Alexei Volkov
R-value is a measure of how well a material resists the flow of heat and is used to describe the effectiveness of insulating materials. The R-value of plastic is a topic of discussion among DIY enthusiasts and professionals alike, especially when considering window insulation. While plastic is not a preferred choice due to its flammability and potential for mold, it can be used as a temporary solution to block drafts. The R-value of plastic depends on its thickness and the presence of trapped air between the plastic and the window, with a taut sheet of plastic having a higher R-value than a loosely placed one.
| Characteristics | Values |
|---|---|
| Definition | R-value is a measure of how well a two-dimensional barrier, such as a layer of insulation, a window, or a complete wall or ceiling, resists the conductive flow of heat. |
| Formula | R-value has units of (square foot)(hour)(Fahrenheit degree)/(British Thermal Unit). |
| British Thermal Unit | A British Thermal Unit is enough heat to raise the temperature of one pound of water by one Fahrenheit degree, or about 1055 Joules. |
| R-value vs U-value | U-value is the inverse of R-value. The higher the R-value, the lower the U-value. |
| Plastic bags as insulation | Plastic bags are not recommended as insulation due to their flammability and potential for trapping moisture, which can lead to mold. |
| Alternatives to plastic insulation | Closed-cell polyurethane foam and fiberglass insulation are suggested as alternatives to plastic insulation. |
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What You'll Learn
Plastic over windows
The R-value of plastic over a window depends on the presence of a layer of still air between the plastic and the window. If the plastic sheet is pulled taut against the window without any trapped air, the R-value of the window remains unchanged. However, if there is a layer of still air between the plastic and the window, the R-value increases. For example, an air-pane assembly with an R-value of 1.2 can be improved to an R-value of 1.9 by adding a layer of plastic with trapped air.
The effectiveness of using plastic over windows to improve insulation and reduce drafts varies. Some people have noticed a significant difference, especially in older homes with single-pane windows. Others have observed that it may not lead to dramatic reductions in energy bills, and the energy savings may not meet the manufacturers' claims.
To install plastic over windows, you can purchase window insulation kits or custom-fit plexiglass window insulation panels. The kits provide window-sized plastic film pieces, while the plexiglass panels offer a sturdy and reusable option without the need for tape. When installing, ensure you properly size the film, clean the window frame, and apply the adhesive and film. You can use a hair dryer to tighten the film and prevent wrinkles.
While plastic over windows can be a temporary solution, it is important to address the underlying issues causing drafts and energy losses. This may involve sealing air leaks, improving weatherstripping, or upgrading to more energy-efficient windows.
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R-value calculations
R-value, also known as the
R-value is calculated by measuring the heat flowing through a building element and combining this with the inside and outside temperatures. The mathematical formula for calculating the R-value is:
> The R-value for the building element is the temperature difference between the two temperature sensors divided by the heat flux through the heat flux sensor.
The R-value can also be calculated by measuring the initial and final width and length of the specimen using calipers (manual method) or by measuring the axial and transverse strain (automatic method). The automatic method is preferred as it allows the user to calculate properties such as ultimate tensile strength or strain at break.
R-values are additive for layers of materials, and the higher the R-value, the better the performance. A low U-value, or conversely a high R-value, usually indicates high levels of insulation.
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Insulation alternatives
R-value is a measure of how well a material resists the conductive flow of heat. The higher the R-value, the better the performance of the material as an insulator. Plastic is one such material used for insulation, particularly over windows, to improve the R-value of a window assembly.
However, there are several alternative materials that can be used for insulation. These alternatives are often sought due to the health issues and environmental impact associated with traditional fibreglass insulation. Some alternatives include natural fibres, recycled materials, and innovative solutions. Here are some insulation alternatives in more detail:
Natural Fibres
Sheep's wool is a popular natural insulation option that can regulate temperature and absorb moisture. It is fire retardant and can be safely installed by oneself. The compressed wool fibres form millions of tiny air pockets, providing effective insulation. Other natural fibres used for insulation include cotton and hemp. Cotton is made from recycled textiles and offers soundproofing and non-toxic benefits. Hemp is a new addition to the insulation market, known for its strong thermal performance, mould resistance, and ability to absorb moisture. It is fully biodegradable, recyclable, and resistant to rodents and insects.
Recycled Materials
Recycled denim is one of the most sustainable insulation alternatives, made from discarded fabrics. Cellulose is another recycled material used for insulation, primarily made from recycled newspaper. It is environmentally friendly, has a low Volatile Organic Compound (VOC) index, and is healthier for humans than many other materials. Other recycled materials used for insulation include recycled plastic and mycelium.
