In a rational attic, we may now think about its ventilation.
Most people wouldn’t know there is anything wrong with the tidy attic ventilation over this well-built 1998 remodel of a solid 1912 bungalow home. It’s a sixteen foot wide bump-up of the roof along knee wall lines. There is the usual row of ~40 sq in NFA high vents, and misunderstood, insufficient soffit vents.
All vents are freshly vacuumed in this now-accessible attic.
Think whether these soffit vents are sufficient.
The rule book here is written by Joe Lstiburek: A Crash Course In Roof Venting
Aim for 1 sq ft net free ventilation area for each 300 sq ft of attic space, distributed 60% at inlet paths, and 40% at exit paths. For 700 sf attic area, there should be 1.4 sf in soffits, and 0.93 sf in roof peak vents.
Each soffit path is three holes 2 1/8” diameter, treated as 25% unobstructed by wire screens, net free area 8 sq in. There are five vent sets each side, ten total, 80 sq in total, 0.56 sf. Vs. 1.4 sf requirement, the found soffit vents are acceptable only if in every bay, rather than every fourth or whatever. A 250% area increase is needed at soffits. The five high vents are acceptable.
I think this pre-fab soffit vent from a local lumber supplier matches the job photo. Their invoice calls it simply Bird Block. It is sold in 22 1/2” lengths of 2x4 or 2x6, with holes the same size despite opportunity to be much larger in 2x6. It is here that I measure the 2 1/8” hole diameter. The screen is 1/8” mesh wire, without many staples. Birds do push and peck through this, to nest just above. Such screens are unacceptable. An exterior screen is better, for example Construction Metals FV0416G, 4”x16”.
A 250% area increase in only the existing vent locations is possible, and allows alignment with the exterior screen. Add holes up to 3” diameter. Chisel webs to connect holes. The goal is nfa of at least 21 sq in.
There are five roof peak vents, Norwesco 556042 38 nfa 7 in.
Five at 38 sq in nfa each, give total 1.3 sq ft, vs required 0.93 sq ft.
A Beaverton, Oregon home owner responded to birds-in-attic by adding 4”x16” exterior soffit vent screens, vents every fourth bay. This did nothing for insufficient attic ventilation.
I then enlarged each screen location to 21 sq in nfa, eleven places, 231 sq in nfa total, if all screen resistance is discounted. Where this attic is 600 sf, the enlarged soffit vents should add up to at least 600/300 * 1 * 144 * 0.4 = 115 sq in.
In a Damascus, Oregon attic, there was no ventilation despite two powered roof fans. Soffit vents were fully blocked by insulation batts and by foolish placement of roof paper. All found insulation, fouled by vermiculite, was hauled out and disposed. Each bay has two soffit hole sets, in series.
First, enlarge and baffle the attic ventilation paths at the attic edge. Found minimum path areas are 7.1 sq in in each bay, in two holes at 2 1/8” diameter. in 2x4 blocks of the attic edge, and again in screened holes of horizontal plywood soffits. I achieved negligible resistance at 2x4 blocks within the attic, expanding or adding holes.
Exterior soffit repair will be up to the home owner. Each existing exterior soffit hole might be enlarged to at least 2 5/8”. Or, add at least one more 2 1/8” hole each bay, to be hidden by the exterior screens. Cover perhaps-rough hole cuts with Construction Metals 4”x16” screens.
New holes, not enlarged or merged, are easier on the drill.
A “wheelbarrow” soffit vent baffle was invented for this job. This adjustably deals with misalignment and variability of roof and floor joist spaces.
Baffle, topside view. Set screws 1. Set handles 22” apart by gage, equalize angles and set screws (2). The 2x4 blocks at (2) wedge against the roof sheathing. All is secured with a 3" deck screw at each barrow handle.
I created storage in the attic of this year-2000 home in Happy Valley, Oregon, in 2011. Without doing the math on inlet capacity at soffits, I then consented to install a homeowner-purchased Costco solar-powered roof fan. Consider now whether I did right. Soffit vents seem to have 4x16 screens. I can’t discern the opening behind the screen.
I could rarely look over a soffit baffle, avoiding dangerous trekking and trampling through 9" deep cellulose.
Here is one photographed view of a soffit screen from inside, over the master bathroom, where I fixed bath fan and gas fireplace ducts. This is a fresh-air intake for the silly gas fireplace. Soffit cuts scale as 2” x 11.2”. If 25% screen-obstructed, they are about 17 sq in nfa.
