Strawbale house

Strawbale house

http://www.piperscreekfarm.com.au/piperscreekfarm.com.au/Strawbuilding_Info.html

Straw-bale info

Myths and FAQ’s

Wont you get mice in your walls? This is one question I often get asked by people when I say I am building a strawbale home. They screw up their nose and look at me like I’m nuts and they say “but won’t you get mice in your walls”. Well the answer is not as many as they will. Brick veneer homes offer a far better place for mice to live because the walls have cavities in them, straw on the other hand, after the walls have been compressed are hard and should not have any cavity spaces. Provided walls are well rendered and the top and bottom plate sealed you should not get mice living in your walls.

Wont your house go rotten? Straw will only rot if it is exposed to moisture. If a home or wall is constructed properly (see notes on render below) you should not have any problems with your wall going rotten, but beware water penetration is fatal!!

How long will it last? This is another question I love. I love the look of amazement on peoples faces when I explain that there are strawbale houses in Nebraska that are over one hundred years old. (The average life span of a project house built in Australia is only about 30 years).

But what about bush-fires, surely they burn easily? I just love this question. Actually rendered strawbale walls have three times the fire resistance of brick veneers. (Check out tests by the CSIRO). The main reason strawbale homes are less combustible is because there are no cavities. For fire to burn it needs three things, ignition, fuel, and air, no cavity means a lack of air to help the fire combust.

But hang on, hay stacks can spontaneously combust can’t they?Well yes hay stacks can catch fire due to a build up heat in the bales but we are building our house out of straw. Generally hay is baled when the grass / hay is still quite green to preserve as much calorific value as possible for the animals that will be eating it, if hay is stored too green microbial activity (like what happens in your compost heap) can lead to very high temperatures which can in some instances cause combustion. Straw on the other hand is baled dry and as long as it is kept dry should not pose any risk of spontaneous combustion.

So what’s the difference between hay and straw? As I said above hay is baled green as a means of storing calories to feed to animals when needed. The aim of hay is store as much leaf area as possible so as to give high calorie per volume of bale. Straw on the other hand is actually the stalk of a grain plant, so in this instance the calorific value of the crop has been stored in the grain and not in the stalk. Once the grain is removed there really is very little feed value in the stalk, and the stalk is completely dry. The stalk of grain crops, (straw) is actually a cellulose material meaning it has the same fiber construction as timber. On top of this straw is hollow, this is one off the reasons it is such a good insulator. John Glassford from Huff and Puff construction has a great saying, “Hay is for horses, Straw is for houses”. I can’t come up with a better saying myself so I’ll just have to steel his.

It must be cheap building with straw-bales? Is it cheap, thats what everyone wants to know. Well that depends. How cheap you can build a house really comes down to how much of the construction you can do yourself. If you are paying someone $65.00 and hour to build your house it will cost a few dollars. A strawbale wall will work out a fair bit cheaper than a brick veneer wall but a house is more than just walls, the cost of the roof and the slab will still be the same, the wiring, the plumbing etc will still all cost the same as if you builder a brick veneer. The key thing is to do as much of the work yourself as possible. Get some friends on board and set up a deal where you can work for each other at a reasonable rate. I do this with a couple of mates and it means we can work for each other for a week or more at a time and get a nominal wage, knowing that they will return the favor when I need a willing cheap worker. This works well for us as we often need more than one person to complete a job efficiently.

Straw is only half of the story – thermal mass and good design is paramount

Super insulated walls.
A cavity brick wall only offers a metric insulation value of R.5 then we add an Australian standards batt of R1.5 so most house walls in Australia are lucky to get an R value of 2.0. A strawbale house where the bales are laid flat will have an R value of least 7, and thats before you take into account the heat sink ability of the render.

Good Design
Good house design is vital! If you build a strawbale house with a bad design, it may become an oven or a fridge. Good design is of the upmost importance and starts with good orientation and designing for winter sun in and summer sun out. Also watch out for some of the you bute e glass windows that the manufacturers will tell you will increase your houses performance. These windows are great for poorly designed houses but if you have designed your house to get the heat out of the winter sun, then some of this glass actually stops the UV rays entering and heating your house. Check out the SHGC rating of the window, this stands for Solar Heat Gain Co-efficient, a window with a SHGC of 1 will allow all of the suns energy in to heat your house, so if a window has a SHGC of say 0.35, then you will only get the benefit of one third (roughly) of the sunlight hitting your windows. These types of windows are great if you have to have a window in a west wall and you can’t shade out the summer sun, but if you design a house to capture the winter sun and install these windows, your house wont warm up like you expect it to.