Innovative Solutions
Aerogel is an innovative material known for its exceptional thermal resistance and moisture control. It is 90% air, yet heat has difficulty passing through it. Its slim design makes it ideal for spaces with limited room for thicker insulation. Icynene is another innovative insulation solution, a spray foam made from castor oil that expands about 100 times its volume when sprayed into a wall or ceiling. It seals leaks and drafts and cancels noise. ThermaCork is another innovative insulation material that is natural, renewable, recyclable, and biodegradable.
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Plastic strain ratio
The plastic strain ratio, or Lankford coefficient, is a measure of the plastic anisotropy of a rolled sheet metal. It is considered a direct measure of sheet metal's drawability and is useful for evaluating materials intended for forming shapes by deep drawing. The r value is the ratio of the true strain in the width direction to the true strain in the thickness direction when a sheet material is pulled in uniaxial tension beyond its elastic limit.
The plastic strain ratio is calculated using the ASTM E517 Standard Test Method for Plastic Strain Ratio r for Sheet Metal. The r value is dependent on the ratio of width and thickness changes as the sample is pulled in tension. The word "plastic" in the phrase "plastic strain ratio" implies that you have exceeded the specimen's elastic limit and that only the strain that induces plastic flow is considered in the calculation.
Because it is difficult to measure thickness changes accurately, it is assumed that the volume of the specimen remains constant and the thickness strain is expressed as et = ln(Lowo/Lfwf). After substituting et into Equation 1 and inverting it to eliminate negative values, the plastic strain ratio is given by Equation 2. Equation 2 enables the calculation of the plastic strain ratio either manually with a set of calipers or automatically with the use of two extensometers.
If the automatic method is used, the specimen can be pulled until it fractures, which enables the determination of the ultimate strength, yield strength, and elongation in the same pull. To calculate the plastic strains using the automatic method, the elastic strains must be calculated and subtracted from the measured strains. The r value is much more sensitive to errors in width measurement than errors in length measurement.
Because rolled sheet metals develop planar anisotropy, sample orientation can be significant to the measurement of the plastic strain ratio. Therefore, test specimens must be cut at 0 degrees, 45 degrees, and 90 degrees respective to the rolling direction, and the cut direction must be reported with each result.
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R-value for shrink film
R-value, or thermal resistance, is a measure of how well a two-dimensional barrier, such as a layer of insulation, a window, or a complete wall or ceiling, resists the conductive flow of heat. The higher the R-value, the better the material is at stopping hot or cold air from passing through.
R-values are additive for layers of materials. For example, if one material has an R-value of 12 and it is attached to another material with an R-value of 3, the combined R-value of the two materials is 15. The R-value per unit of a barrier's exposed surface area measures the absolute thermal resistance of the barrier.
The R-value of plastic sheeting is typically between 0.5 and 0.85, while the R-value of a pane of glass is 1. This means that the plastic film has a lower ability to resist heat transfer than glass.
Window insulation kits that use plastic shrink film are easy to install. The plastic is cut to fit the window and attached using double-sided tape or glue sticks. A hair dryer is then used to shrink-wrap the plastic film to fit snugly across the window. This creates an airtight seal that is supposed to reduce the transfer of hot or cold air through the window.
While plastic shrink film can provide some level of insulation, it is important to note that it may not have a significant impact on energy bills. The effectiveness of window insulation kits in reducing heat transfer also depends on other factors, such as the presence of air leaks and the overall thermal resistance of the building assembly.
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Frequently asked questions
The R-value of plastic depends on its application and the number of layers used. Plastic bags used as insulation in an 8x10 shed, for example, will have a different R-value compared to plastic sheeting taped over a window. Plastic bags are also a vapour barrier and will trap moisture, which can lead to mould. Plastic sheeting taped over a window with a layer of trapped air between the plastic and the window will have a higher R-value than plastic pulled tightly against the window with no air layer.
R-value is a measure of how well a two-dimensional barrier, such as a layer of insulation, a window, or a complete wall or ceiling, resists the conductive flow of heat. The higher the R-value, the better the performance.
To calculate the R-value of plastic, you need to know the temperature difference between the two sides of the plastic and the exposed surface area. The formula for calculating the total rate of heat flow through the plastic barrier in BTUs per hour is: Temperature difference (in Fahrenheit degrees) divided by the R-value, multiplied by the exposed surface area.
