Ice Dams and How to Prevent Them

Building Science, Home Maintenance, Insulation, R-Value, Roof Health

Does your house have icicles hanging from the roof during the winter? If so, there is something going on that could potentially cost you thousands of dollars. It is called an ice dam and here’s how to prevent them.

What is an ice dam?

If the attic isn’t properly insulated, during the winter the warm air will escape and melt the first layer of snow on your roof. Many uneducated contractors will tell you to increase your attic insulation to prevent ice dams. While this may slow down the buildup of ice dams, it will essentially increase the speed of your heat loss. Consequently, the heat loss will make your home less efficient.

DIYers (do-it-yourself) like to solve the problem by putting heat tape on the roof, but that can be incredibly damaging to the shingles. Furthermore, the tape does not solve the problem, which is inefficient insulation and heat loss.

The layer of snow that is against the roof will melt and run down the roof shingles. You can’t see this happening because it is under the pile of snow. Once the water reaches the overhang where it is colder, an ice dam starts to form. This process happens over and over; the ice dam gets bigger; and the water gets higher. Eventually, water will get under the shingles!

As the melting continues, the ice dam and water lift the shingle more and more. Eventually, it hits the top of the shingle. That is when water can get into your house, and potentially cause thousands of dollars in damage.

How to prevent ice dams

Whether you are creating a conditioned or unconditioned attic, we recommend the same things.

Seal the gaps – First you need to air seal any gaps or holes in the floor or roof. There are several products that we suggest you use. Please contact us so we can help you decide which product is best for you.
Use correct R-value – Make sure you have enough R-value. Remember, fiberglass doesn’t have a constant R-value so we don’t recommend it. In southern New Hampshire, you need a minimum of an R-49.
Ventilate the attic – If you plan to insulate the attic floor, make sure to allow for proper ventilation to prevent condensation. If you are not sure you have adequate insulation, give us a call and we will come out and take a look.

Using a spray foam in the attic is the best choice because it fills the holes and crevices. We recommend closed cell spray foam for several reasons:

It has a higher R-value per inch than open cell spray foam.
Closed cell spray foam is a vapor barrier and water can’t permeate it.
It sticks to the walls and fills all the crevices and holes.

Learn more about Closed Cell Spray Foam by watching a video on our website. Contact us with questions.

Did You Know? Insulation R-Value Changes

Building Science, R-Value

In the world of building construction, improvement, and insulation, we talk about R-value all the time. Usually, we talk about it as if it’s a constant number. We see R-19 stamped right there on the product, so that’s what it is, right? Well, did you know that the insulation R-value changes?

What Is R-Value?

R-value is a measure of resistance to heat flow through a given thickness of material. Furthermore, it is important, but it’s just one of a few key factors that determines the effectiveness of an insulation material. R-value is tested in an air-tight vacuum chamber, which means that typical insulation materials like fiberglass and cellulose do not actually perform with that value unless paired with intense air sealing measures.

^[1]

How Is R-value measured?

Because of scams involving R-value claims that didn’t match reality, the U.S. Federal Trade Commission (FTC) devised the R-value Rule to protect buyers of insulating products. In fact, in 2013 the FTC handed down its largest fine ever ($350,000) to a company claiming that its paint had an R-value of 100.^[2]

So, the R-value Rule requires testing insulation using one of four American Society of Test Methods (ASTM) standards. The basic requirement is that the mean temperature must be 75° F (24° C) with a temperature difference of 50° F (28° C) across the insulation.^[3]

According to Building Science Corporation, most R-value testing is done with a temperature of 50° F (10° C) on the cold side and 100° F (38° C) on the hot side.^[4]

The Lowdown on R-value

The main point here is that R-value isn’t some constant number for a given material, and installation isn’t the only factor that affects it. As you can see above, temperature has a significant effect as well.

The team at The Green Cocoon knows that the performance of the insulation is affected by how well it is installed. Energystar.gov has developed guidelines for grading the quality of installation. Moreover, they’ve developed a table that shows what levels of insulation are cost-effective for different climates and locations in the home.^[5]

Compression

Improperly installed insulation loses its R-value. For example, fiberglass can lose up to 50 percent of its prescribed R-value when compressed. Additionally, when you compress fiberglass batt insulation, the R-value per inch goes up, but the overall R-value goes down because you have less inches or thickness of insulation.^[6]

Fear not, closed cell spray foam is the only insulation material that has an almost constant R-value. The reason is because you cannot pass air through it or compress it.