Walls that breath?
A lot of people talk about mud brick or strawbale walls breathing. This happens in a couple of ways. Firstly strawbale walls (if you use a natural or lime render) are permeable, this means that air can travel through them. (never ever use concrete for render as it is not permeable and will cause the bales to rot). The other way walls breath, and this can only happen with a natural render is that the wall moderates humidity. So what does that mean. Well in a normal house you will notice in winter that the air can become moist, or damp, and condensation in spots is common. With an earth or lime render or mud brick wall the wall will actually suck up this moisture and hold it. Then when you have a hot summer day, the moisture held in the wall will be released, having a cooling effect on the house as well as adding moisture to the dry air. And so the wall breaths. If you are struggling to believe this take a shovel full of moderately dry clay or soil. Pour a couple of cups of water onto it. Where does it go? It soaks into the soil. Now put that shovel full of soil in front of the fire or out in the sun and watch the water evaporate from the soil. Thats how it works.

Healthier air
One of the huge benefits of a strawbale home is that the air inside is much much healthier. I am not really sure how this happens but a lot of books talk about the wall acting like a big heat exchanger allowing the cold outside air to enter the wall and the warm inside air to leave through the wall. As the two flows of air pass each other in the wall the heat is exchanged, keeping the warmth inside the house but allowing fresh air to move inside. I’m not sure exactly how this works but one of my mates who just completed his strawbale house at Beechworth said to me the air in the house is different to their old house, he could not explain it but said it was alive, it was a different texture it wasn’t stuffy at all. My personal theory is that as the hot air rises inside your house it create a higher air pressure at the top of the wall, and air flows out through the top half of the wall. This creates a slight vacuum inside and clean air is drawn in to the lower part of the wall. I believe the heat is exchanged as it passes through the internal render. Either way it has been established that strawbale homes have roughly 3 air exchanges every hour.

Thermal Mass
Thermal mass is the other key to good house design, and it is simple and incredibly powerful. Understanding thermal mass is very important when designing your house. Thermal mass is about the ability of a product to hold and store heat energy (or cold energy). Essentially utilizing thermal mass inside your house is like charging a battery. With a battery we charge it with power to use later.

Thermal mass is any building material that will hold a substantial amount of heat (or cold). Things like, a concrete slab, a rammed earth wall, internal render, a slate bench top. All of these things absorb heat either form the sun with good design, or from your fire or other heating. All of these materials have a thermal lag which means they gain and loose energy slowly. Think about this. Put a brick and a tin tray in the oven for an hour. Take them out of the oven and measure their temperature every ten minutes. you will find that after the first ten minutes the steel tray will be cool, having quickly released its heat and returned to room temperature. The brick on the other hand will take maybe an hour or two hours before it releases its energy and returns to room temperature. (This little experiment highlights how crazy we are for building all those brick veneer houses, they are being built inside out!!! the bricks should be on the inside) With this in mind a house should be designed with as much thermal mass inside the strawbale envelope as possible. Now the incredible thing about thermal mass is that it also keeps your house cool in summer. Why? Well its the same thing in reverse. If your house contains a lot of thermal mass and the weather becomes hot your walls turn into air conditioners, firstly they start to release the moisture they are holding so the air feels cooler and then they start to release coldness. WHAT? Yes thats right, as the air inside the house warms up the wall will start to slowly warm up to but due to thermal lag this takes a long time, essentially the wall wants to stay at the same temperature as the air but this will take days so the wall will feel cool as it stays at say 19 degrees and the air temperature is sitting on maybe 30 degrees on a hot day. So I hope that you can see that when you combined benefits of thermal mass and the humidity regulating properties of natural materials they work together to create a much more livable environment. Of course the reality is you wont let the hot air into your house if you can help it and your house will stay cool for days despite the hot outside weather.

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