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Old 07-15-2011, 03:11 PM   #1
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New Roof


I'm replacing the ancient, unventilated roof on my house. There are two gable vents on one end of the house, and absolutely zero other ventilation sources. Add to it that the house was built in the 1920's, so I have no overhangs.

The big question I have is, which is better, ridge vent or a powered vent attic vent? I've seen some solar ones, and I think that's a pretty easy solution, but is it reliable? I feel like a ridge vent isn't going to be enough to really draw the heat out. The house is only 1,400 square feet and the attic for the most part is a big hip a gable running from one end to the other on one side. It definitely bakes up there in the summertime.

Also, I'm thinking of going for the tech shield radiant barrier decking. The only question I have is that the original roof on this house was wood shingles on those 1x4 lath strips with a 4" space between. My current roof is on top of the old wood shingles...ugh. So we're ripping that off. Standard practice around here is to just layer the decking right on top of those lath strips - will that negate the effectiveness of the radiant barrier decking since it would have contact with so much wood? I understand how the techshield works, and I've had several people swear by it, but at the same time it seems like it would casue excess heat to build up in the roof material itself?

I want to do this right. I live in Oklahoma, and we suffer from 106 degree summer days to below zero winter days, plus hail in the spring. Trying to ventilate to help with energy efficiency and to help prolong the life of the roof. Any other advice?

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Old 07-15-2011, 11:28 PM   #2
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A ridge vent (properly installed) is the best option for almost any gabled roof. It's passive, has no moving parts, needs no electricity, works day and night.

My research suggests that Shingle Vent II is currently the best, and that's what I'm getting for my 2300 square foot "loft bungalow".

A ridge vent is perfectly situated to evacuate the most heat from your attic, because:

a) it's at the highest point and,

b) it spans the whole top of your house (attic)

All it needs is the same thing that any other type of roof vent needs, to work effectively:

- enough INTAKE ventilation from the bottom edges of the roof and,

- an unobstructed path for air movement from eaves to ridge, between each and every pair of trusses or rafters.

Think about it.

Hot air rises. Its natural tendency is to exit through a hole at the very top of the enclosed space (in this case, a long narrow slot), as long as compensating volume of air is allowed to enter from somewhere else in that space to keep the flow going. That flow will happen. It's basic physics. Problems and sub-par functioning occur when:

a) Your inlets are higher than the area of the eaves - hot air in the attic still rises to exit the ridge vent, but it's mixed with outside air that came in partway up the contained volume; basically, you are short-circuiting part of the flow that you need. The air that enters a high inlet vent does not pass through the full volume of the attic and so does not pick up heat from that full volume. It competes to some extent with the large volume of overheated air that it bypasses.

b) The venting is unbalanced - a two-inch wide slot along the length of your roof needs the equivalent of two one-inch wide slots, one at each eave, the full length of the roof. Any less than that much input surface area results in less-than-optimum exhausting. You can have _more_ input venting than output, but you should never have _less_ input vent area.

c) The lower venting is intermittent. The best flow is from a full-width slot at the bottom feeding up toward a full-width slot across the top. If you skip the lower-level venting between some pairs of trusses or rafters, then you allow that entire area of roof deck to remain much hotter than the areas where air is moving bottom to top and drawing heat with it.

Mushroom and tower vents (like Venmars) are limited by their locations, in two ways:

a) They are not quite at the top of the roof, so there's always a dead (hot) space above them, unlike a ridge vent.

b) They leave large swaths of roof unventilated, therefore large areas inside the attic are stagnant or have less-than-ideal movement of hot air. Usually, there's a six-to-twelve-foot strip up each side of a roof that gets poorer venting because the tower vents (or worse, the poor flat little mushrooms) are grouped more toward the middle... guess where the shingles start to fail...

Power vents have their own problems. Lower-level input venting is just as critical for them as for any other style of vent. They make up, somewhat, for not being the full width of the roof, but they have moving parts and they need power.

If you don't have your intake vents low enough (many people install a power vent on the roof, but leave their gable vents uncovered), you have an exaggerated version of the short-circuiting air-flow problem. Air comes in high, flows directly to the power vent, and gets blasted out. You get a small hurricane between the gable vent and the power vent, leaving the rest of the attic space stagnant. And hot. Rain and dirt are sucked in along with the unintended high-entry-point input air from the gable vent. The same can happen if you leave old mushroom vents in place. They need to be removed (and the holes sealed, of course) for the same reasons.

