here is a 2 part question. first, if i add soffet vents, do i need a vent between every roof joists. 2nd Q. can a ridge vent be added without re-roofing the roof?
The ideal is for a balance of the same area of soffit vents as you have ridge vent. You do not need to vent between every rafter, but the more spread out the vents are the more even the air flow. The best natural flow comes from continuous soffit vents and continuous ridge vents.
You can add a ridge vent without re-roofing. It will cost you a couple of circular saw blades as you need to cut through both the shingles and the plywood to make a 2" cut at the ridge. Once you make the cut, your ridge vent is installed on top of the ridge shingles.
I would never suggest anyone cut thorugh the shingle with a ciruler saw. One of my guys did this with my brand new saw and now the whole thing is filled with melted tar and the guard is stuck open.
A cheap pair of sheet metal snips or a hook blade on a utility knife will make quick work of it.
Unless you go with a "cobra" type vent and then you will have to nail ridge caps over that. I also like to put the "rafter mates" on every bay. Not that much cost and it insures that the hot air does not just sit against the sheeting and natural convection carries it to the ridge vent. Just my $.02 worth
That's interesting Joe, I've been doing this for ....well, let's just say a long, long time. I use a circ saw to cut through roofs for vents, tie-ins for roofs of additions and whatever. Never had any issues with the saws - ever. Can't think of why our experiences are different. Don't do anything special except that I'm a fanatic about setting the blade depth only as deep as it needs to be. Or maybe I tend to cut roofs when they are cooler in the morning - guess a really hot roof would tend to gum things up.
But anyway, never a problem with this.
Just don't look forward to do any finer cutting with the blade after cutting in a ridge vent. Pretty much toast.
People say that the best roof venting occurs with continuous soffit vents and a continuous ridge vent, because theoretically, heat loss from the house warms the cold air coming in the soffit vents and causes it to rise up to the ridge vent.
But, I remember when I was 12 years old or so, and me and my friends would smoke cigarettes in our attic because we didn't want our parents to see us smoking.
We NEVER saw the smoke rising vertically in the attic. What I recall seeing is a horizontal wind across the attic where the wind piled up against one side of the house, came in through the soffits on that side of the attic and then blew out the soffits on the other side of the attic. And, that makes perfect sense because you rarely ever get a calm enough day in the winter for a soffit/ridge vent system to work as advertised. More often than not, there's always SOME wind.
So, to my way of thinking, even though the soffit/ridge combination works best on paper, having soffits on both sides of the roof will give you "wind driven" roof ventilation, and that's more reliable than a soffit/ridge combination because in 100 days, 90 of them will have some wind, even if it's occasional gusts.This message has been edited. Last edited by: Nestor,
Nope, for two reasons.
And the ventui.
A ridge vent can have air passing over it almost 360 deg. Passing air causes a slight vacuum drawing out the hot moist air.
Just like how a chimmney or a sewer vent works.
A soffit on a good day can only have air passing over it on one side of the house.
Hot air rises, so unless there's no other venting it's not going to fall back down to the lowest point with little vacuum to suck it out.
Ditto what Joe said - attic ventilation works by heat, not wind. The attic heats up, hot air rises and escapes via the ridge vents, since air exiting the attic other air must be pulled in so cooler air enters via the soffit vents. The more complete air path you have from soffit vents to ridge vents the more even cooling effect you will have.
It's possible that a windy day can create some high or low pressure areas around the house that will change the air-flow, but it's not windy all the time.
"attic ventilation works by heat, not wind. The attic heats up, hot air rises and escapes via the ridge vents, since air exiting the attic other air must be pulled in so cooler air enters via the soffit vents."
No. Attic ventilation works by heat loss through the ceiling insulation. By warming up the cold air in the attic, you lower it's relative humidity and any frost or condensation in the attic will evaporate into that warmer air. How that warmer wetter air gets OUT of the attic doesn't matter. How colder dryer air gets into the attic to replace it doesn't matter either. The whole point of attic ventilation isn't to get a convective current going, it's to remove moisture from the attic. Any way you can do that is fair ball.
The ONLY supposed advantage in having a soffit/ridge vent system in your attic is that it will theoretically give you 100 percent sweep efficiency of the attic air space. But, for that to happen you need the air in the attic to be dead calm in order for the heat loss from the house to cause a convective air current to form from the soffits to the ridge vent.
And, what I'm saying is that 99 days out of 100, your typical attic is too drafty to have a such a convective air current form.
So, if you seldom get calm enough days for a ridge/soffit vent system to give you that 100% theoretical sweep efficiency, where's the advantage in having a ridge vent?
Most of the time, there's SOME wind, and that means that a soffit/ridge vent system isn't going to provide that theoretical 100% sweep efficiency, and won't work significantly better than any other system consisting of holes in the attic, like gable vents, roof vents or turbines.
If you're planning on converting your attic to living space, then I agree the best option would be to ventilate the underside of the roof with soffit vents and a ridge vent and those styrofoam pans that fit between the rafters.
But, if you're planning to leave your attic as an attic, you should check to see if you even need any more ventilation than you already have before you start spending money providing more. Just go up to your attic on the coldest nights of this coming winter and look for any frost or condensation forming on the underside of the roof deck or on the rafters. Also, do you ever have ice dams forming on the overhang of your roof?
If the answers to these questions are: A) can't see no frost or condensation anywhere, and B) what's an ice dam?
...then you don't need any more attic ventilation than you already have, and the money you spend putting in more attic ventilation would be better spent partying your face off, buying lottery tickets or being sent to some Nigerian in Africa who tells you a distant relative has died and left you a fortune.This message has been edited. Last edited by: Nestor,
I just reread much of this thread, and I'm realizing that neither Jaybee nor Joe Caption were thinking about attic ventilation in winter. They're more thinking attic ventilation is for getting rid of the hot air in the attic in summer.
