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Attic Ventilation Challenge

Posted by leeindallas (My Page) on
Sat, Mar 21, 09 at 2:10

Hi,

I'm wondering if anyone might have any advice for an attic ventilation challenge I have. I'm in North Texas where summers often exceed 100 degrees. I'm really worried about the damage poor ventilation is going to do to a newer roof.

Background:
My house was built in '91 with a ridge vent and no intake vents. I got the roof replaced last year as part of the purchase of the house. Realized immediately that there was a problem with not having enough intake air to the attic. Needed this fixed last summer, but never got around to it. Determined to get it corrected this spring.

Attic Construction Details:
Hip roof with ridge vent the entire length of the peak.

Two sides of the house (back and one side) mostly do not have insulation blocking air to the soffits. Adding an adequate number of 8x16 aluminum soffit vents will not be a problem here.

It's the front and another side of the house that's the problem. (Sides of house are about twice the length of front/back.) Vaulted ceilings along this side have insulation about 1/3 the way up the roof. I haven't tried, but suspect that trying to fish rafter bafles [http://www.ornl.gov/sci/roofs%2Bwalls/insulation/fact sheets/attic floors.pdf] would be impossible due to long protruding roofing nails.

Question:
What is the best way to correct the ventilation problems in this space? I'm ruling out rafter baffles. Assuming that power vents would interfere with the passive ventilation that already exists. SmartVent [http://www.dciproducts.com/html/smartvent.htm] doesn't help me since my problem is not a lack of soffit but rather blocked space between the soffit and the attic.

Placing an intake booster on the side of the house with the blocked soffits seems like a good idea in theory - Especially because of how my attic is constructed. Truss webs have 2-foot wide particle board running along each side of the house for support. This creates tunnels that inhibit air circulation even more.
[http://www.flickr.com/photos/83143405@N00/3371102989/]
[http://www.flickr.com/photos/83143405@N00/3371923974/]
[http://www.flickr.com/photos/83143405@N00/3371924004/]

Placing an intake booster [http://www.canyonrock.com/Content/Documents/21202.pdf] in the tunnel might push static air out and over the tunnels to mix with the moving air in the attic? At least it seems like a reasonable expectation. Biggest concern would be possibly introducing moisture if fan is running while it's raining.

Another problem I'm having is a garage that has zero ventilation. [http://www.coolmygarage.com/] I'm also thinking about purchasing a garage power venting solution that might help kill two birds with one stone.

Thanks in advance for any thoughts you might have.


Follow-Up Postings:

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RE: Attic Ventilation Challenge

Hello,
I need more informations about your house to help you.
I live in Europe, but I know american ways of house constructions.
This is a special part of knowledge.
Maybe you can ask any structural engineer of your area?


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RE: Attic Ventilation Challenge

Hello, Margi.

Thank you for your long distance reply. You mentioned you might need more information to help. I've added a few more pictures to show the roofline and where soffit vents can be placed. This will give a better idea of my airflow limitations within the attic.

http://www.flickr.com/photos/83143405@N00/3373101142/

http://www.flickr.com/photos/83143405@N00/3373101158/in/photostream/

http://www.flickr.com/photos/83143405@N00/3373101188/in/photostream/

http://www.flickr.com/photos/83143405@N00/3372282575/in/photostream/

Thanks also for the suggestion to seek out professional advice. After more research and gathering of advice online, I may consult with a professional.

Regards,

-Lee


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RE: Attic Ventilation Challenge

Lee,

now I see the problem.

You have big area of roof, this area between roof and slab doesn't have isolation, this is almost closed area and isn't well ventilated.
You have 2 ways to solve your problem.
First - you have to enforce more speed and bigger exchange of air in this space.
Second, this which is used in more cold areas( for example in Poland where I live), is isulations not at slab but under a roof ( but with mim 1,5-2 inch airspace between isolation and top side of truss or rafter).
You can find a person which is a member of ASHRAE (The American Society of Heating, Refrigerating and Air-
Conditioning Engineers). This person probably will know building standards of your area.

If you have more questions write to me.
You can see my email adress in my profile.


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RE: Attic Ventilation Challenge

Margi, Thanks for your reply.

