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Insulation & R-Value


What is insulation, what it does and how to know what thickness insulation you need for your home. South Africa insulation R-value requirements and zones.


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We’ve recently got back from our first camping stay on our land (it was awesome). We’ve been shown a budget wendy house that’s premade in panels and erected in one day on site. It’ll be the first building we put up on our land to live in while we build the main house. We’ll need to insulate the wendy house to make it comfortable to live in. I’ve done some research into insulation and learned a bit along the way! This article is about why you need insulation, what we’ve chosen to go with, and why.

What Is Insulation

Insulation is kind of like padding that goes between the roof and the ceiling, and in some cases between the walls of a building.

Insulation’s main job is to slow down the transfer of heat between the inside and outside of the building. This helps control the temperature inside the house, preventing it from becoming too hot or too cold.

What does insulation do

Do You Have To Insulate Your House

Yes. There’s legislation in place in South Africa that says you have to meet minimum insulation values in your building. Also, you want to, because it will make your house 100 times more comfortable to live in.

Insulating the roof is a must. If you’re building a wood house, or a cladded timber frame house, or any kind of modular build that isn’t standard brick, then insulating your walls is a must. If you’re building with brick (which has natural insulation properties), it’s still recommended that you insulate the walls, but not many people do.

Insulating walls of building

Benefits of Insulation

Insulating the walls and roof of your building adds a lot of positive benefits, such as:

  • By blocking heat transference, the building is more comfortable, not getting severely hot on a sweltering day outside, and not getting freezing cold when the temperature drops outside.
  • This saves on energy consumption & bills as less heating and cooling is required (fires, heaters, fans, aircons etc). Even if you’re off grid and not paying physical money for energy consumption, energy still has to be generated at the cost of something.
  • It also saves on greenhouse gas emissions for the same reason, making your home more eco-friendly and sustainable.
  • Insulation dampens sound, both externally and internally, creating a more peaceful environment.
  • And it reduces condensation build up, which helps prevent mould and rot in the building infrastructure.
  • The structural integrity of your building is increased.

What Thickness Insulation Do You Need?

Choosing a type of insulation and thickness mostly comes down to what sort of climate you’re in.

If you’re in a super mild environment, you’re not going to need as much insulation as some-one who is in a place that gets freezing temperatures in winter, or super hot summers.

(Honestly though, with climate change, I don’t think anywhere will be “mild” all year round for much longer.)

Insulation helps control temperature changes

Anyway, to take out any guesswork, you just start with the R-value for your area. That’s the minimum requirement. From there, you can add extra depending on whether you want to lessen noise between rooms or from outside, or want to add extra on the side of the house facing the sun etc.

What Is An R-Value & What Is Your Zone?

Ok, so the R-value is a standard which measures how well insulation can prevent the flow of heat into and out of a building. The higher the R-value, the greater the insulation performance. If you’re interested in how R-value is calculated, you can read more about it in this article by Home Insulations. All insulation is given a R-value rating, and different types of insulation have different ratings.

Different areas have different R-value requirements, depending on their mean temperatures (ie. what your area’s general climate is like). SANS legislation and Energy Efficiency in New Buildings in South Africa came into place in 2013. It created legislation for zones in South Africa, setting minimum R-value requirements per zone. This helps you know what R-value you need to have as a minimum in your building, based on where you are.

Map of South Africa R-Value Climate Zones

Find where you are on the map to see what zone your area falls under:

South Africa Map of Insulation R Value Zones

Table Of R-Value Requirements Per Zone In South Africa

Once you have your zone number, you can have a look at the table below to see what legislation says your minimum required insulation R-value must be for your area.

Table of R Value Requirements Per Zone in South Africa

Our Zone & R Value

We are in the Overberg region of the Western Cape which is Zone 2. Our total R-value needs to be minimum 3.7. A normal roof (with roof tiles and a rhinoboard ceiling) is roughly 0.3 to 0.35 R-value, so the legislation says that insulation to the minimum R-value of 3.35 must be installed. (Obviously if your roof differs from the standard then you’d need to take that into account. Ours is metal, so we don’t get an added R-value from it.)

What Insulation We’ve Chosen

We looked at glasswool (also called mineral wool or batt), polystyrene and polyurethane foams, perlite and cellulose (made from recycled cardboard and paper).

Each of these options have positives and negatives, including environmental impact, cost, health impact and safety.

After looking at all these options, our choice was very easy to make. Yay for simple decisions!

We are going with a product called Isotherm. We like it because:

Isotherm Insulation Benefits

  • It’s manufactured here in South Africa (in Cape Town in fact). Local is lekker, we like supporting the local economy. Plus, less carbon footprint for transportation.
  • Isotherm is made from recycled plastic, a big fat green tick in our book!
  • Some insulation is really difficult to install, like glass based insulation that requires special gear. They can make you very itchy, and inhalation is dangerous. Isotherm is safe and doesn’t irritate the skin, eyes or lungs.
  • It’s dust free and allergy free.
  • Made from plastic, it’s lightweight. Some insulation (like paper based insulation) can create a structural hazard/safety risk in your roof if it gets too heavy (say if there’s a small, undetected leak in your roof).
  • Plastic is also by nature fire resistant, mould & fungal resistant, termite & other noo-noo’s resistant, and moisture resistant.
  • It has a 30 year guarantee.
Isotherm insulation

Choosing Insulation Thickness

Now we know we are in Zone 2, we need a minimum of 3.7 R-value insulation, and we’re going with Isotherm to insulate. The 145mm has an R-value of 3.77 which is perfect. Bearing in mind that the ceiling and walls will add to the overall R-value, and we’ll add sisilation as well. Sisilation is a high quality reflective foil insulation. It looks like a supersized roll of tinfoil and kind of does the same thing. Shiny side up!