Building materials are classified into four general classes based on perm ratings: impermeable (0.1 perm or less), semi-impermeable (1.0 perm or less and greater than 0.1 perm), and semi-permeable (10 perms or less and greater than 1.0 perm). A perm rating is a recognized rating system that measures the amount of water vapor that can pass through a material. A higher perm rating indicates more readily vapor can diffuse through the material.
Vapor permeability is measured in units called perms, which are determined by standardized industry tests. The minimum measure of permeability required by current building codes is around 5 perms, but DuPont building scientists believe this threshold is too low to ensure consistent performance. A perm rating of 1.0 indicates that 1 grain of water vapor will pass through 1 square foot of the material, provided that the vapor pressure difference between the cold side and the warm side of the material is equal to 1 inch of mercury (1 inch Hg).
A perm rating of less than 0.1 is considered a vapor barrier, while a perm between 0.1 and 1 is considered a vapor retarder. A perm between 1 and 10 is considered vapor permeable, with materials with lower perm ratings being better at stopping the movement of water vapor.
Vapor permeability is dependent on paint, with latex paint generally in the 2-3 perm range and elastomeric paint highly variable. One of the lowest perm ratings for a building material is polyethylene sheeting at.04-.06 perms for 4-6 mil poly. In comparison, raw drywall has a perm rating of.06 perms for 4-6 mil poly.
In summary, building materials are divided into four general classes based on perm ratings, with each rating indicating the degree of permeability they allow for water vapor diffusion. Understanding these classifications can help in inspecting, diagnosing, and installing or repairing buildings effectively.
📹 STEGO IQ | Below-Slab Vapor Barriers: What Does a Perm Rating Mean?
Stego IQ defines permeance, and how perm ratings from test methods are critical to building materials, such as vapor barriers.
What is a standard perm?
Basic perms are a popular technique used to add body, volume, and curl to straight or minimally wavy hair. They involve expert stylists wrapping hair sections around perm rods and applying a perming solution, reshaping the hair to achieve a uniformly curled appearance. These perms offer volume boost, versatility, and longevity, with the ability to last up to six months or longer when properly maintained.
Spiral perms are an advanced perming technique characterized by distinctive, long, and springy ringlet curls. Stylists wrap hair sections around perm rods in a spiral pattern, applying perming solution to reshape the hair into spiral curls. Both methods can enhance the overall look and maintain hair growth and aftercare.
What are the 3 types of permeability?
Permeability is a crucial concept that influences fluid flow through porous materials across various industries and geological processes. It is measured by the ability of a rock to pass through a fluid phase, such as oil, gas, or water, in the presence of other fluid phases. There are three main types of permeability: Effective Permeability, Absolute Permeability, and Relative Permeability. Permeability quantifies the ease of liquids, gases, or chemicals passing through a material, with high permeability allowing fluids to flow more easily. It is relevant for professionals and researchers in hydrogeology, petroleum engineering, and resource management.
What is 1 perm?
The United States perm is a unit of water vapor transmission, defined as one grain per hour, per square foot, or per inch of mercury. This is equivalent to 0. 659045 metric perms, or 57. 2. 135 ng/m²/Pa. The fundamental SI unit for permeance is the kilogram per second per square meter per pascal.
What is the perm rating of materials?
Perm is a measure of a material’s ability to allow water vapor to spread through it. A higher perm number indicates more water vapor can pass through the material. A perm rating less than 0. 1 is considered a vapor barrier, while a rating between 0. 1 and 1 is a vapor retarder. A perm rating between 1 and 10 is semi-permeable, and a rating greater than 10 is considered permeable. Permeability occurs when vapor pressure, a combination of humidity and temperature, differs on one side of a material.
What is a good perm rating?
A vapor barrier is considered effective if it has a vapor transmission rate of 1 Perm or less; conversely, a material with a rate above 1 Perm is deemed inadequate.
How is perm rating measured?
Permeability, or perms, is a measure of a material’s ability to control diffusion. It is defined as the ability to pass one grain of water vapor per hour through one square foot of flat material at one inch of mercury. Perm ratings are a standard measure of water vapor permeability, with higher ratings indicating more easily water vapor can diffuse through the material. Perm ratings range from 0. 1 to 10, with a perm rating of less than 0.
1 being considered a vapor barrier, 1 to 1 being a vapor retarder, 1 to 10 being semi-permeable, and 10 to 10 being permeable. Manufacturers of air/vapor barrier material report their permeability, which is used by design professionals to specify materials.
What is permeability of building material?
Permeability refers to a material’s ability to allow air or moisture to pass through it, and construction materials require varying degrees of permeability to ensure safety for occupants. Misselecting the right products can lead to moisture vapor infiltration, mold, leaks, occupant complaints, and high energy bills. Product selection can be challenging, as confusion often arises when labeled as “permeable” without specifying whether this pertains to water or air.
