Article Reposted from Nick Connor
Polyurethane foam insulation is available in closed-cell and open-cell formulas. Polyurethane foam can be used as cavity wall insulation or as roof insulation. Thermal Engineering
Polyurethane FoamPolyurethane foam (PUR) is a closed cell thermoset polymer. Polyurethane polymers are traditionally and most commonly formed by reacting a di- or poly-isocyanate with a polyol. Polyurethane foam insulation is available in closed-cell and open-cell formulas. Polyurethane foam can be used as cavity wall insulation or as roof insulation, floor insulation, pipe insulation, insulation of industrial installations. Insulating panels made from PUR can be applied to all elements of the building envelope. Another important aspect is that PUR can also be injected into existing cavity walls, by using the existing openings and some extra holes.
Thermal Conductivity of Polyurethane Foam
Thermal conductivity is defined as the amount of heat (in watts) transferred through a square area of material of given thickness (in metres) due to a difference in temperature. The lower the thermal conductivity of the material the greater the material’s ability to resist heat transfer, and hence the greater the insulation’s effectiveness. Typical thermal conductivity values for polyurethane foams are between 0.022 and 0.035W/m∙K.
In general, thermal insulation is primarily based on the very low thermal conductivity of gases. Gases possess poor thermal conduction properties compared to liquids and solids, and thus makes a good insulation material if they can be trapped (e.g. in a foam-like structure). Air and other gases are generally good insulators. But the main benefit is in the absence of convection. Therefore, many insulating materials (e.g. polyurethane foam) function simply by having a large number of gas-filled pockets which prevent large-scale convection. Polyurethane foam typically is divided into two categories; Open cell with densities less than 1.5 lbs/cubic ft. (larger size and number of gas-filled pockets) and closed cell with densities greater than 2.0 lbs/cubic ft. (smaller size and fewer gas filled pockets). Closed cell foam adds water proofing and structural stability to a building and is the recommended foam for this memo.
Alternation of gas pocket and solid material causes that the heat must be transferred through many interfaces causing rapid decrease in heat transfer coefficient.
Example – Polyurethane Foam Insulation
A major source of heat loss from a house is through walls. Mathematically the loss is determined by the thickness of the wall and insulation, if any, the resulting thermal conductivity of the wall and it component parts and the convection heat transfer coefficients. These coefficients are determined by the temperature differences between the interior and exterior surfaces which depend on ambient (outside air) and interior temperatures.
A simple calculation with the below assumptions follows:
Wall area – 3 m x 10 m in area (A = 30 m2). The wall is 15 cm thick (L1) and it is made of bricks with the thermal conductivity of k1 = 1.0 W/m.K (poor thermal insulator).
Indoor and the outdoor temperatures are 22°C and -8°C
The convection heat transfer coefficients on the inner and the outer sides are h1 = 10 W/m2K and h2 = 30 W/m2K, respectively.
- bare wall – no insulation with 15 cm of brick
Assuming one-dimensional heat transfer through the plane wall and disregarding radiation, the overall heat loss is calculated as follows:
The overall heat transfer coefficient is then (U) times the heat flux (q) of the material
U = 1 / (1/10 + 0.15/1 + 1/30) = 3.53 W/m2K
The heat flux can be then calculated simply as:
q = 3.53 [W/m2K] x 30 [K] (h2) = 105.9 W/m2
The total heat loss through this wall will be:
qloss = q . A = 105.9 [W/m2] x 30 [m2] = 3177 W
- composite wall with thermal insulation – brick plus polyurethane foam (PUF) 10 cm
Assuming one-dimensional heat transfer through the plane composite wall consisting of PUF with a thermal conductivity of .028 W/m.K (high thermal insulator) and no thermal contact resistance and disregarding radiation, the overall heat loss is calculated as follows:
The overall heat transfer coefficient is then (U) times the heat flux (q) of the material
U = 1 / (1/10 + 0.15/1 + 0.1/0.028 + 1/30) = 0.259 W/m2K
The heat flux can be then calculated simply as:
q = 0.259 [W/m2K] x 30 [K] (h2) = 7.78 W/m2
The total heat loss through this wall will be:
qloss = q . A = 7.78 [W/m2] x 30 [m2] = 233 W
As can be seen, an addition of a thermal insulator causes significant decrease in heat losses and results a stable interior environment requiring much less energy for heating and cooling.