People with poor or NO lower-edge input vents make the mistake of installing a power vent, thinking (mistakenly) that if it's got enough power, it'll be able to overcome the inconvenience and just suck any hot air out of the attic... the brute-force method. Doesn't work. Invariably the houses with no proper lower-edge inlet vents are old and leaky. So the power vent in the attic sucks mightily and the input air has to come from somewhere, so it gets sucked out of your house. The house becomes a low-pressure zone. This whole arrangement is bad for three reasons:

1) Outside air (usually hot air) rushes in through every crack and crevice, heating your indoors when you least need extra heat in there.

2) Household air being sucked up into the attic carries moisture from cooking and bathing. That moisture gets into the insulation and/or it condenses onto the underside of your roof deck when nighttime cools the outside. Mould and rot happen.

3) A power vent that isn't matched with sufficient, proper input venting is under stress, and its motor burns out.

Go with a ridge vent and find some way to get the same amount of unobstructed input venting to match. Proper attic venting must be balanced. That product you mentioned sounds like a good one to try.

For the ridge, I like the Shingle Vent II because it's got good baffling (testing showed no entry of 110 mph horizontally blown rain), because it works as well as the others in still-air (windless) conditions, and it has that extra deflector to provide additional suck (Bernoulli effect) whenever a bit of wind blows across it.

Good luck. Don't take my word for anything. It's not gospel. But if it makes sense and meshes with what you hear and read elsewhere... take the hint....

- kevin


Last edited by kevinmcl; 07-15-2011 at 11:34 PM. Reason: clarity
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Old 07-18-2011, 08:29 AM   #3
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i'm ripping off the flat roof from my kitchen and replacing with a pitched roof, vaulted ceiling in the kitchen with 2 velux window's.

Pitch approx 22 degrees. From House to to edge of flat roof approx 5000mm. What is the clculation for working out the rafter size
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Old 07-18-2011, 09:27 AM   #4
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Lots of triangle and geometry sites on the web.
For example:

http://www.csgnetwork.com/righttricalc.html

For that one, you'd specify angle "A" as 22 degrees, and side "b" as the ceiling of your kitchen. The calculator fills in the length of "c", the roof. But of course, add some extra length to "b" to account for overhang/eaves.



If you meant the height of the rafters, rather than their length, then that's a different story. Two-by-twelves on sixteen-inch centers oughta be plenty tough enough. Or engineered joists/rafters. Whether you add additional depth would depend on how much insulation you want to apply. But leave air space between insulation and roof deck, bottom-to-top - from the soffit to where your new sloped roof will join the wall of the house. Then provide exit ventilation. Second best would be several flat mushroom vents just below where the new roof meets the house. Best would be a "vertical wall flashing vent" located AT the join line - similar to a ridge vent, but adapted to installation where a roof butts against a vertical wall. This will keep your kitchen from turning into an oven in summer, and will keep your shingles from cooking and curling.

Sounds like a project.
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Old 08-06-2011, 11:41 AM   #5
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Built a new home 2 years ago with sprayed in cellulose insulated walls, radiant barrier sheeting, 14" deep attic insulation and plastic air channels from soffit to attic. Both the front and rear porches have perforated vinyl ceilings with air channels to attic, so there should be adequate air source.

We're having an unusually hot summer and the upstairs is hard to cool, so I checked the attic. Have really nice full length shingled ridge vents on a 10/12 roof, even the dormers have ridge vents. Would you believe, the clowns didn't even cut the sheeting prior to installing the ridge vents? Solid sheeting covered by really nice vent system. If not for the radiant barrier, I guess the shingles would have melted here in South Mississippi.

I placed a remote thermometer in the attic to monitor the temperature and see any before and after changes. I've searched the forum for outside/attic air temperature range recommendations, but can't find a meaningful answer. I'm going to try for 10 degrees.

High temp of day: Outside: 93.6 Attic: 112.5 Var: 18.9
6:00 PM Outside: 90.7 Attic: 101.5 Var: 10.8

I'm not getting the 140-150 degree temperatures others report, I guess due to the Georgia Pacific Thermostat® OSB Radiant Barrier Sheathing. My downstairs is 2100sqft and the upstairs is 1300sqft. The house is a 1 1/2 story similar to a Cape Cod. I do know that, while the barrier can reflect up to 95% of the direct sun heat, it also reflects any attic heat downward into the insulation.

Will wind turbine vents work for this, or should I get power vent? Re-doing the ridge vents would be costly. My heat variance is about 20 degrees and I need to get some air circulation in the attic.

After 3 days of reading pros and cons, I am totally confused. Everyone seems to be emphatic that their chosen method works best. It stands to reason that something has to work better the others. I suppose a power vent has to work:

http://www.amazon.com/Broan-356BR160...xp_grid_pt_0_2

Any advice would be appreciated.

Last edited by nicksteel; 08-07-2011 at 03:11 PM. Reason: Update
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