Ridge Vent / Gable Vent Combined?
fitz4
| Posted in Energy, Heating & Insulation on
I am getting several proposals to reroof the Education Wing of our Church. One contractor routinely adds a ridge vent when there is none, as at our church. Another contractor said that a ridge vent combined with the existing gable vents would be less effective than the gable vents alone. I am inclined to agree with the second contractor, but would welcome other opinions.
Replies
What about
ridge venting and eave venting? That's standard. Roofers generally aren't going to talk about adding eave venting because it's carpentry work (although there is vented drip edge they can install). What pitch is the roof, what's in the attic, and what is the roofing material? How large is the attic and how large are the gable vents?
My house has gable vents only, and they work very well, but we get plenty of breezy weather.
Well enough alone?
Thanks for your reply. I have read that eave venting combined with a continuous ridge vent is the best practice, and the ridge vent would be easy for the roofer to add. I'm just trying to determine whether adding a ridge vent without adding eave venting would "short-circuit" the action of the gable vents. The gable vents may, in fact, be working adequately. I need to check their size, as you suggest.
The roof pitch is 8 in 12.. There's about 6" of insulation in the attic, and the wooden trusses. It's basically open. The failing shingles are asphalt "jets", probably 25-30 years old and past their life expectancy. I don't know how to tell if ventilation issues hastened their demise.
The size of the attic? The T-shaped Ed. Wing has a 30'x50' base crossed by a 30'x36' top. The two attics are open to each other. Where it all adjoins the church, the attics are open to each other. So, the attic of the 50' base of the "T" borrows from the fellowship hall's (32'x46') gable ventilation at one end, and from the two gable vents at the top of the T at the other end.
This is beginning to sound like a lot of attic.
There is no
I'm just trying to determine whether adding a ridge vent without adding eave venting would "short-circuit" the action of the gable vents
firm rule about this. You have to assess attic performance on a case-by-case basis. My guess would be that you are not hurting anything with the gable vents. Your relatively steep roof is a good thing if you can get eave and ridge venting in place.
The main thing you want to avoid is having a lot of interior air percolating up into the attic from the space below. It's not worth talking much about roof venting unless you deal with air sealing first. I have seen well-vented attics with a fair amount of moisture in them because the attic floor was leaking a lot of warm, moist air.
What type of underlayment are your roofers proposing to install? Felt? One of the newer roof wraps?
Did some measuring
and found that one section of the building has close to adequate gable venting, by the book. The other section has no vents dedicated to its attic, but it's connected to the vented attic. My thinking at the moment is to add ridge and soffit vents to the unvented section. What the whole building has going for it is that it has very little introduction of moisture from human activities normal to a residence, and , on average, very few people there at all.
I agree with you about air sealing to reduce the venting load. I may be overly worried about the whole issue, as the building appears to be performing well-- maybe it ain't broke!
There will be 6' feet of ice and water shield at the eaves, and ice and water shield in the valleys. Elsewhere, 30# felt.
Well enough alone?
Thanks for your reply. I have read that eave venting combined with a continuous ridge vent is the best practice, and the ridge vent would be easy for the roofer to add. I'm just trying to determine whether adding a ridge vent without adding eave venting would "short-circuit" the action of the gable vents. The gable vents may, in fact, be working adequately. I need to check their size, as you suggest.
The roof pitch is 8 in 12.. There's about 6" of insulation in the attic, and the wooden trusses. It's basically open. The failing shingles are asphalt "jets", probably 25-30 years old and past their life expectancy. I don't know how to tell if ventilation issues hastened their demise.
The size of the attic? The T-shaped Ed. Wing has a 30'x50' base crossed by a 30'x36' top. The two attics are open to each other. Where it all adjoins the church, the attics are open to each other. So, the attic of the 50' base of the "T" borrows from the fellowship hall's (32'x46') gable ventilation at one end, and from the two gable vents at the top of the T at the other end.
This is beginning to sound like a lot of attic.
It would help to know where this place is.
