Take the 2-minute tour ×
Home Improvement Stack Exchange is a question and answer site for contractors and serious DIYers. It's 100% free, no registration required.

I am getting ready to redo the roof on our house (shingles and some sheathing) and I am wondering what kind of venting I should add to the roof. The roof currently is not vented at all and as a result there is mold and and the sheathing is rotting and sagging.

I have been doing some research and it seems like the best way to go is to use soffit/ridge vents instead of an attic fan. However, the roof is only a 2/12 pitch and I am wondering if the shallow pitch will have any adverse effect on the vents. At one point on the roof it is about 30' from the soffit to the ridge with a 2/12 pitch.

Any advice?

share|improve this question
    
Some sort of venting is required. Whether that's gable-end vents or ridge vents seems to be a case-by-case decision; I asked my contractor whether we should switch to ridge vents and the answer was "no". –  keshlam Jul 8 at 16:48
    
I appreciate the feedback. In my case, we don't have any ventilation at all so I am trying to decide which method is better. It seems like ridge venting would be the most efficient method and that way you don't have to use an electric fan that can break. It also seems like it would provide more uniform ventilation throughout the attic. –  BWDesign Jul 8 at 16:54
    
This article from Green Building Advisor might be worth a read. Insulating Low-Slope Residential Roofs –  Tester101 Jul 9 at 10:23

4 Answers 4

up vote 1 down vote accepted

Before I added my ridge vent when we had consecutive days of 100F the attic could get up to 120-125 easy - and my roof is heavily shaded. I would come down completely soaked - like swimming pool soaked - when doing 15 minutes of electrical work.

I just added a ridge vent to my attic a few months ago. Measured attic temperature after a couple of days in the high 90s and the high temp was around 7PM at low 90s but basically it stayed in the mid 80s. So 25F temperature change, being conservative.

We did cut out 3 inches on each side of mine so a total of 6 inches.

I am not sure what an attic fan would do in comparison. The air needs to get out of the attic. Hot air will escape upward. The attic fan will take hot air out of your house and put it in the attic, with no where to go. I could almost see the attic fan as barely making a difference due to the heat introduced from the component in this situation. And from an energy stand-point it doesn't make sense to run something that is a low difference maker. In this situation I could see just using your AC as being more efficient.

You would ALWAYS put the ridge vent before the attic fan. Now would you want to install an attic fan with the ridge vent is a really good question. I could see the benefit of having the attic fan going with the hot air having an easy escape route.

share|improve this answer
    
I really like the idea of just using a ridge vent and soffit vents, but my concern is that the shallow pitch and long distance from the soffit to the ridge will not allow for efficient upward movement of air. The attic fan would go on the gable end and push outside air into the attic space to keep it the same as the outside temp. It wouldn't pump in heat from the house. –  BWDesign Jul 8 at 18:39
    
That is a very old school design. Powered gable venting is one of the least effecient methods of moving air in your attic. It doesn't matter how shallow your attic is. The hot air will work its way up. The only time I wouldn't recommend a ridge vent is for cooler climates. With a low pitched roof I would make sure that the shingles you put over vents (cap shingles) do stick out a bit longer to keep driving rains from going in. If you google "building science attic venting" you will see scientifically that ridge venting is most efficient in your climate. –  DMoore Jul 8 at 18:53
    
Ok. I appreciate your insight. I think we will go with ridge venting. I will look into the ridge cap and see what I can find for a shallow pitch. –  BWDesign Jul 8 at 18:59
    
I usually would recommend keeping all other venting open with ridge vent especially with the low pitch. Ask a couple of roofing contractors what they think. Also I wouldn't go spray foam. It will be 30 years before you would get a return on your money. Also if you have electric in your attic or any other issues, things are very hard to work on after sprayed. –  DMoore Jul 8 at 19:07

I am starting to research getting a new roof for my home a colonial currently with gable vents an electric fan and soffits. The previous owners painted over the exterior of the soffits and within the attic filled the soffits with loose insulation. It has been a nightmare to fix for me.