What is a perm rate?
A perm rating is a standard measure of a material’s water vapor permeability, with higher ratings indicating more easily diffused water vapor. Class I impermeable vapor retarders (or vapor barriers) have a perm rating of less than 0. 1, Class II semi-permeable vapor retarders (0. 1-1) have a perm rating between 0. 1 and 1, Class III permeable vapor retarders (1-10), and highly permeable vapor retarders (>10). The U. S. Department of Energy provides more information on vapor barriers and vapor retarders.
What is a Class 2 perm rating?
Class I vapor retarders are characterized by a very low permeability, with a rating of 0. 1 perms or less. Such materials include sheet polyethylene and unperforated aluminum foil. Class II vapor retarders are rated between 0. 1 and 1. 0 perms. Examples of Class II vapor retarders include kraft facing on batts. Class III vapor retarders have a rating greater than 1. 0 perms and less than 10 perms. Examples of such retarders include latex and enamel paint.
How do you determine the permeability of a material?
Permeability is a crucial property in chemical processes, affecting efficiency, safety, and quality. It is determined by the flow rate or pressure difference across a material sample, and is calculated using Darcy’s law. This article provides an overview of different methods of measuring permeability, their advantages and disadvantages, and how to choose the best one for your application. It is essential for chemical engineers to understand how to measure permeability to ensure the best results in various chemical operations. Experts who contribute quality contributions may be featured in the collaborative article.
What is class III perm rating?
Class III vapor retarders, such as gypsum board, fiberglass insulation, cellulose insulation, board lumber, concrete block, brick, 15-pound asphalt coated paper, and house wrap, are used to control moisture in various areas of a home, including foundations, ceilings, crawlspaces, floors, slab-on-grade foundations, and walls. The use of a vapor retarder depends on the climate and home construction.
📹 What are Vapor Barriers, Retarders & Perm Ratings?
In this video, I’m going to explain the science of vapor movement, the perm ratings system and rating of specific building materials.
Belinda, As always you have described the subject very well. Short introductions such as this are extremely useful, but I just wanted to elaborate on the rate of permeability. Diffusion of water vapour is, as you mentioned, dependent on the thickness of the material and the temperature at the time, but there is one other factor that should be considered and that is moisture content of a material. Wet masonry is less permeable, but conversely wet OSB board is more permeable, so the laboratory data given on the product is not necessarily true to in-situ performance. BTW the US perms are far better to understand than the Sd measurements (and others) we use in Europe.
Really looking forward to your follow-up with actual wall assemblies! I’d love to understand more about the way these assemblies should change when moving between climate zones. For example, I’d expect differences between Texas (Zones 2/3) and Seattle where I am (Marine 4). We deal with way too much moisture up here to have the same humidity considerations!
i think you should be teaching architecture, or maybe construction management. you cut through the material (no pun intended) that would put most people to sleep and give concise yet comprehensive information that people who are designing and building structures need to know, and you make it easy to understand even for a laymen.
1 perm = equalls 1 grain of water passing a plane in space the size of 1 gross of square inches in 3600 seconds. A grain of water?? a grain of sand, a grain of rice, wheat, ??? Just about enough to give me a migraine! I can barely fathom it even though that is about how tall I am. Man this U.S. unit system is a basket full. Belinda thank you for the attempt to shed light on this subject, some of the editors at Fine Homebuilding struggle with this. I do get some of it. Temperature/dewpoint spred can be used to estimate how high the clouds are above the ground (divide by 3F(U.S. sys) or 2C (metric sys) x 1000 (feet) or 300 (M)) So if clouds are forming in our walls we want that to happen at a point where they can eventually excape inward or outward I guess.
I enjoy your articles. I have a question. I live in central Florida. Doesn’t freeze. High heat/high humidity. My home is 100 years old with 11’ walls. I caulked all joints, edges & corners. Then painted 3 coats of quality paint. I painted the inside with quality paint. I have old double hung windows. I installed tight fitting wood/glass storm windows on the outside that can be opened from the inside. I installed press first wood/shrink wrap interior windows inside against the double hung windows. The house is up on piers. Air can freely blow under the house. I attached thick plastic film to the joists under the house and caulked. I have 18″ of insulation in the attic with a small ventilation fan to move the air outside. I don’t get air flow through my walls. Many salespersons have told me that replacing my double hung windows and blowing in insulation through the side walls will save me a lot of money. That the energy saving will pay for the cost in just a year. Last year I used my furnace for 17 hours. I used the air conditioner for 93 hours total for the year. I am comfortable with just ceiling fans. A friend with a new super insulated house needs to run his air conditioner most of the summer. Mostly to eliminate the humidity. What is going on? Is my home design superior to new construction. Are triple glazed windows and insulated walls worth it for?