Just as important in maximizing energy efficiency is ensuring structural integrity of a building and the risk of fire loss is minimized. The key element in structural integrity is the roof; its resistance to external forces, wind, floods, hail, rain, lightening, snow, flying debris and burning embers, If a roof fails due to any of these issues, the damage is usually catastrophic in terms of the structure and any personnel inside.
Fire losses in terms of deaths and dollars have increased steadily from 2012 to 2021, according to FEMA, to 3800 deaths and over $15 billion in damages, excluding large dollar loss fires.
Fires, especially wild fires, are desperate for fuel and oxygen and it will consume everything in its path which has those two elements. So; if you work or live in any of those areas, it is important to have class A rated fire protection for your structure. A well-insulated roof exterior will prevent heat from igniting interior items, but the exterior insulation needs class A rated fire coatings to prevent ignition of the roof and subsequent compromise of the interior contents.
In addition to interior thermal efficiency, the home insulation and protective materials should be low or no maintenance, resistant to mold, mildew, water and class A fire rated protection. While many insulating products lose their effectiveness due to age or being compromised by water intrusion, closed cell spray polyurethane foam does not. In addition, it provides seamless monolithic protection for the structure providing both an air and vapor barrier and increasing the racking strength of the structure several times greater than other insulating materials. Fire rated drywall, Class A rated intumescent coating and or rockwall insulation offer lifetime fire protection for the interior of the house.
A roof can restrict a fire’s oxygen intake. Fires need oxygen to survive. If an internal fire breaks through the roof, it gets a fresh supply of oxygen to amplify the damage to the building itself. Fire rating are based on the following factors with a description of the requirements for a Class A rating:
- Flame penetration.How easily or quickly do flames penetrate through the roof into the attic? Have a flame spread of no more than six feet.
- Flame spread. How easily or quickly do flames spread across the roof surface? Be exposed to fire for 2-4 hours before igniting.
- Ember generation. How easily or quickly does the roof covering lose integrity, dislodging or breaking and releasing embers for further fire spread? Support a burning brand of 12″ x 12″ and 4.4 lbs (2,000 g) without losing integrity. Resist a gas flame turning on and off for at least 15 cycles.
Class A is the highest roof fire rating possible for a roof covering. It’s a desirable rating for all roofs, but it’s considered even more essential for buildings in locations where wildfires are more common. Certain roofing materials more prone to fire may also obtain a Class A rating when used in combination with other fire-resistant materials (for example, when treated with fire retardants, or used with a fire-resistant underlayment).
UL 790 and the FM Approvals ASTM E 108 are deemed by the code requirements as appropriate testing standards.
The roof coating along with slope, the coating substrate, combustible nature of the roof deck and whether the roof is insulated determine the fire rating of the roof system.
- Although there are exceptions, most fire ratings are done for slopes of under 3/4 inch for commercial roofs, and coatings tend to be recommended for application to a roof with 2 inches or less slope. Special coatings may be needed for high slope transitions.
- The substrate to which the coating is applied could affect the flammability of the roof system. When coating over an existing roof the type of substrate; BUR, mod bit, concrete, metal, asphalt or another type should be noted.
- Most coatings are tested over noncombustible decks. Additional and challenging tests are required for the use of combustible decks.
- Components of the existing roof being coated can affect the flammability of the roof system and can reduce the allowable slope for a given system.
It is important to note that the application conform with the manufacturers recommended thickness and application rate and other factors such as; Proper application parameters, final dry-film thickness, the use of granules or gravel, use of reinforcements and even the number of coats. For best results, Inspections certified by the manufacturer should be performed during and after completion of the installation.
For more information about roof-coating fire ratings, check out FM Approval’s RoofNav online database for up-to-date roofing-related information or the UL Online Certifications Directory.
