What is a good perm rating for vapor barrier?

A perm rating of less than 0.1 is considered a Class I impermeable vapor retarder (which is also considered a vapor barrier); perm rating between 0.1 and 1 is considered a Class II semi-permeable vapor retarder; a perm rating between 1 and 10 is a Class III permeable vapor retarder; and a perm rating greater than 10 is …

What is the recommended perm rating?

According to the International Code Council (ICC), a housewrap must have a perm rating of at least 5. Higher perm numbers do not necessarily mean the housewrap performs better.At a certain point, the perm number becomes irrelevant and will perform similar to its lower perm number counterparts.

How much water is a perm?

Well, a US perm is one grain of water vapor per hour per square foot per inch of mercury. In metric it is 57.2135 nanograms per second per meter squared per Pascal (57.213 ng·s·m²·Pa).

How do you calculate water vapor transmission rate?

The water vapour transmission rate (WVTR) is the rate of water vapour permeating through the film. In this work, the WVTR is determined from the slope of the regression line of sample weight versus time graph whereby the slope is then divided by the area of the film being exposed to the transmission (Equation 10).

How many perms do I need for a vapor barrier?

So, according to the Building Science Corporation, “A vapor barrier is defined as a layer with a permeance rating of 0.1 perm or less.”

When should you not use a vapor barrier?

You may find that vapour barriers are often not required in warmer climates. And, if installed in the wrong climate or on the wrong side of building materials, a vapour barrier can cause more harm than good. This circumstance may prevent water vapour from drying, which in turn can cause rot and mold. (Source: Dupont.)

What is Class 1 vapor barrier?

Class I. Class I covers materials most frequently referred to as vapor barriers. These vapor retarders have a permeance level of 0.1 perm or less and are considered impermeable. Examples include polyethylene film, glass, sheet metal, foil-faced insulated sheathing and nonperforated aluminum foil.

How do you calculate a perm?

Perm Calculations You multiply the highest 3 numbers by the bet you want, then multiply the lowest numbers, 1 x 2 x 3 = 6, and divide the smaller number from the larger number, which equals 10.

What is a good water vapour transmission rate?

WVTR is the standard measurement by which films are compared for their ability to resist moisture transmission, with lower values indicating better moisture protection….Relevance to package performance.

What is a good MVTR?

The amount of protection is indicated in part by the material’s MVTR, which is typically expressed as g/100 in2 /24 hrs. Currently, the best barrier materials on the market today have an MVTR of <. 02 g/100 in2 /24 hrs.

What is class 2 vapor barrier?

Class II vapor retarders have a permeance level between 0.1 perm and 1 perm and are considered semi-impermeable. Examples include unfaced expanded polystyrene, fiber faced polyisocyanurate and asphalt-backed kraft paper facing on fiber glass batt insulation.

How big is a perm of water vapor?

Well, a US perm is one grain of water vapor per hour per square foot per inch of mercury. In metric it is 57.2135 nanograms per second per meter squared per Pascal (57.213 ng·s·m²·Pa).

What do you need to know about a perm?

A perm is simply a unit of measure for the amount of water vapor that will pass through a given area of a material over a period of time when there is a static vapor pressure difference between the two

Which is a measure of the water vapor transmission rate?

The moisture vapor transmission rate (MVTR), also called the water vapor transmission rate (WVTR), is a measure of how much water vapor mass can penetrate an area of a material in a given time period, so is given in units of mass per area per time.

How is the permeance of water vapor determined?

Vapor Drive Permeability is a property of a material. Permeance depends on the thickness of the material and therefore is a performance evaluation and not a property of a material. Permeability = Water Vapor Permeance X Thickness Permeability should only be calculated if the test specimen is homogeneous (not laminated) and at least ½” thick.