I will most likely do as you said in the first solution and increase the speed and exchange of air. I'm very familiar with ASHRAE after having worked in the heat and air supply business for many years when I was younger. I know that there are many members in the area of the U.S. that I live in. I will look into finding someone with building construction knowledge for this area.

I really appreciate your help!

-Lee


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RE: Attic Ventilation Challenge

The previous poster's suggestion - insulating under the roofline - is my thinking also. Probably an open or closed cell spray foam - bringing the attic into the "building envelope." Since your HVAC is in the attic, you could get significant energy savings from this. The only issue is your shingles - the manufacturer will let you know if their warranty covers this, and you can decide from there.


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RE: Attic Ventilation Challenge

Not sure if this will help, but here goes:

I had a similar problem, where I couldn't jam baffles in place after the fact. This was for a space from behind knee walls (finished attic) up to the ridge. My crude but creative approach at the time was to use 4" corrugated gutter extenders, split them with a utility knife, flattened them out by stepping on them, and then push them in place with a stick that had a drywall nail protruding from the leading end to catch it. Crude, but it worked.

Thinking about it today, another approach might be to install foam baffles in this manner: 1) insert heavy duty cardboard (force it over the roof nails 2) slip the baffle behind it, 3) remove cardboard. 4) repeat....Probably would work. This is assuming you have some room to crawl around in the ridge area, of course.

Good luck.


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RE: Attic Ventilation Challenge

First, you need to separate "attic ventilation" from your desire to "cool the attic air", then understand the purpose of each and then understand their effect on the roofing shingles.

Attic ventilation is for the purpose of allowing moisture to escape from the attic space with an area ratio of 1:300 which involves very little air movement. Studies have shown that this level of ventilation has little if any meaningful effect on the temperature of the attic air or the temperature of asphalt shingles. But it does allow excess water vapor to escape and insulation to dry out if there is a leak or if water vapor has entered from bathrooms or rain and/or snow blowing through oversized vents. Don't try to make this system do more than that.

If you want to cool the air in attic space you will need to move a great deal more air but there are two problems with doing that. First, that amount of air may draw moist air into the attic. Second, the major source of heat in an attic is the radiant energy (invisible infra red light) that is emitted from the hot back side of the roof sheathing. As radiant energy travels through air it does not raise the temperature of the air but when the radiant energy strikes the attic floor, it heats the floor and the floor then heats the attic air. Exchanging this hot air with less hot outside air by fan might lower the temperature of the attic air but it will do little to cool the hot attic floor and roof sheathing.

Adding a radiant barrier under the rafters or on the attic floor will help to reduce the heat reaching the interior of the house by directing it back up toward the roof sheathing but that will not help cool the roofing shingles.

Roofing shingles directly exposed to the sun are already as hot as they're going to get and there is little you can do inside the house that will reduce the shingle temperature by more than a degree or two. If you want to keep the hot attic air from increasing the cost of cooling the house add more insulation and move any AC ducting out of the attic.

I prefer to include the attic space in the conditioned portion of the house by insulating between the rafters and installing an air vent path between the insulation and the roof sheathing. Aluminum foil on the back of the sheathing will help to reduce the heat that reaches the attic but this design will not reduce the temperature of the shingles.

More effective ways to increase the life of your asphalt shingles are to use a lighter shingle color, trees, etc. for shading, face the smaller side of the house toward the south, or move farther north.


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RE: Attic Ventilation Challenge

Homebound, I'm glad to hear how you faced this same challenge. I thought about the cardboard, but not sure if even a very stiff piece will help get me past the long nails. About your experience with the corrugated gutter extenders... Is this similar to what you're talking about.

http://a248.e.akamai.net/f/248/38435/6h/www.improvementscatalog.com/images/en_US/local/products

I'm trying to visualize how you got this to work. By ripping the 4" channel, I'm assuming you ended up with two pieces whose profile looked like a carrot symbol "^". You then slid those pieces with the point facing the insulation and legs sliding along the roof decking. Sounds like that might work. I just hope the gutter extenders are fairly economical because it would take a few to get the job done. I really appreciate you taking the time to share.


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