While attic ventilation may do that, having hot humid air in your attic won't do your house any harm. Where I live, not having enough attic ventilation in winter can cause serious damage.
That's because without sufficient attic ventilation, the warm moist air that rises into your attic from below will form frost all over the underside of your roof. That frost doesn't do any harm...
...until spring, when it melts. (I have personally seen a picture of a house in Winnipeg that had three inches of frost collected on the underside of it's roof!) The melt water then drips off the roof onto your ceiling insulation, getting that insulation wet. Insulation works by keeping air stagnant and so wet insulation takes forever to dry out because of the minimal air movement through it. And, since that wet insulation is in close contact with your wooden ceiling joists, you have all the elements needed for wood rot fungii to start feeding on your joists.
And, on top of all that, the vapour barrier under your ceiling insulation can help to keep your ceiling drywall dry, thereby keeping you blissfully unaware of the wood rot that could be happening right above your head.
And so the easiest way to check for sufficient attic ventilation is to simply look for frost on the underside of the roof on the coldest days in the winter. As long as there's no frost or condensation on the underside of the roof in winter, there won't be any melt water in spring to cause damage.
Also, insufficient attic ventilation causes the roof to be warmer than it otherwise would be in winter, and that can cause snow on the roof to melt. When that melt water reaches the roof overhang (which doesn't get any of the heat loss from the house and is therefore colder) it freezes again forming a ridge of ice on the roof generally directly above the exterior walls of the house. That ridge of ice prevents the melt water from leaking off the roof, and every time enough melt water accumulates to leak onto the top of that ice ridge, that water freezes making the ice ridge taller and able to contain deeper water behind it. Eventually, the water level gets to the same elevation as the top of the ice and water shield around the perimeter of the roof, and at that point water can leak under the shingles and dribble over the top of that ice & water shield to leak through the roof and into the house. The result is water damage to the ceiling drywall and/or exterior walls of the house.
My sister was lucky. She had an ice dam develop on her roof, but the melt water just leaked down the underside of the roof onto the soffits, and from there leaked down the stucco on the outside of her house causing no damage. She had a big icicle about two feet wide and an inch thick on the side of her house all spring until it melted.
Ice dam formation is highly dependant on the weather. It has to be warm enough so that the roof over the heated part of the house is above freezing, but the overhang temperature is below freezing, and those conditions have to persist for long enough for enough snow to melt to create the ice dam, and for the water to accumulate until it can drip over the ice and water shield. In MANY homes, they will use TWO 40 inch widths of ice and water shield around the perimeter of the roof instead of just one to avoid the possibility of water damage from ice dams.
I can see how someone in Texas or Florida wouldn't be concerned about attic ventilation in winter, and only think it was for getting rid of heat in summer. But, hot humid attics won't harm a house. The primary reason for ventilating an attic is to prevent water damage to houses in northern climates. But, that same ventilation would also serve to get rid of excess heat in attics in summer as well, tho.This message has been edited. Last edited by: Nestor,
A Vapor Barrier and "Adequate Insulation for your area", are equally important and should go along with the venting! Both for heat loss in the winter and halting heat gain/cool transfer to the interior in the summer. A properly vented and insulated attic will be close to, if not have the same, temperature and humidity as the outside air.
Whatever type of attic venting a house has, either ridge vents or turbine, lower air intake vents in the attic also need to be adequate in number and size, to create the air flow.
The vapor barrier and deep enough insulation are the homes first line of defense to prevent the condensation from the warm/moist air inside an occupied home, in the winter. (Venting helps to remove any moisture that may seep through)
There are numerous sites that show current recommended depth of attic insulation based on your zip code as well as charts that give you the square feet necessary of vent openings based on square footage and types of roofs. Owens/Corning is one, there are many others.
This is much easier than checking your attic for frost in the winter.
What does mold and fungus need to live?
Heat, food, moisture.
Lower the heat, get rid of the moisture and it can not live.
Guess what frost is moisture.This message has been edited. Last edited by: joecaption,
I'm with you on this one. We just use the oldest circle saw the company has and use it for just that purpose.
"Why isn't everyday Earth Day ?"
The ones that were suppost to use I picked up at yard sales. That way if it falls off the roof and smashes, I just go get another one.
I found one of those cheap Trades Man brands that Lowes sells as there house brand that had been returned for $10.00. I soon found out why, they had made the hole in the plate where you would remove the blade 7", kind of hard to remove a 7-1/4" saw blade, like trying to fit a qt. of jello in a pint jar.
Jeeze Nestor. Please stop assuming that you know more of what I am thinking in my posts than I do.
Kind of like me thinking that you Googled all your attic ventilation information.
You can "think" all you want but nothing I've read that you have posted has nothing to do with the facts based in real life.
Storys about as a kid smoking in the attic hardly have anything to do with how an attic vent system works.
You have been suggesting completly wrong info that is going to cause major damage to someones home, your info is based on what you "think" happens, and it's wrong, so please stop.
Hate to be so hard but it had to be said.This message has been edited. Last edited by: joecaption,
I always thought that a long reply by anyone on any trade was a "GOOGLE". This message has been edited. Last edited by: CommonwealthSparky,
"Why isn't everyday Earth Day ?"
Good, average or formerly great, the oldest saw finds its way up to the peak, along with the newest guy on the crew.
"Why isn't everyday Earth Day ?"
thanks for the info. I was thinking of going with a continious soffit vent and baffles between every rafter.
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