Math shows the soffit vents of this year-2000 home in Happy Valley, Oregon are inadequate even for static ventilation, 1.9 sf, vs. required 3 sf. They are very inadequate where one Costco solar-powered fan is now running.
Innovation In Continuous Soffit Vents:
What is the required width of a continuous soffit vent? Such vents might be be interrupted at roof or floor joist crossings, but in a first approach, treat as continuous at the soffit outer edge, for the home example at Page 2.
A screened continuous gap 3/4” wide will provide ample attic ventilation, certainly enough to feed the potential draft of a solar-powered roof fan.
A stamped aluminum continuous vent sold beside the deficient Bird Block, by my visual estimate, is only cumulative 3/8” width, in a pattern of irregular slots, not screened.
A better continuous soffit vent:
This was found in a Lake Oswego, Oregon condominium complex, built 1970. The gutter is screwed to 2x6 backers spaced out 3/4” by 1x4 boards at 48” pitch.
Imagine a simple condominium unit 42 feet wide with these soffits full-length on both sides. Per foot of unit length, the soffit nfa is 12*2*0.75*0.6 sq in = 10.8 sq in. The floor area is 42 sq ft. Soffit nfa is 10.8/42 sq in per sq ft, 0.26 sq in per sq ft. Soffit nfa is 77 sq in or 0.54 sq ft for every 300 sq ft of floor area. This is very close to Joe Lstiburek's rule, 0.6 sq ft per 300 sq ft floor area.
At 3/25/2015, a thoughtful Portland home owner has called me for review of her soffit provisions for attic ventilation. Surely something is wrong. I will want her to know of this post, and of all of my writings, Label: Attic Ventilation .
I wonder what one finds today in a search: "fix attic ventilation inlets." Here is something new to me but of long standing, B. R. Stewart at Texas A&M, 1995. The report says:
Providing for Natural Ventilation
Natural ventilation caused by a chimney effect or by wind movement is the most economical ventilation method. The quantity of ventilation air depends on opening size, temperature rise and wind movement. Tests have shown that effective natural ventilation can be achieved by providing inlet and outlet open vent areas of approximately 1 square inch per square foot of attic area, when roof slopes of 3/12 to 5/12 are used. This vent area should be the net open area rather than gross vent size. Some prefabricated vent materials may have only 60 percent net open area. Thus; if a vent area of 10.4 square feet is required, the gross vent area would be 10.4 / 0.6 = 17.3 square feet. [National standards appear to require only 1/2 this amount of open vent area. The open vent area calculated by these formulas is to be divided roughly equally between inlet and outlet.]
Suppose a 1,500-square-foot home requires the above soffit vent area. If the total soffit length of the home is 100 feet, the vent width for a continuous soffit vent would be (17.3 x 12) / 100 = 2.1 inches. Ventilators should be purchased on the basis of net ventilation opening.
Roof overhangs of 12 inches or more provide ample space for inlet (soffit) vents. Outlet vents should have the same net area as inlet vents. This is easily provided by continuous ridge vents or individual roof vents for gable-roofed homes. The roof ridge length may not be long enough on a hip roof to use a continuous ridge ventilator. In this case, several individual roof vents, well spaced near the ridge, should be used.
One square inch per square foot, each at inlet and outlet is 300 sq in per 300 sq ft., 2.1 sq ft nfa at soffits for every 300 sq ft of attic floor area for moderately-pitched roofs. Joe Lstiburek said: Aim for 1 sq ft net free ventilation area for each 300 sq ft of attic space, distributed 60% at inlet paths, and 40% at exit paths. This more-modern specification is 0.6 sq ft at soffits for every 300 sq ft of floor area, 0.29 sq in per sq ft, a lot less! A B. R. Stewart rule has increase from Lstiburek's, by times 2.1/0.6 = times 3.5. Such soffits would be quite holey and a hard sell. Surely they would work better where a powered roof ventilator is added. I know the condominium with novel continuous soffit vents by Lstiburek's rule. The attic runs very hot absent a powered 14" roof ventilator. That fan is noisy and it runs about 1000 hours each year, drawing say, 200 watts. Electricity cost at 11 cents per KWH is $22 per year and rising. It would be better to have natural circulation although I have fully sealed the attic floor. The practical cure is to install smaller solar-powered fans that will be better-matched to soffit inlet area, and I will do that in time.
At May, 2017, add a better study of some very common birdblock:
Potential path air flow is regulated by the screen net free area, 20 sq in x 0.75 = 15 sq in.
This is often found in every fourth bay, 8’ apart for usual 24" oc framing. Such bird block is needed in every frame bay.