[1] Building Performance Institute (April 4, 2017). What Does R-Value Mean? Retrieved from bpihomeowner.org.

[2] FTC.gov (January 31, 2013). FTC Action Leads to Court Order: Home Insulation Marketer to Pay $350,000. Retrieved from ftc.gov.

[3] Electronic Code of Federal Regulations (March 31, 2020). R-value Rule. Retrieved from ecfr.gov.

[4] Building Science Corporation (April 12, 2013). Info-502: Temperature Dependence of R-values in Polyisocyanurate Roof Insulation. Retrieved from buildingscience.com.

[5] Energy.gov. Recommend Home Insulation R-Values. Retrieved from energy.gov.

[6] Insulation Institute. The Facts About Compressing Fiber Glass Insulation. Retrieved from insulationinstitute.org.

Understanding and Managing Thermal Bridging

Building Science, R-Value, Thermal Bridging

When you are ready to insulate your living space, you may be contemplating what R-value you will need. And why not? It is the most widely accepted measure of insulation and often aligns with a minimum code requirement. But assuming that your wall assembly will realize the R-value of your insulation ignores a crucial variable in building science—thermal bridging. Understanding and managing thermal bridging will make your living space more comfortable. And, it will save you money!

What is thermal bridging?

Thermal bridging is when a particularly conductive (or poorly insulating) material allows heat flow across a thermal barrier. Physics dictates that hot wants to go to cold and a thermal bridge is a perfect invitation. The best analogy is to think studs within an insulated wall. The insulation on either side of a stud will do its job of limiting heat flow. But, the wooden stud, which has a much lower R-value, will not. These bridges can become expressways of heat loss.

How does this work in a home?

To understand and manage thermal bridging, let’s continue with the stud example. The typical R-value of a 2×4 wooden stud is 4.4, while the insulation on either side is much higher—typically greater than R13. The result of this bridge is heat being transferred and a significant loss in the wall’s effective R-value. In a wood stud wall with R20 insulation, thermal bridging can reduce the effective R-value to as low as R15. One of the worst thermal bridges is a metal stud. This can reduce effective R-value by as much as 80 percent. Along with studs, other typical building materials that create thermal bridges are metal fasteners, plates, headers, and windows. Typically, you can observe these all around a house—roofs, walls, fenestrations, for example.

The impacts of thermal bridging

If you are losing heat through thermal bridges, the efficacy of your climate control system decreases and uses more energy. Furthermore, these bridges can create cold spots within your walls where condensation can easily form. This moisture can produce a whole host of issues to the wall cavity—mold and mildew or rot, to name a few. Excessive moisture also challenges the structural integrity of your wall.

How do we prevent or limit thermal bridging?

Luckily advancements in building science and common sense can really help limit thermal bridging. Realistically, most homes will have some thermal bridging happening, but there are tools at our disposal to help prevent it.

Exterior insulation – According to Havelockwool.com, a better term would be “out-sulation.” This means that by wrapping the exterior of your home in insulation you can prevent (or dramatically reduce) the common thermal bridging via wall studs. Cork is a great option. You can spray foam the exterior, add rigid board or Rockwool as well.
Advanced house framing or Optimum Value Engineering – The goal of this building technique is to reduce lumber used in framing, which reduces thermal bridging. There are a lot of strategies incorporated, including spacing wall studs up to 24 inches on center, eliminating headers in non- load-bearing walls, designing on two-foot modules to make the best use of common sheet sizes, and to reduce waste.
Insulated studs – The company T Stud produces an R19 stud that is a thermally broken, insulated wall stud assembly. They also have the BareNaked Tstud™, which is an uninsulated, thermally broken wall stud that you can insulate with the insulation of your choice. These studs can not only reduce thermal bridging, but they are incredibly strong.
“Alternative” Wall Assemblies – Structural Insulated Panels (SIPs) are a common alternative to stick-built construction and can help limit thermal bridging. These panels have an insulated foam core between two rigid board sheathing materials, which when installed properly, can provide a continuous air and vapor barrier. There are issues, however, including the use of foam. Bamcore makes a bamboo wall system that is an environmentally friendly, extremely high-performance option.

Understanding and managing thermal bridging is easy

In conclusion, understanding and managing thermal bridging is a must because thermal bridging happens in every home. And, if you are constructing a new building, you can take care of that very easily. If you have older construction or renovation, you can use the best insulation possible in between the base to mitigate the amount of thermal bridging happening.

This article is based on an article from Havelockwool.com.