof framing costs versus insulation system costs:
|Framing cost per lineal foot*||Least expensive R49 (material and labor)*||Framing material cost per square foot 24″ on center*||Framing material cost per square foot 16″ on center*||Total framing material and installed insulation 24″ on center*||Total framing material and installed insulation 16″ on center*|
|11 7/8″ I Joist||$1.56||$3.36 (2)||$1.15||$1.52||$4.51||$4.88|
|14″ I Joist||$1.84||$1.80 (3)||$1.36||$1.80||$3.16||$3.60|
*as of 2/15/2012
(1) R30 Closed cell rigid polyurethane Spray foam with R19 unfaced fiberglass batt insulation
(2) R51 Blown-in-Blanket fiberglass insulation and vapor barrier
(3) R49 Knauf Ecobatt 13 ¾” and vapor barrier