Keeping bugs and birds out of vented attic
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| Posted in General Questions on
I’m building a house in central Arizona with open eaves, 24″ overhangs, a 4:12 roof pitch, standard 2×4 trusses, and a vented attic. I like the AccuVent baffles, which block off the openings between the trusses, provide a ventilation channel above the 11″ of cellulose I’ll be blowing in, and allow the cellulose to cover the entirety of the top plates pretty much to the full height of the truss heels, which would give me around R-14 there as compared to R-38 where the cellulose will be at full depth.
1. Without soffits, how might I keep birds and especially bugs from getting in? I know many builders install wood blocking with screened holes or a venting mesh on top, but the AccuVent provides blocking, and the wood would not allow the insulation to cover the whole top plate nor as much air flow. The only solutions I’ve come up with are to attach galvanized wire mesh to the outside of each truss bay or to attach aluminum screen across the undersides of the exposed truss tails all the way along the eaves. Attaching 48 separate pieces of mesh from the outside would be a pain and fairly funky looking. The screen would be easier, but even funkier, and birds might damage it if they could manage to do so from below in mid-air. Is there a better way?
2. The overhang drops low enough to block most of the wind from reaching the eave openings, but not as much as the vented soffits that are usually used with AccuVent. Do you think the AccuVent would get blown loose? I’ve seen at least one discussion and article here about site-built baffles, but at the rate I work, it would probably take me a month (literally)!
3. I decided against soffits because they added more expense and trouble than seemed worthwhile, especially as horizontal soffits for a 24″ overhang would require their own framing support. One easier and less expensive option I’ve just considered is a soffit of OSB or plywood that attaches directly to the undersides of the truss tails, which are a little longer than 24″ because of the angle, so that if I attach 24″ strips of OSB or plywood, they would leave roughly a 1.4″ gap at the fascia or the house end where air could enter through wire mesh. Does this sound feasible?
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Replies
Jeff,
I do a lot of houses with exposed rafter tails. The detail I've settled on is to stop the finished blocking between the trusses or rafters 1 1/2" below the roof sheathing (usually 2"x t&g if visible) and attach a U shaped piece of perforated flashing above to block the opening.
The easiest way I've found for doing wood soffits is to run the vents the same way as the boards. That way no additional framing is required. You just fasten them to the rough fascia and a 2"x4" ledger on the wall, and periodically use strips of the same perforated metal
Thank you, Malcolm. The photo helps a lot; I'm afraid it's well beyond my time and materials budget to do anything that looks that good, not to mention that the rest of my house would be racked with envy! Sometimes my questions would be best suited to ReallyInexpensiveAndPrettyFunkyButStillGreenBuidlingAdvisor.com, if there were such a thing. For your exposed rafter tails, is your blocking taking up 1-1/2" of the top plate's upper surface area?
Jeff,
I've got a meeting to go to tonight, but either later or in the morning I'll scan a section from one of my drawings and post it.
Thank you, Malcolm.
Jeff,
Here is the detail. This one had rafter tails applied to the trusses and dropped so that there was t&g visible at the overhangs not the sheathing.
Thank you very much for the drawing, Malcolm. Having the blocking angle outward like that is a great solution. I got a very helpful reply from AccuVent to my email asking whether it might blow loose if applied in an open eave; the engineer there recommended extra staples, but cautioned that blown-insulation might not offer enough additional support to help resist wind pressure. AccuVent looks to be the best prefab baffle, so I'm hoping to find a way to make it work. What do you do for a baffle? Your blocking would greatly reduce the wind pressure on the blocking part of the AccuVent, but the wind coming through the metal vent above the blocking would enter the AccuVent fairly strongly.
If I decide to install a vented soffit by attaching OSB or plywood to the undersides of the truss tails with a strip of wire mesh for ventilation, which of those materials would be better, or is there another choice that would be comparably inexpensive and quick?
Jeff,
Most of my projects are either raised-heel trusses, or dimension lumber which often don't need any baffles. Unless you are using an assembly where the baffle has insulation directly against it for an extended length, I'm not sure what type you use makes much difference. I'm also beginning to doubt whether air-sealing the baffles makes much difference either.
We are luck are in the our lumberyards here supply perforated metal vent in any shape you specify for a variety of uses - like the bottom of rain-screens or between trusses. They work well as protection from both pests an mechanical damage. A lot of the older buildings I've worked on relied on insect screen. Almost invariably over time it gets damaged, and then is very difficult to replace.
I completely understand not using expensive materials for the sake of it. My only caution would be that sometimes using things like OSB for exterior exposure can provide a false economy of savings if they need replacing pre-maturely later on.
Thanks again, Malcolm.
The wire mesh I would use with a vented soffit is galvanized and much heavier than window screen.
I'm sure the wood you used for your soffit is well worth its cost in relation to the overall quality of your building, and it certainly looks much better than OSB or plywood. I get what you're saying about avoiding replacement later on, but it seems to me that the OSB attached to the undersides of the trusses shouldn't be much more exposed than the OSB on top of them, especially if I install the mesh at the outer edge. It will never see the sun nor any rain, what little we have, and I can't imagine much condensation either.
Jeff,
The other (perhaps unfortunate) part of the equation when you are choosing finish materials is that because of the present exorbitant cost to build a house, it becomes a huge part of your total financial worth. Much as we all want to build to suit ourselves first, it's also a good idea to keep an eye on that time when you will sell to someone else. Depending on the region, the average tenure of a homeowner is between seven and thirteen years.
Good and important point, Malcolm, and I agree. Mine will be one of the most energy-efficient and healthiest houses in the area, and if I ever sell it, that will probably have to outweigh its many deficiencies in the finish realm.