It's located in Berkshire County
of Massachusetts (far western MA).
Ridge and soffit vent
is the best combination. However you need to verify that you have enough air flow coming through the soffit, this includes not only the soffit itself but also the bird blocks at the truss heels. A lot of older buildings do not allow for good air flow at the heels of the trusses and sometimes they are closed off due to insulation as well. If this is the case you may need to install baffles at the heels of the trusses. They are fairly inexpensive and easy to install in most cases. With a roof this steep you should look into the ridge venting and the location of the gable vents i.e. if the gable vents are close to the ridge and of adequate size the ridge vents may not be necessary, if they are to small and in the middle or down low on the gable then the ridge vent venting will become more useful. The purpose of the venting is to allow air and moisture to escape, so you can see that having good venting at the lowest point (soffit/eave) and the highest point (ridge) will accomplish this best.
One section
of the building seems to have enough gable venting for its size, but the other section may need the soffit /ridge venting you describe. I don't see any sign of interior moisture having frozen on the underside of the sheathing and then dripping onto the insulation as it melts. I don't know if it gets too hot in there in the summer, but adding venting should prevent that.
**IF** you have soffit vents, it's generally recommended to close off the gable end vents when installing ridge vents. The theory is that air will be drawin in the gable ends rather than up from the soffit. (I've never read of any actual testing of this, it's just "conventional wisdon".)
If, on the other hand, you do not have soffit vents, I'd say you want as much venting as you can get -- ridge + gables. Only in relatively ideal conditions will you get "flow through" ventilation from one gable to the other, but the ridge vents provide some purely convective ventilation, and some "siphoning" when the wind is at right angles to the ridge.
Of course, the ideal thing is to add the soffit vents and eliminate the gable vents, but adding new soffit vents is usually (but not always) difficult and expensive (compared to ridge vents).
But a lot of variables here, in terms of location/climate, shape and structure of the roof, use of the building, etc. Generally a building like a church has relatively low occupancy and fewer problems with "moisture drive" from the living space into the attic area, reducing moisture as a reason for being concerned about attic ventilation.
Just like a balloon, hot air is less dense and rises. The air in the attic gets warm. In some circumstances, gable vents may work very little. The hot air inside the attic creates a pressure and won't let sufficient air enter through the gable vents. Since outside air entering is often cooler, it falls to the bottom. In many cases, gable vents, alone, may not provide any ventilation other than a small area directly near the vent.
Once we started tightening buildings up and providing better insulation, back in the 70's, we discovered that the old gable vents, that were standard up to that point, were almost useless. In snow country, warm air in the attic causes ice dams and a lot of ensuing problems. Insulation is only one component of a properly done attic, without ventilation you can have all kinds of trouble, not only ice dams but mold growth and degradation of the insulation from absorbing moisture.
A continuous ridge vent, coupled with continuous soffit vents takes advantage of the warm air rising. They work like a chimney once the flow gets going. Gable vents won't have much of an effect on this flow except to join in. Chances are, they won't do much more than as I stated before.
With only a ridge vent and the gable vents, you won't get the volume of flow you would with the addition of soffit vents. Without them, the gable vents will provide the only new air coming into the attic, so leave them. Plan on adding continuous soffit vents in the future, do the ridge vent now while doing the roof.
6" of fiberglass insulation isn't enough. Adding another 6" and making sure there are no air leaks coming from the heated or conditioned space will start to pay back the investment immediately. It sounded like two connecting buildings are open to each other in the attic. This violates fire codes since fire can spread from one stucture to the other in the attic.
Soffit vents later
Your suggestion to go ahead with ridge vents now, and to add soffit vents in the future may be the way to go. I'm thinking I should use the lightest color shingle that will look OK on the structure to help reflect heat.
The addition in question was built in 1956 (and is connected to a fellowship hall dating to 1890, which is connected to the church, cornerstone 1824). I don't know how tight the building is re: air escaping from the occupied space into the attic, but I think it's relatively tight. More insulation would definitely help.