For a good understanding of R-value you should refer to the Oak Ridge National Lab website.
Laredo, Texas, follows the 2009 International Residential Codes (IRC) that mandate the following R-values:
- R-30 for the roof;
- R-13 for the walls; and
- R-19 for the floors.
R-value measures the thermal resistance of a material or group of materials and is the reciprocal of heat transfer through the materials.
- The greater the R-value, the greater the resistance.
- R-value not only takes into account the material, but its surface and associated air space.
Since the R-value also relates to all components of the construction, not just the insulation, it can be manipulated through the use of other materials, such as the finishes.This can be best understood in terms of flooring, where carpet and a fibrous pad have an R-value of 2.08, whereas a carpet and a rubber pad have an R-value of 1.23.
Knowledge of the thermal properties of materials will help in the design of more creative solutions for the thermal property of your walls. Consult ColoradoEnergy.org for a list of the R-values of a range of building materials.
Heat gain through the roof
As Laredo is in Zone 2 of the Energy Star map and has a Hot Arid climate, more than twice the heat gain of the east and west walls occurs through the roof.
- With that in mind, the most important area to insulate would be the roof.
- As a green architect in a similar climate, I have firsthand experience with the benefits of overinsulating the roof, and I recommend the same to you.
Given the required R-value of 30, we often use extruded polystyrene where we have limited space, which gives the required R-30 in 6 inches. To achieve a higher R-value, high density spray foam (closed-cell) will give you a total R-value of 42. The Energy Star website suggests roof values for your area up to an R-60.
Insulating the attic
Attic spaces allow you to either put the insulation at the plane of the ceiling or the roof.
- If you put the insulation at the ceiling joists, then you would be wise to insulate the ducts, whereas putting insulation in at the roof rafters would be effectively increasing the area that you are conditioning. That would be a decision based on your attic space.
- Spray-in foam works well at the roof rafters and lay-in batt insulation at the ceiling joists.
Home foam insulation has the advantage of sealing cracks and so preventing air infiltration (25% of heat loss occurs from air infiltration). As to the best insulation for the walls, a combination of materials might be more cost effective: first using 1/2 " spray foam and then adding batt insulation would seal the walls and give you an R-13 wall.
Other techniques to reduce solar gain
Other methods of reducing solar gain would be with landscaping and deep overhangs that would shade the exterior walls.
Deciduous trees in the summer would offer additional shade to the roof. As the roof is critical, you might consider installing a cool roof, which is light-colored and will reflect the heat rays of the sun.
Insulation materials, by R-value and relative cost
Below is a list of insulation materials, their R-values and relative cost.
|Type (calculated for 4" walls u.o.n.)||R-Value p/in||Cost (2004)|
|Cotton Batts (ie. Recycled denim)||3.7||$.70- .75 p/sq.ft|
|Loose-fill Fiberglass||4||$.21 p/sq.ft|
|Extruded Polystyrene||5||$.50 p/sq.ft x 2"|
|Fiberglass Batts||3-4||$.38 p/sq.ft|
|High Density Polyurethene Spray Foam||7||4 x's cost of Fiberglass batt|
|Mineral Wool||4||$.19 p/sq.ft|
|Low-density Polyurethane Spray Foam||4||4 x's cost of Fiberglass batt|
|Foil-faced Polyisocyanurate||7-8||$.40 p/sq.ft. for 3/4" sheet|
For more information:
Read Mick Dalrymple's Q&A "What type of roof should I choose in a hot climate?"