Water vapor flows thru building structures, resulting in a latent load whenever a vapor pressure difference exists across a structure. The latent load from this source is usually insignificant in comfort applications and need be considered only in low or high dewpoint applications. Water vapor flows from high to lower vapor pressure at a rate determined by the permeability of the structure. This process is quite similar to heat flow, except that there is transfer of mass with water vapor flow. As heat flow can be reduced by adding insulation, vapor flow can be reduced by vapor barriers. The vapor barrier may be paint (aluminum or asphalt), aluminum foil or galvanized iron. It should always be placed on the side of a
structure having the higher vapor pressure, to prevent the water vapor from flowing up to the barrier and condensing within the wall.
Basis of Table 40
- Water Vapor Transmission thru Various Materials
The values for walls, floors, ceilings and partitions have been estimated from the source references listed in the bibliography. The resistance of a homogeneous material to water vapor transmission has been assumed to be directly proportional to the thickness, and it also has been assumed that there is no surface resistance to water vapor flow. The values for permeability of miscellaneous materials are based on test results.
NOTE: Some of the values for walls, roofs, etc., have been increased by a safety factor because conclusive data is not available.
Use of Table 40
- Water Vapor Transmission thru Various Materials Table 40 is used to determine latent heat gain from
water vapor transmission thru building structures in the high and low dewpoint applications where the air moisture content must be maintained.
Example 8 – Water Vapor Transmission
Given:
A 40 ft × 40 ft × 8 ft laboratory on second floor requiring inside design conditions of 40 F db, 50% rh, with the outdoor design conditions at 95 F db, 75 F wb. The outdoor wall is 12 inch brick with no windows. The partitions are metal lath and plaster on both sides of studs. Floor and ceiling are 4 inch concrete.
Find:
The latent heat gain from the water vapor transmission. Solution:
Gr/lb at 95 F db, 75 F wb = 99 (psych chart) Gr/lb at 40 F db, 50% rh = 18 (psych chart) Moisture content difference = 81 gr/lb
Assume that the dewpoint in the areas surrounding the laboratory is uniform and equal to the outdoor dewpoint. Latent heat gain:
Outdoor wall = (Table 40.) = 10.4 Btu/hr Floor and ceilings =
= 259 Btu/hr Partitions =
= 777 Btu/hr Total Latent Heat Gain = 1046.4 Btu/hr
40x8 100 x 81 x .04 x 81 x .10 2x 40x40100 40x8 3x 100 x81x1.0
TABLE 40- WATER VAPOR TRANSMISSION THRU VARIOUS MATERIALS
PERMEANCE Btu/(hr) (100 sq ft) (gr/lb diff) latent heat
With With 2 Coats Aluminum
DESCRIPTION OF MATERIAL OR CONSTRUCTION No Vapor Vapor-seal Foil Mounted
Seal Unless Paint on on One Side
Noted Under Smooth of Paper
Description Inside Cemented
Surface* to Wall†
WALLS
Brick -- 4 inches .12 .075 .024
-- 8 inches .06 .046 .020
-- 12 inches .04 .033 .017
-- per inch of thickness .49 - - - -
Concrete -- 6 inches .067 .050 .021
-- 12 inches .034 .029 .016
-- per inch of thickness .40 - - - -
Frame -- with plaster interior finish .79 .16 .029
-- same with asphalt coated insulating board lath .42 .14 .028
Tile—hollow clay (face, glazed)--4 inches .013 .012 .0091
--hollow clay (common) )--4 inches .24 .11 .025
--hollow clay, 4 inch face and 4 inch common .012 .011 .0086
CEILINGS AND FLOORS
Concrete--4 inches .10 .067 .023
--8 inches .051 .040 .019
Plaster on wood or metal lath on joist—no flooring 2.0 .18 .030
Plaster on wood or metal lath on joist—flooring .50 .14 .028
Plaster on wood or metal lath on joists—double flooring .40 .13 .028 PARTITIONS
Insulating Board ½ inch on both sides of studding 4.0 .19 .030
Wood or metal lath and plaster on both sides of studding 1.0 .17 .029
ROOFS
Concrete--2 inches, plus 3 layer felt roofing .02 .018
--6 inches, plus 3 layer felt roofing .02 .018
