Cob House Insulation: How to Keep Your Home Warm in Winter

A photograph of a cob wall with straw on its surface. A drawing of an orange house with warm waves inside appears in the lower center, and the words "Cob House Insulation: How to Keep Your Home Warm in Winter" are written across the top.

When winter chills start creeping in, your home’s insulation is one of the most crucial things to consider.

After all, it’s never fun wrapping up indoors or watching your energy bills skyrocket. To avoid these issues, cob house insulation is essential, especially if you want a comfy indoor environment with low demand for central heating.

Let’s be frank — although cob is one of the best sustainable building materials, it’s a poor insulator. This poses a significant challenge, especially in winter when outdoor temperatures drop considerably.

Therefore, failure to insulate your cob house will result in an uncomfortably cold interior and hefty bills.

In the rest of this article, I’ll discuss cob house insulation. You’ll know the best ways to insulate your cob house and keep it warm in winter.

Let’s get started!

Cob House Insulation: The Basics

A closeup of a cob wall with straw on its surface. In the center is a drawing of a stylized house with red curvy lines and arrows to indicate heat.
The high thermal mass of cob helps to even out diurnal fluctuations in temperature.

Cob is a poor insulator due to its composition.

The two main components of cob, sand and clay, have low conductive resistance to heat loss (R-values).

Straw is the other constituent in cob and is highly insulative but accounts for only a small percentage of the cob mixture.

So, why doesn’t cob incorporate a higher amount of straw?

While it’s possible to have a straw-heavy mix, such a mix will not have the appropriate load-bearing capacity for thick cob walls.

Moreover, cob’s strength and durability come from the bond created by its unique sand, clay, and straw mix. Adding more straw for insulation would weaken this bond and consequently impact the cob’s structural integrity.

Straw is only added to the mixture to increase a monolithic cob wall’s shear and tensile strength.

According to the Cob Code Appendix AU on Cob Construction, cob has an R-value of 0.22 per inch of thickness. This is quite a low resistance to heat loss and far lower than the R-values of other natural materials such as hempcrete and straw bale.

An R-value of 5.28 for a 24-inch cob wall is nothing to write home about.

Coupled with its poor thermal resistance, cob’s massive thickness [18-24 inches (45.72-60.96cm)] is an energy efficiency nightmare.

Therefore, creating an adequate insulation layer for your cob home is essential if you’re in a cold climate.

Cob Insulation and Thermal Mass

Before insulating your cob house, let’s discuss thermal mass and insulation.

Cob insulation and thermal mass are the two points of note regarding cob houses.

Thermal mass is the material’s ability to absorb, store, and release heat; cob has excellent thermal mass. This is advantageous in some circumstances as it can absorb and store high amounts of heat from the sun and slowly release it when the house gets cold.

A point worth noting is that heat applied to one side of a high thermal mass material like cob will eventually reach the other side.

On the other hand, insulation is a barrier that slows the heat transfer rate from one area to another. Simply put, insulators are non-conductive.

So, if your house is in a warmer climate, you can take advantage of cob’s thermal mass to even out fluctuations in temperature between the day and night. However, in a cold climate, you’ll need to insulate cob for comfort and efficiency.

Let’s explore ways to insulate your cob house.

Cob House Insulation: The Best Techniques to Use

The following are the best techniques for cob house insulation:

Erecting a Dual Wall System

A workshop where cob walls are being made for testing. In the foreground are a dual cob wall system with an outer leaf for strength and a less dense inner layer for insulation.
A dual cob wall system with an outer leaf for strength and a less dense inner layer for insulation. Image courtesy of MAKE IT LONDON LTD.

Most builders and homeowners prefer a dual wall system for cob house insulation.

A dual-wall system provides the best insulation because:

  • It allows more room for insulation.
  • It lowers thermal bridging.
  • It eradicates moisture problems when building with cob and vapor-impermeable insulations like foam boards.

As the name suggests, a dual cob wall system entails building two interlinked walls.

The interior side of the wall is cob while the exterior is made with a high insulation value fiber mix such as:

  • Straw bales.
  • Hempcrete.
  • Straw clay.
  • Compressed hemp mud mixture.

