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Prevent critters/bugs from entering attic soffit vent area

Prevent critters/bugs from entering attic soffit vent area

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  #1  
Old 10-04-15, 10:01 PM
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Prevent critters/bugs from entering attic soffit vent area

In modern construction, and likely my own home, I am planning on using a product similar to this : vent baffle.

I will have soffit (solid and vented) to the required calculations so my intake vent net free area matches my exterior ridge vent net free area. However, I have a concern that the soffit may work loose or fail over the years and that critters, bugs, spiders will enter into the attic from the soffit area and up the attic baffle chute and nest in the attic unbeknownst to me. Is the a common concern? Is there a simple way to prevent this?

I found that 'back in the old days' builders would install what was called bird blocking between the trusses. They would install a 2x4 with 2 or 3 holes cut in it, with screening stapled to the inside to allow ventilation but prevent birds from entering and nesting. This would cause a problem when blowing in cellulose to a desired R-38 because it would easily cover those blocks with the vent holes. Now, most modern construction no longer leaves the eaves / truss tails exposed and instead calls for vinyl or aluminum soffit with perforated pieces for the venting requirements. I can see this being nice for 10 years or so, until a nail or screw works loose and flaps in the wind and makes it easy for something to enter the soffit.
 
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  #2  
Old 10-05-15, 05:28 AM
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On the front of my cape I installed continuous vented vinyl soffits (12" wide) so had more than enough NFA. Before it went up into the vinyl channel I covered the entire bottom of the soffit area with screening, stapled and siliconed in place. The reason for this (For me) was to block all of the insects that can easily get past the standard perforated vinyl material.

I haven't seen a lot of 10 year failures and would consider that normal maintenance. But soffits are a pain as once a critter enters they have access to all rafter bays. And it may not be the soffit point they enter, but mice will head there to build their potty.

Batts, mice, birds, hard to keep them out.

Note, if you add screening over any air path it will change the NFA.

Bud
 
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Old 10-05-15, 07:25 AM
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So Bud, did you leave the truss cavities open from soffit to interior of attic or did you use something like I posted, or even build bird blocking?

Also, wouldn't you want to keep your intake balanced with exhaust nfa?
 
  #4  
Old 10-05-15, 08:23 AM
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The baffles provide space above them for air flow (and critters) directly below the bottom of the roof. The tail portion of that baffle is a wind block to keep the incoming wind/air from going directly through the insulation. In my case I detailed rigid insulation over the top of the top plate and provided a path to the existing joist cavity above the inadequate insulation (another part of this project).

But no bird blocking as I HOPE I have addressed all of the potential access points.

As for balancing the high and low ventilation, since you are in a colder climate, they would recommend more lower than higher. BUT, the guidelines are ancient and have evolved without any testing or additional thought. The trade off involves the shift in attic pressure that increases or decreases the air leakage from house to attic through the leaks in the ceiling below. if you have aggressively addressed those leaks the balance can differ considerably, 75/25 to 25/75. If you have not taken care of ALL of the leaks then try for more lower NFA then higher.

Bud
 
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Old 10-05-15, 10:55 PM
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Bud, stapling rolls of screendoor screen (most available option) between each truss cavity would be quite the chore. I am hoping I can just avoid this problem by proper soffit installation and upkeep. Issue would be with the house sides that are on the walkout basement...what a difficult task it would be to work on soffit at those points. If I had truss bottom chords that stuck out level with the top of my wall, I could attach screen mesh there, but since the truss tails are the only thing that sticks out past the wall, they are at the slope of the roof and would be complicated to work with.

We talked before about air sealing my walls (caulking the plates, and between every stud and interior side of osb joints). My ceiling would be addressed with caulking the plates, and making sure my light boxes are sealed to the drywall. I'd hate to have a pressurized attic that always took conditioned air to achieve the ventilation. Are you suggesting more intake less exhaust won't cause this problem?
 
  #6  
Old 10-06-15, 06:26 AM
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You will always have a pressurized attic when there is a temperature difference between inside and outside and so does your house. In winter there is a positive pressure at the top and a negative pressure at the bottom, all with respect to outside. Since the house experiences the same, the ceiling plane has a positive pressure on the house side and a negative pressure on the attic side, thus any leaks experience a small air flow. Note, these pressures are very small, but when we can easily reduce those leaks we do save, but those savings are not worth a major renovation.