Shingles, sheathing, rafters--plus plaster on wood or metal lath 1.5 .18
Wood –1 inch, plus 3 layer felt roofing .02 .018
--2 inches, plus 3 layer felt roofing .02 .018
MISCELLANEOUS
Air Space, still air 3 5/8 inch 3.6
1 inch 13.0
Building Materials
Masonite--1 thickness, 1/8 inch 1.1 .17 .027
--5 thicknesses .32
Plaster on wood lath 1.1
--plus 2 coats aluminum paint - - .12
Plaster on gypsum lath 1.95 - -
--ditto plus primer and 2 coats lead and oil paint - - .13
Plywood--1/4 inch Douglas fir (3 ply) .63
--ditto plus 2 coats asphalt paint - - .087
--ditto plus 2 coats aluminum paint - - .13
--1/2 inch Douglas fir (5 ply) .27
--ditto plus 2 coats asphalt paint - - .041
--ditto plus 2 coats aluminum paint - - .12
Wood--Pine .508 inch .33
--ditto plus 2 coats aluminum paint - - .046
--spruce, .508 inch .20
Insulating Materials
Corkboard, 1 inch thick .63
Interior finish insulating board, ½” 5.0 – 7.0
--ditto plus 2 coats water emulsion paint 3.0 – 4.0
--ditto plus 2 coats varnish base paint .1 – 1.0
--ditto plus 2 coats lead and oil paint .17
Part 1. Load Estimating | Chapter 5. Heat And Water Vapor Flow Thru Structures
TABLE 40- WATER VAPOR TRANSMISSION THRU VARIOUS MATERIALS (Contd)
PERMEANCE Btu/(hr) (100 sq ft) (gr/lb diff) latent heat
With With 2 Coats Aluminum
DESCRIPTION OF MATERIAL OR CONSTRUCTION No Vapor Vapor-seal Foil Mounted
Seal Unless Paint on on One Side
Noted Under Smooth of Paper
Description Inside Cemented
Surface* to Wall†
MISCELLANEOUS Insulating Materials, cont.
Insulating board lath 4.6 – 8.2
--ditto plus ½” plaster 1.5
--ditto plus ½” plaster, sealer, and flat coat of paint .16 - .31
Insulating board sheathing, 25/32” 2.6 – 6.1
--ditto plus asphalt coating both sides .046 – 1.0
Mineral wool (3 5/8 inches thick), unprotected 3.5
Packaging materials
Cellophane, moisture proof .01 – 0.25
Glassine (1 ply waxed or 3 ply plain) .0015 - .006
Kraft paper soaked with parafin wax, 4.5 lbs per 100 sq ft 1.4 – 3.1
Pliofilm .01 - .025
Paint Films
2 coats aluminum paint, estimated .05 - .2
2 coats asphalt paint, estimated .05 - .1
2 coats lead and oil paint, estimated .1 - .6
2 coats water emulsion, estimated 5.0 – 8.0
Papers
Duplex or asphalt laminae (untreated) 30-30, 3.1 lb per 100 sq ft .15 - .27
--ditto 30-60-30, 4.2 lb per 100 sq ft .051 - .091
Draft paper--1 sheet 8.1
--2 sheets 5.1
--aluminum foil on one side of sheet .016 --aluminum foil on both sides of sheet .012 Sheathing paper
Asphalt impregnated and coated, 7 lb per 100 sq ft .02 - .10 Slaters felt, 6 lb per 100 sq ft, 50% saturated with tar 1.4 Roofing Felt, saturated and coated with asphalt
25 lb. per sq ft .015
50 lb. per sq ft .011
Tin sheet with 4 holes 1/16 diameter .17
Crack 12 inches long by 1/32 inches wide (approximated from above) 5.2
*Painted surfaces: Two coats of a good vapor seal paint on a smooth surface give a fair vapor barrier. More surface treatment is required on a rough surface than on a smooth surface. Data indicates that either asphalt or aluminum paint are good for vapor seals.
†Aluminum Foil on Paper: This material should also be applied over a smooth surface and joints lapped and sealed with asphalt.
The vapor barrier should always be placed on the side of the wall having the higher vapor pressure if condensation of moisture in wall is possible.
Application: The heat gain due to water vapor transmission through walls may be neglected for the normal air conditioning or refrigeration job. This latent gain should be considered for air conditioning jobs where there is a great vapor pressure difference between the room and the outside, particularly when the dewpoint inside must be low. Note that moisture gain due to infiltration usually is of much greater magnitude than moisture transmission through building structures.
Conversion Factors: To convert above table values to: grain/(hr) (sq ft) (inch mercury vapor pressure difference), multiply by 9.8. grain/(hr) (sq ft) (pounds per sq inch vapor pressure difference), multiply by 20.0 To convert Btu latent heat to grains, multiply by 7000/1060 = 6.6.