These types of insulation are suitable for a cob house because:

  • They’re vapor-permeable.
  • They allow for air movement.
  • They provide a strong thermal barrier.
  • They reduce the risk of condensation and moisture buildup.

You need a forming system to build the best double cob wall. This system allows you to create sections of the cob wall in blocks that can then be interlocked with each other.

You should install the forming boards from the bottom up.

Insulation Sandwiching

This involves sandwiching insulation material between two layers of cob wall.

The insulation materials can be sheep’s wool, foam boards, or fiberglass.

The two sides of cob walls are tied together with rebar to ensure the walls remain straight and upright.

This method is suitable for repairing existing cob walls as it improves a home’s energy efficiency without rebuilding it from scratch.

It’s important to note that when using this technique, you should leave a small gap around the edges for ventilation.

The main downside to this method is that most insulating materials are impermeable. Therefore, they can lead to condensation within the walls. Condensation weakens the walls, causing a structural problem.

The best way around the condensation problem is to opt for highly permeable insulation. Since sheep’s wool is vapor-permeable, it’s the best option for creating a cob wall sandwich insulation.

Alternatively, you can stabilize the cob with about 10 percent Portland cement, which acts as a vapor-impermeable barrier. This could be used as an extra precaution against condensation.

A cob wall is being built with cavity wall insulation.
Cavity wall insulation is being built into a cob wall during construction. Image courtesy of Pinterest.

Interior Stud Frames Filled With Insulation

For this method, you build the cob walls normally and then construct an interior stud frame siding using 2×4 studs.

Once the stud frames are in place, fill them with insulation material such as fiberglass, sheep’s wool, or foam boards.

The main benefit of this method is that it provides a thermal barrier and helps create an airtight wall. It also allows for more flexibility in terms of design and aesthetics.

However, it can be expensive due to the cost of the interior stud frames.

Frequently Asked Questions (FAQs)

Is cob energy efficient?

Although cob has a high thermal mass, it’s not energy efficient due to its poor insulation. Therefore, when building with cob, you must insulate the walls to make them energy efficient.

What is the advantage of cob wall?

The advantage of cob walls is that they have a higher specific heat capacity, are highly durable, and sturdy. With proper maintenance against water, cob walls can serve you for generations to come.

Can you build with cob in winter?

Although you can build with cob in winter, places with daytime temperatures below freezing point would not be suitable for cob construction due to freezing and thawing of the materials. If you must build a cob wall in such temperatures, ensure to have sufficient insulation around it.

Does cob hold heat?

Due to its high specific heat capacity of 1205 Jkg−1K−1, cob can hold, store and release heat slowly. This helps keep the interior temperature consistent throughout the day and night.

What is the best insulation for cob house?

A closeup photograph of white and brown sheep's wool against a black background.
Sheep’s wool insulation is a sustainable option for insulating cob walls.

Strawbale is the best insulation for a cob house because it’s a sustainable material and surrounded by air pockets that act as barriers to heat flow.

What are the disadvantages of a cob house?

The disadvantages of a cob house are:

  • Cob is a poor thermal insulator.
  • They are highly susceptible to water damage.
  • They are labor-intensive to build.

Is cob good for cold weather?

Cob is not good for cold weather because its high thermal mass requires longer hours of sunlight exposure to absorb heat effectively. Therefore, cob needs additional insulation to make it suitable for cold weather.

Are cob houses damp?

Cob houses should not be damp but sometimes condensation can occur if moisture from the interior cannot escape. The best way to prevent this is by adding a vapor-permeable insulation layer and using breathable waterproofing agents.

Moreover, ensure the cob walls are well-ventilated with sufficient gaps around them.

Final Thoughts

Cob house insulation is necessary to make the walls energy efficient.

The main consideration for insulating cob walls is to ensure the walls can breathe and release vapor efficiently. That way, you can rest assured the walls won’t develop condensation.

Before you leave, check out this Havelock insulation review to find out if it’s ideal for your cob house.

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