With more intake the attic pressures shift with less pressure across the larger vent area and more pressure across the larger vent area, intake and exhaust air flow must balance. But the resulting lower pressure at the attic floor slightly reduced the pressure difference across the ceiling.

These differences are minor and nothing to lose sleep over. I point them out only to emphasize the 50/50 guideline does not need to be followed perfectly and if there is a balance other than 50/50, which way would be better. Note, when you kick the ac on in the summer the house pressures reverse, low at the top and high at the bottom, while the attic remains the same. The result is a reduced pressure across the ceiling. All of this analysis is far more appropriate for new construction where we actually have some control. With a retrofit, we do our best and that is just fine.

The ventilation resulting from these pressure differences is only part of the total ventilation as wind is a major contributor.

As for losing conditioned air to the outside, all houses leak or are deliberately ventilated to meed a desired air exchange. Where you are concerned about a little conditioned air escaping into the attic, you house is losing lots of air everywhere else. A typical home must exchange all of the inside air every 3 to 4 hours. Less causes air quality issues. A tight home that requires 5 hours for a full replacement will require a ventilation system to make up the difference.

Are you confused yet?

Bud
 
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Old 10-06-15, 06:50 AM
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Bud, sounds like I should vent with baffles on every truss space but control the intake more with the amount of vented soffit I use. Any tips to keep critters out when my truss tails come down at a slope rather than having a horizontal bottom chord to staple screen on? Just staple it at an angle to the truss tails instead?

In terms of air sealing, I prefer a passive system for venting and not an air handler. Should I hold off on construction adhesive used (cheaper) to seal all my gaps and cracks in the framing of exterior walls?
 
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Old 10-06-15, 07:29 AM
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I have one section of truss tails where I covered the bottom with 1/2" plywood and drilled 2-2" holes at each rafter bay. I cut between those holes to create an elongated slot. I then covered the full length of this row of slots with a 3" wide vent strip sealed at both ends. The holes on the vent strip are small enough to not have to worry about insects getting in.

Seal everything you can as it is next to impossible to get an existing home too tight. Basically, there are good leaks and bad leaks, if a leak can ever be considered good. But leaks that allow infiltration in through your insulation bring along dust, pollen, and other outside pollution. If it came to the point you needed more fresh air, you can also use these natural pressures to bring in fresh air through a filter system.

You can have a blower door test done to put a number on the air exchange, but mother nature will give you her own indication. A house that is too tight will have high humidity in the winter and the windows will fog up on cold days. Of course that also depends upon the windows. But again, becoming too tight would be very rare and require far more work than you have on your list.

Bud
 
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Old 10-06-15, 09:11 AM
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Bud, do you have any photos of that soffit you made?

I know there is a lot of controversy about sealing bottom plates against the subfloor, top plates to each other, and on each side of studs. There are long horizontal gaps in my OSB that I can see my housewrap from the inside through the gap. They range from 1/8" to maybe 3/16". I am tempted to run a bead of sealant and fill the gaps. This would cause dispute with those who believe it impedes the sheathing's ability to a.) breath or b.) expand/contract. It seems silly to use all the construction adhesive to seal my plates, each side of studs against the osb, all trimmers, sills, headers to the sheathing, but to just leave the horizontal gaps in the 7/16 OSB.
 
  #10  
Old 10-06-15, 10:27 AM
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Not sure what I have attached.

Name:  IMG_0200.jpg
Views: 8465
Size:  30.7 KB

Yaa It made it. Now I have to write down how I did that.
Dinosaurs can't jump.

Bud

And here is the vent strip I covered the openings with:
http://www.lowes.com/pd_16592-228-SV...d=3015100&pl=1
 
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Old 10-06-15, 06:46 PM
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Re the soffit photo I'd be interested to know the kind of 3" wide vent strip you used over the inside surface, how you attached it, and how you "sealed at both ends".
 
  #12  
Old 10-06-15, 07:14 PM
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Suobs, the link above is basically the vent strip I used, although that picture shows a slight curl on the left and right flange. Mine were flat. I ran mine over the bottom of the soffit and used small ss screws about every foot. I will add more at about 6" intervals after I cover the bottom with metal trim.

To close the ends I cut a corner out of each side to leave a 3/8" tail and then heated the bend point and folded that tail down to close the opening.

With that vent strip, one could install 2 pieces of plywood for the soffit leaving a gap that the strip would either cover or fit into. I chose to cut the slots to keep the plywood all one piece.

Bud
 
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