I’m confused.There’s many suggestions to use xps/eps ridged board insulation against concrete foundations with batts insulation in front to bring r-value up. But it’s not recommended to use poly tarp against foundation walls with batts. Aren’t both systems using a non permeable barrier? Why is one recommended but the other not?
I would like a house without plastic vapour. So many codes require them. Natural materials. What the condensation dew pint calculation in the codes does not take into account is the phase change from gas to liquid energy requirements for this. This goes to the position of the vapour barrier. American houses all appear to have HVAC ventilation. Not common at all in Australia.
Something else I didn’t know I wanted to know that helps me learn a little bit more, even if I’ll forget it within a week. 🙂 Would cement or concrete walls be considered Class IV permeable? I ask because I used to live on Guam and we lived through a typhoon back in the ’70’s where the wind was so strong is pushed water through the walls! Thank you for another informative article. I look forward to any follow-ups you do, especially if you manage to slip in a shipping container’s wall example. 😀
I wish you covered ICF construction in this analysis. I have been wondering for a bit about this. Matt Risinger has a article where he talks about the importance of a wall to be able to dry from the inside out- on second thought I may not have fully grasped the concept in that article. I think I see the importance of that ability though. There is going to be moisture on the inside of a house from showers, cooking, human respiration, etc, and that needs to be managed. Im guessing in an ICF house that is mostly vapor impermeable, and this moisture could cause problems if not dealt with. I hope mechanical ventilation such as a HRV, or an ERV, or a dehumidifier would be up to the task. What are your thoughts?
Thank you for your thoughtful presentation. I have encountered a problem building an outdoor sauna in Seattle. Right now this small structure (67 sq. ft) has a slab on grade, walls framed with 2×6 studs, 1/2 inch CDX plywood sheathing covered with Tyvek, and a sloped roof covered with 11/2 in. CDX and asphalt shingles. The sauna would likely be used not more than five times a week year around, which would involve the 9 kw heater bringing the interior air temperature up to 180 degrees F over a 30 minuet time period, maintaining the temperature for 30 minutes, and then cooling. There would be no heating in the sauna at other times. I intend to use this primarily as a dry sauna, with relative humidity below 30% most of the time. A subsequent owner may decide to use it as a wet sauna where relative humidity could be near 100% for up to 30 minutes at a time. To assist in the cooling and dehumidifying the sauna after use, an exhaust fan will be operated for about 30 minutes, and, if necessary, a dehumidifier may potentially be added to the system. Sauna manufacturers routinely recommend filling the wall cavity and ceiling joist space with rock wool insulation and the use of a foil vapor barrier on the interior face of the studs and between the ceiling joists. The foil (with a perm rating of near zero) must be carefully installed with all seams taped with a special metallic tape. The foil is recommended to reflect radiant heat back into the sauna and to stop water vapor from passing into the wall cavity and attic where in cold weather it might be expected to condense on the interior face of the plywood sheathing and promote the growth of mold or cause wood rot.
In my zone they require a 6 mil poly with unfaced batts . I would not do this on my own house knowing what I’ve seen in renovations (mold) .I prefer a more permeable product such as typar (not tyvek too high perm rate). Living on the coast we typically get higher temps with higher humidity during winters then most zone 5 areas.
I live in a colder climate, Canada, and have a question about insulation/vapour. We build walls with 2x material, have insulation in between them, vapour barrier on the inside, sheathing and housewrap on the outside. If we install sheet insulation on the outside of that, will moisture get trapped in the walls?
Great info Belinda, Happy new year! We have solid wooden walls in my house and all the functions of vapour control, wind and rainproofing and insulation are well coped with for my subarctic climate, without any man made materials at all. The industry has not been kind to this building system here in Norway and it is cultural, instead of technical criteria that allows its continued use. A shame, since it is sustainable and makes for a fantastic indoor climate with good acoustics too. The calculation of R value is specifically designed to favour mineral wool insulation, but I find the true insulation properties of timber houses to be better than equivalent mineral-wool insulated houses here. The study of vapour movement is not new at all, the specific traditional dimensions of timber houses here have a lot to do with vapour movement and were perfected seven or 8 hundred years ago, the same is probably true for traditional buildings in Dubai..? Industrial companies make data that serves them, and their lobby power is formidable. We have a responsibility to give credit to our forbears where it is due, many ancient building systems are excellent and remain little changed due to them working. I have a film on my website about untreated wooden walls and how they weather if you are interested. all the best, Lucas