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.
R21 insulation achieves a higher R-value despite being thinner than R19 because it's denser and has an optimized structure that reduces heat transfer more effectively. While air is still the primary insulator, how well the fiberglass traps that air (and limits its movement) makes all the difference.
Blown fiberglass installed in an attic has an R-value of 2.2 per inch, so 12 inches of it would give you an R-value of 26.4 in your attic installation project.
Spray Foam Insulation R-Value
The R-Value for spray foam insulation can vary depending on the product, manufacturer, and other variables. Open cell spray foam insulation is R-3.6 to R-3.9 per inch. This R-Value is normal for open cell spray foam. Closed cell spray foam insulation is R-6 to R-7 per inch.
Vacuum insulated panels have the highest R-value, approximately R-45 (in U.S. units) per inch; aerogel has the next highest R-value (about R-10 to R-30 per inch), followed by polyurethane (PUR) and phenolic foam insulations with R-7 per inch.
Thermablok® Aerogel is a revolutionary advancement in thermal technology offering the thinnest insulation available to prevent thermal and cold bridging. Classed as a Super Insulation, Aerogel has the highest insulation value of any known material with the lowest thermal conductivity value of any solid (0.015W/mK).
The optimal R-value for most homes is 30, but you can go as high as 38 if you live in a tropical climate. Anything higher than 38 indicates an over-insulated space.
Spray foam insulation approximately has R-6, whereas fiberglass insulation has an R-value of between R-3.8 and R-5. In general, fiberglass insulation is less effective at stopping heat transfer than spray foam insulation. This makes spray foam insulation a better choice when it comes to energy efficiency.
It is possible to over-insulate your house so much that it can't breathe. The whole point of home insulation is to tightly seal your home's interior. But if it becomes too tightly sealed with too many layers of insulation, moisture can get trapped inside those layers. That's when mold starts to grow.
For standard 2-inch x 4-inch walls, fiberglass batts now come in low, medium, and high-density products ranging from R-11 to R-15. Spray foam insulation in the same wall cavity can provide R-values from R-14 to R-28, offering superior insulation in a limited space.
When uniformly compressed below the labeled thickness, some reduction in R-value will occur in fiber glass batt insulation. The chart below displays the compressed R-value of standard batt products installed across several nominal framing cavity depths.
R15 insulation offers slightly better thermal insulative properties than R13. Some of its formats are a fraction of an inch thicker than R13, making it more effective at helping with energy efficiency. However, the difference is minimal.
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.
The most common application of R19 Insulation is in a vertical wall framed by 2×6 studs. As mentioned above this is a common code requirement in some parts of the United States. R19 Insulation can also be used in other areas around the home, including floors and crawl spaces.
Depending on where you live and the part of your home you're insulating (walls, crawlspace, attic, etc.), you'll need a different R-Value. Typical recommendations for exterior walls are R-13 to R-23, while R-30, R-38 and R-49 are common for ceilings and attic spaces.
Removing old insulation is generally recommended if it's damaged, contaminated with mold, or infested with pests. However, if the insulation is in decent shape and just needs a boost, adding a new layer on top can be a cost-effective option.
Never place insulation close to water heaters, oil burners or anything hot. The backing is especially flammable and will be a major fire hazard for your home.
As a general guideline, it's advisable to install double layers of thermal insulation throughout your house. Nevertheless, this may not be feasible in all areas due to space limitations. Typically, it's not practical to add two layers of insulation within the walls of your home.
Spray foam insulation is a good insulator in terms of keeping a room warm or cool. But its many downsides outweigh the benefits. These include, making it more difficult getting a mortgage, potential health issues, environmental damage, ventilation issues and a lower home valuation.
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. It contains 70% recycled material, making it a greener product than fiberglass at 20-30% recycled material..
Directly against electrical wiring or fixtures: Spray foam insulation should not be applied directly against electrical wiring, fixtures, or recessed lighting unless they are rated for contact with insulation. Contact with insulation can cause overheating and pose a fire hazard.
The best kind of rigid foam insulation is polyisocyanurate, also known as polyiso. It has the highest R-rating of any rigid foam insulation and comes with a reflective foil on both sides, allowing it to act as a radiant barrier as well.
Mold and Mildew Growth
Having too much insulation in your attic can allow for the accumulation of moisture. This leads to mold and mildew growth, which can be extremely bad for your health.
Exterior Garage Walls
Typically these will be constructed from standard 2-by-4 studs which will define the amount of insulation that you can have installed. You should be targeting an R-Value of R-13 or R-15 in these areas to properly maximize your insulation in this application.