1/6/2012. Attics: Ventilation. -- Tony Gill Simonson Management Services --

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1 Attics: Ventilation Jack Martin, Bucks County Community College -- Tony Gill Simonson Management Services -- 1

2 Purpose of Attic Ventilation Photo Source: U.S. Department of Energy Removes solar heat during hot weather. Removes moisture vapor during cold weather. 2 2

3 Passive Attic Venting: Convection How does it work? Natural Convection Ridge Vent Outlet (Heated Air) Warm air rises, escapes high h vents Inlet (Cool Air) Cooler air is pulled through soffit vents Heats, rises, escapes ridge vents 3 3

4 Passive Attic Venting: Wind How is it supposed to work? WIND Bernoulli Principle Positive pressure on windward d side Negative pressure on leeward (protected) side Air is sucked out leeward vents 4 4

5 Passive Attic Venting: Wind How does it work? WIND Bernoulli Principle Positive pressure on windward d side Negative pressure on leeward (protected) side Air is sucked out leeward vents 5 5

6 Power Venting How does it work? Fan-Powered Roof Vent Attic fan mounted in central hallway used in non-air conditioned homes. Roof vent (pictured) used in air conditioned homes. Neither is recommended as weatherization measure. Potential for moisture condensation is high. 6 6

7 Code Issues Based on presence of ceiling vapor retarder Thermal Boundary - Ceiling No vapor barrier: 1/150 One square foot of net free vent area per 150 square feet of attic. Vapor barrier: 1/300 One square foot of net free vent area per 300 square feet of attic. 7 7

8 Applied Ventilation Formula Calculate the number of roof vents required for this ranch home with a gable roof. Ceiling is air sealed and insulated. Ceiling has vapor retarder. 60 x 30 ceiling area. Each vent has a net free area of 60 square inches. How many roof vents does it take Photo Source: U.S. Department of Energy 8 8

9 Applied Ventilation Formula Ceiling = 60 x 30 = 1,800 square feet. Formula for attic with vapor retarder = 1/300. 1, = 6 sq. ft. required NFA ventilation. Convert to inches: 6 sq. ft. x 144 sq. in./sq. ft. = 864 square inches square inches NFA/vent = Vents! 9 9

10 Ground Rules and Specifications Vented attics should not communicate with the conditioned space. - The ceiling air barrier must be continuous with no leakage. - Specify appropriate attic sealing as part of the work scope. Specify that vents be placed low and high on roof. Specify eave chutes when soffit vents are present and loose fill insulation is called for. Specify that all mechanical ventilation ducts and plumbing stacks are vented directly to the outside

11 Venting Configurations In a perfect world. Continuous Ridge Vent Continuous Eave Vent (Soffit Vents) 11 11

12 Real World Vent Placement Roof vents placed high on roof Continuous Soffit Vent Photo Source: Pennsylvania Weatherization Training Center 12 12

13 Vent Placement Gable vents are attached similarly to roof vents. Photo Source: U.S. Department of Energy 13 13

14 Attic Window Vent Photo Source: U.S. Department of Energy 14 14

15 Eaves Chutes Eave Chute Photo Source: Pennsylvania Weatherization Training Center; Product photo by J&R Products, Inc

16 Fiberglass batt as air barrier? Thanks to Chuck Prauza! 16

17 Air enters the soffit venting, travels up the rafter bay & exits the ridge vent, right? Sure, some of it does. But some simply follows the floor stringer bay across the house to exit the soffit venting on the other side! SO WHAT? Conductive heat loss is a function of time, t emp diff, & area. Allowing the floor & ceiling to see exterior air greatly increases the area the home s heating plant must warm. 17

18 How about a ranch style or other low slope roof design? Ice dam This is a most difficult insulating/air sealing task. You must pull back any existing insulation & install rigid blocking over the wall plate. It must be sealed to the dry wall below but can not block air movement up the rafters. Insulation baffles are required. Finish by blowing the cavity full of cellulose. 18

19 Tight fit! 19

20 Bendable bat wing Bendable bat-wing rafter chutes often make your life easier! 20

21 Code Issues Thermal Boundary Roof Deck Attic ventilation not required when the roof assembly is the thermal and pressure boundary

22 Hot Roof Hot Roof simply means the insulation is directly in contact with underside of the roof deck. There is no venting. The technique is used when: The roof cavity is too small to allow venting & sufficient insulation The roof design (i.e. Mansard, cathedral, cross perlin post & beam, intersecting dormers) doesn t allow adequate venting. The attic can not be airsealed from the living space Guess what? Done right, it s code approved! 22

23 Conversion vented attic to hot roof 23

24 Ell attic same house 24

25 Common Misconceptions Attic ventilation always removes moisture vapor during cold weather. Attic ventilation will cure an attic moisture problem. The more attic ventilation, the better. Attic ventilation is not necessary at all

26 Why doesn t venting always work? INSULATING AN ATTIC WITHOUT AIRSEALING IT - WILL NOT AFFECT THE VOLUME OF AIR MOVED INTO THE ATTIC BY STACK EFFECT! The driver is the ΔT, (living space to outdoors). It s the same. The path the holes hasn t changed either. The same pressure over the same holes will move the same amount of air. THE SAME AMOUNT OF WARM, MOIST AIR WILL CONTINUE TO ENTER THE ATTIC FROM THE LIVING SPACE IN SPITE OF THE ADDED INSULATION. 26

27 Why not? INSULATING THE ATTIC WILL LOWER THE ATTIC TEMPERATURE BECAUSE THERE IS LESS CONDUCTIVE HEAT LOSS FROM THE HOUSE TO WARM IT. ATTIC SURFACES PARTICULARLY THE UNDERSIDE OF THE ROOF DECK - WHICH MIGHT NOT HAVE BEEN COLD ENOUGH TO ALLOW CONDENSATION & SUPPORT MOLD GROWTH PRIOR TO INSULATING NOW WILL BECAUSE THEY HAVE BEEN COOLED TO BELOW THE DEW POINT! 27

28 Add venting? THE CONVENTIONAL SOLUTION -ADDING MORE ATTIC VENTILATION WILL MOST LIKELY CAUSE EVEN MORE CONDENSATION IN THE ATTIC BECAUSE THE ATTIC WILL NOW SUCK HARDER ON THE HOUSE, INCREASING THE PRESSURE ON THE EXISTING HOLES & BRINGING EVEN MORE WARM, MOIST AIR UP FROM BELOW. AIRSEAL THE ATTIC! 28

29 Air sealing matters! Keeps moisture out Keeps pollutants out Keeps heat in Reduces draftiness Increases comfort Makes reliable mechanical IAQ venting possible! 29

30 Determine connectedness and existing net free with ZPD B/Z 1433 Z/O 826 T/P 655 cfm 30 30

31 Real life proof! Mold on Sheathing Frost on Sheathing Photo Source: Pennsylvania Weatherization Training Center Photo Source: Pennsylvania Weatherization Training Center Both conditions occurred after attics were insulated and vented, not before

32 Two smokers; balloon framing 32

33 A puzzler! N Main house Sun porch Family room El l unheated shed Barn 33

34 Summary Keep moisture out of the attic by air sealing. Vent attics where possible and practical. Follow 1/150 and 1/300 code guidelines. Place vents high and low. Use the right vent for the job. Avoid turbine or power vents. Don t expect vents to solve an attic moisture problem