My first trial of hard-covering a well-insulated skylight is continuation of work reported in this blog post:
Daring To Collapse and Rebuild Crummy Loose-Fill Fiberglass Insulation
This skylight in a far corner was reached, with displacement of the surrounding floor insulation, when an adjacent bath fan was replaced. A non-contact draping of R11 kraft faced batts, has had no insulation value.
Here is framework attached to the skylight. Employ 1/2" plywood and 2x2 to enclose from above, with a 7 1/2" insulation thickness and a minimum of framing thermal shorts.
Note materials now ready to clad the skylight. I have stripped the kraft facing from found insulation and will set it to fill the 1 1/2” depth of side on-flat 2x4. Rumpled kraft facing prevents intimate contact against drywall and is harmful, absent a base layer of conforming loose-fill. Facing is select 3/8" plywood. The sides are from one diagonal cut of 24" width plywood. The face has attached 2x2 edge nailers.
All found insulation is reset, stripped of kraft facing. A full covering of new R25 batts is needed to fill the space of framing for the plywood covering.
Done at the skylight except for a third layer of insulation on the floor, R25 covering all 2x4 truss bottom elements, floor total about R46. For now, the floor is generally R10, and demanding improvement.
Is this what a skylight should look like, from the attic? Know skylight shaft area near roof sheathing is still not insulated. I could stuff some in above the plywood. Should I? I won’t, protesting that skylights in shafts are not windows, a bit inefficient and weird now. I will prefer LED lighting.
Plywood temporarily set on these truss bridges to the skylight, has been taken away. I will use access under these bridges to complete the sealing and insulation of the attic floor, before fitting and setting a permanent walkway. Home builders should make their work easier, and access safer for anyone, by including such level walkways. The bridges and an anchored walkway are beneficial to truss stability.
Suspicion about insulation at the back wall of the skylight was justified. A negligible kraft faced R11 batt was set, not in contact with the drywall and again having no insulating value. A careful cutout, full stuffing of insulation and drywall patch with flexible grout took about two hours, of nine hours total spent in insulating the skylight.
Payback Math For R30 Attic Wall Insulation: This skylight has insulation area 30 sf, of total 220 sf attic walls. The cost of all attic wall insulation was $890; and that of the skylight was $175. Deduct a $.30 per sf rebate, $66, for all walls net cost of $824. Savings for insulating 220 sf from zero to R30 are $2.4 * 220 * (1/3-1/33) = $160 per year. Payback time is 824/160 = five years. R30 hard-covered insulation of attic walls is a very good investment.
Please find details of all attic wall insulation in this home, with concurrent attic floor sealing, in a pdf photo album:
Insulation and Hard Covering of Attic Walls
This attic was improved after the home owners were inspired by my display and message at the Portland, Oregon 2014 Better Living Show . Recent installation of a new bath fan had been an unpleasant experience of hostility in darkness above a silly attic hatch. An excellent and very beautiful attic ladder enabled many more improvements. Just pull down, to deploy the ladder. The entire investment of $3925 will be repaid by electricity and winter heat savings alone, in seventeen years. The cost of the investment, and more, is fully added to home resale value. Insulation improved from R18 to R38 is largely protected by hard covering, as should be required of all insulation.
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.
I really like this remodel. Attic-wall closets for the lower roof not raised, should not be ventilated. The closet to left, facing South, will run hot. Just well-insulate its floor and attic wall.
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
http://www.finehomebuilding.com/PDF/Free/021221068.pdf
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.
