Inside the Slide: R-Value Changes With Temperature
Here is a chart we have referenced in presentations about attic insulation performance. It illustrates a couple of things: the difference between two seemingly similar loose fill attic insulations, and how their R values vary depending on how cold the attic is, and the interior house temperature is a warm 70F. Also it is assumed that the attic ceiling plane is 100% airtight with no air leakage from the house to the attic.
- The perception: Attic insulations are the same and perform consistently over a variety of conditions.
- The reality: They aren’t and they don’t. Insulation efficacy varies depending on the temperature difference across the insulation layer, installed density, nodule size (clump size and configuration) and important elements such as: consistency of depth in the body of the attic and over top plates, attic ventilation rates and windwash baffles. So, it depends on a variety of factors.
- The data: The difference in the two loose-fill insulations is their density. Both are rated at R-38 during a standard 75 F mean test (Typically 100F on one side and 50F on the other). The insulation represented by the blue line (more dense and a different nodule size) performs better, but attic insulation is affected not just by the size and density of the particles. Temperature differential affects it, too: at -20 degrees, the R-value of blown fiberglass is much higher than it is at 40 degrees. This seems surprising, but is related to air densities and the physical composition of the fibers within the insulation.
- Best Practices with loose fill: The second Ci photo/detail, attached includes a high-heel truss what allows for near full depth attic insulation placed over the top plate of the exterior wall with 2 inches extra for the installation of the insulation wind baffle with an end dam to hold the insulation in place over the outer edge of the top plate.
In fact, many insulation materials can gain R-value as the temperature goes lower (think cold attic and warm house) and lose R-value as the temperature is hotter. This is pretty good news for cold climates, not so great for hot climates or really cold climates. Several insulation materials, such as spray polyurethane foam (SPF) exhibit quite stable R-values at varied temperatures and perform double duty as an air seal or air barrier, depending on their type.
Unfortunately for really cold climates (Alaska and most of Canada) thermal effectiveness of loose fill insulation can drops off pretty dramatically below -20F, so builders in very cold climates face a little bit of thermodynamic Double-Jeopardy — Higher delta T, or temperature difference, means higher R value is desired, and is a good reason we see insulation values in the R-50 + range in cold climates. Lets also remember that all of this information is again assuming there is no air leakage through the insulation materials.
- The solution: The solution is to add more insulation and a better air-seal the ceiling plane from the house to attic. air sealing opportunities include: at the exterior wall to ceiling interface, along interior walls and the ceiling plane, attic hatches, electrical and mechanical penetrations and of course careful attention to ceiling height changes. If the attic air sealing is inadequate the additional installed insulation can be ineffective to say the least. Many of these decisions regarding total insulation values, types of insulation and air sealing opportunities occur in the design phase and early in the construction phases of a home. That said: the least expensive part of insulating an attic is the additional material. The most expensive part can be getting a truckload of insulation installers to the site and set up to do the work on an additional trip in addition; locating and fixing air leakage paths can be extremely difficult and time consuming. Not to mention the higher long term energy costs associated with than stellar air sealing and insulation.
Slide data source: Marcus Bianchi
Slide detail: Construction Instruction Library