Ventilation of attic- too many wrong vents?


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Old 08-08-10, 12:28 AM
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Ventilation of attic- too many wrong vents?

Too many roof vents? In regards to previous problem: http://forum.doityourself.com/baseme...-problems.html which we still need to address with a enclosed chase within the attic space. Too regulate High heat inside the chase, vents will be needed at top and bottom also I believe.

We have NOT had much of a ice dam build up for 20 years until last year when we had the heat source put into the attic. I believe we have enough ridge vents spanning length of house and continuous soffit vents. However, we have also installed 4 mushroom type vents above the whole house fan 2 in front and 2 on rear roof. We also have 1 gable vent on side wall. We have had this configuration for 10+years. The soffit vents have very small holes so I plan to add at least 8 -undereave vents 16"x8" with louvers with more net open area, this would also give me more cold air air intake. Do the louvers open to the house or to the outside?

Short circuiting?
In my research I have learned we may have too many vents of wrong type? We need the mushroom vents for the fan and they are immediately below the ridge vent (1-2' below). The lone vent on rear roof is kitchen fan vent.

What about the gable vent? Would this take the place of the ridge vent when ridge vent is covered with snow? I could cover GV during summer.
 
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Old 08-08-10, 03:50 AM
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Sounds as if you do have an abundance of vents. The only effective passive venting system is adequate soffit venting and full ridge vents. The mushroom vents motors burn up on a regular basis and are basically worthless. The gable vents are second best, but you probably don't need ridge, gable and mushroom vents all on one house.
I was caught by your statement of having "the heat source put into the attic". What heat source and why?
Once we know some answers, I believe we can start paring down your vent load and make the attic efficient.
 
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Old 08-08-10, 06:08 AM
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A couple of questions.
1. Are the ice dams all over or located in just specific areas. Melting snow contributes the water which turns to ice and the snow usually melts directly above the heat source.

2. I see a large chimney in your photos, is the space around the chimney covered with sheetmetal to prevent air flow from the basement. An open chimney chase will pull warm expensive air from inside the house and heat up the attic.

3. You have some soffit venting, but is it open to the attic. I see a lot of insulation and you need to be sure it isn't blocking the air flow.

4. Exactly how your venting is performing is difficult to say. The blanket assessment that gables short circuit ridge vents isn't always true. The primary force that moves air into and out of attics is wind. If your home is well shielded, then it must rely on heat, ie the very source of the problem you want to eliminate.

5. Last, why is the heating system up there and is it the actual source of the heat? Is it just a chimney that is up there or is there a furnace involved. Whatever the source of heat, it needs to be identified and isolated or eliminated. Just a trickle of heat that escapes through your insulation can be a problem in an attic. the recommended vent requirements of one sq ft per of vent area for every 150 sq ft of attic floor is based upon a well insulated and air sealed home. Use 300 sq ft if not well air sealed. That's total vent area, half high and half low. Low is more important.

Hope that helps.

Bud
 
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Old 08-08-10, 06:58 PM
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"the recommended vent requirements of one sq ft per of vent area for every 150 sq ft of attic floor is based upon a well insulated and air sealed home. Use 300 sq ft if not well air sealed." ----- reverse that (if a vapor barrier is present).....

Be safe, Gary
 
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Old 08-09-10, 12:28 AM
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Thanks Gary, typing and thinking just don't go together well for me at times .

Bud
 
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Old 08-09-10, 10:21 AM
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Answers to questions?

Photos of rear of house are provided in previous problem link:http://forum.doityourself.com/baseme...-problems.html

The heat source in the attic is the new wood burning chimney which is exposed in the attic. there is an insulation barrier firestop at base of chimney. Caulking seals the edges. Insulation covers the stop up to the barrier. However 5' of double wall chimney still exists below the rooftop outside chase. Skin temp of pipe can get higher than 150 degrees
My plan for this is to extend chase downward with 3.5" insulated walls in the attic. For cooling inside the chase -provide cool air from either outside or inside the attic by intake vents at bottom and have an exhaust vent or 2 at top of chase on the roof. The chase on top of roof is uninsualted, just wood with vinyl siding.

Previous to chimney snow barely melted on the roof and we would have a couple of feet always covering all roof vents.

Now the snow primarily melts quickly on North side first where the chase is and also a little to the South. But the North side of house is also much warmer inside (A heat trap)and lack of soffit vent area above bay window.

Ice dams form on North side of house (Bay window to garage) progressively thinner the farther you get from below the chase.

I have baffles on top of the insulation to try to keep the the perforated aluminum soffits clear. It is impossible to get to perforated aluminum soffits from inside the attic as roof is too low. BY putting in more louvered soffit vents myself I hope I can stick my hand into attic and make sure it is clear.

Attic floor area is 1080 sq ft., 48 ft of ridge vent, 2 mushroom vents 1' below the ridge vent on N side of house and 2 mushroom vents 4' below ridge vent on S side of house, these south vents are directly above the whole house fan.

During the winter we cover the whole house fan with visqueen and tape it, we then cover it with batts of insulation.

The mushroom vents are motorless. Vapor barrier is present, I tried to seal obvious holes around pipes etc.

The wood burning fireplace is NOT decorative we use it as our sole heating source.

Hopefully the answers are helpful and not more confusing. Thanks
 
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Old 08-09-10, 12:32 PM
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If you can extend that chimney chase all the way to the attic floor and then vent it to the outside I would think that would help greatly. Be sure the mfg and local codes approve of those measures. More insulation would not be bad.

One of the challenges here is determining where the air is exiting and where the replacement air is entering. If it exits the ridge vent, is the replacement air coming in the soffits or the mushrooms. Remember, it is cold air and the negative pressure in the attic will pull it from the easiest source, which may be the mushrooms.

But removing the heat before it gets inside the attic is the best.

Bud
 
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Old 08-10-10, 01:17 PM
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Originally Posted by Bud9051
If you can extend that chimney chase all the way to the attic floor and then vent it to the outside I would think that would help greatly. Be sure the mfg and local codes approve of those measures. More insulation would not be bad.

One of the challenges here is determining where the air is exiting and where the replacement air is entering. If it exits the ridge vent, is the replacement air coming in the soffits or the mushrooms. Remember, it is cold air and the negative pressure in the attic will pull it from the easiest source, which may be the mushrooms.

But removing the heat before it gets inside the attic is the best.

Bud
Should I be also sealing the mushroom vents during Winter at the same time I seal the Whole house fan access to attic? I could use visqueen on roof decking below these vents.
 
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Old 08-10-10, 01:33 PM
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My personal approach would be to do a smoke test. When the weather gets cold, use a punk stick or toy train smoke to be able to see where the air is flowing. If it is exiting both ridge and mushrooms, it has to be entering the soffits, that's good. If it is exiting the ridge and entering the mushrooms, then it is short circuiting the the air you want from the soffits, cover the mushrooms and maybe the gables. Snow is an issue with ridge vents, so consider that before you cover the gables.

Think of it this way, if you took the roof off, no air would come in the soffits. It is like a siphon, you need the suction from the air going out, to pull the cold air in. When the wind blows, then it can create the pressure and suction regardless of the heat, but the wind isn't always there.

Bud
 

Last edited by Bud9051; 08-10-10 at 01:33 PM. Reason: correction
 

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