The advantage to a ridge vent and soffits is they work 24/7, there are no moving parts for replacement and if you lose power they will still operate.

The disadvantage to gable vents/soffits and an electric fan is the fan is noisy, costs money to operate and needs to be setup to either a thermostat or switch that you have to be aware of. The fan will also draw air from inside the house if your attic is not properly sealed, so if you have an AC the cool air will go much easier into the attic then air from the other gable. Also gables need to be open to allow air flow, which then will allow hornets or other unfriendly creatures to enter your attic. After placing a basic screen over the non fan gable, the air flow was reduced significantly.

share|improve this answer
    
I agree with what you are saying about an electric fan, but my concern is that the shallow pitch and long distance from the soffit to the ridge will not allow for efficient upward movement of air if we use ridge vents/soffit vents. Any thoughts? –  BWDesign Jul 8 at 18:44
    
Look into turbine vents, I do see your concern with the ridge vents in the sense heat wont flow up as easy and might actually push back down. –  treeNinja Jul 8 at 19:41

If you have a 2/12 pitch roof and you are already redoing the roofing and some of the roof decking, let me recommend that instead of ventilating your very shallow and difficult-to-work-in attic, you keep it sealed and insulate the the (new) roof deck, bringing the attic into the conditioned space of the house. You have a perfect opportunity to add a really good level of insulation to the top of your house's conditioned space.

There are two ways to do this: one is spray foaming the underside of the roof decking. You can get enough R-value with 5-7 inches of spray foam, depending on your climate. Either used closed cell foam for the entire thickness, or closed cell for half and then you an use cheaper open cell for the other half.

Alternatively, and this might be much easier and cheaper, you can also add 6-8 inches of polyisocyanurate insulation boards above the roof decking, and then screw down another layer of decking for the roofing material to attach to.

You would want to make sure that this new foam meets your wall insulation to keep a continuous layer of insulation.

share|improve this answer
    
Thanks for the idea, and I like the way you think. In fact, we had consider completely redoing the roof (framing and all) and building a "hot roof" (un-vented) in its place. I really liked that idea until I got some quotes on the closed-cell spray foam insulation which is required for an un-vented roof. At over $4/sq ft, it would cost us over $12k just for the insulation. We live in Ohio and the minumum r value is 30 for the roof. We are trying to complete this project for as little as we can. It seems like the simplest solution is just to vent the roof. –  BWDesign Jul 8 at 18:26
    
Also, if we add a layer of foam board and then another layer of roofing, wouldn't the gap between the layers still need to be vented? The foam board would still allow moisture to build up on the underside of the decking and rot it out. That is the purpose in closed-cell spray foam. It keeps the moisture off the decking. –  BWDesign Jul 8 at 18:31
    
No, because you would be bringing the attic into the conditioned space of the house. When you heat the house in the winter, the heat you generate will enter the attic and prevent condensation. –  iLikeDirt Jul 8 at 19:26
    
As for cost, I feel your pain, but the rigid foam boards would be a much cheaper option than spray foam. Ohio is cold cold cold, so unless your attic floor is superinsulated right now, taking the opportunity to increase the insulation of the top of your house's thermal envelope will likely save you quite a bit on your heating bills. –  iLikeDirt Jul 8 at 19:28
    
And let me paste this in here: greenbuildingadvisor.com/blogs/dept/musings/… –  iLikeDirt Jul 8 at 19:29

I'd suggest giving some serious though to a functional Cupola (or two) (and soffit vents for intake.) Some "old fashioned things" actually make sense.

A ridge vent can be a lot more restrictive than you might think - if you don't have some sort of architectural horror of cupolas, they can be very effective by comparison.

If you prefer a slightly newer fashion, look into "solar chimneys" which use the sun's heat to more directly boost venting.

share|improve this answer

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.