Hello-
Preparing to build our new house near Tucson, AZ. Will probably GC ourselves, but that isn’t the question here. The question is this, the plans are done (at least until the county tells us what to fix before they will give us a permit), and we are happy with them. We supplied the general dimensions and floorplan to a draftsman, who drew the construction drawings. The design is a 2950 sq ft Sante Fe style house (that’s a near flat roof with parapets for the non-desert dwellers in the audience). The ceiling joists are LPIs on 24 inch centers. The drawings call for about a billion (I exaggerate a bit) vents, one in each bay, obviously. On of the insulation contractors that we have had bid on the job says we should not vent the roof, for energy efficiency reasons. In addition, a general that we have talked to about doing the job said that he would leave the vents off, since he thinks that they will let water in in the thunderstorms here in the summer. Obviously being here in the desert the weather is generally very dry, but in the summer monsoon season there is a possibility of an impressive storm almost every day. So the question in a nutshell is, to vent, or not to vent?
Thanks for your inputs,
Jeff
Replies
Dear Jeff,
If you get substantial wind driven rain, it will come in the roof vents. I get a dozen calls a year on this.
When you build an unvented roof, it is absolutely essential that the inside surface of the assembly be permeable. No poly in the ceiling. That way, any moisture that finds it's way into the rafters can get out harmlessly.
Regards, Fred
[email protected]
I can't help with the vent/non-vent question. But I do have a suggestion if you end up venting the roof.
Some brands of I-joists come with knock-outs for running electrical wiring through them. If you knock out most or all of these, you might be able to use them for cross ventilation, and not install so many roof vents.
Even if the I-joists don't have knock-outs, you can check the manufacturer's literature and cut some openings in them where allowed and use them for cross ventilation.
Fred and Boss-
Thanks for the responses.
Fred- It seems that not venting would be a good idea. There is no question that water will get in if it is vented. It make sense that the bottom can't be 'sealed', we hope the top is, and we need to let out any of the moisture that does find its way in. So, should I look at doing something to ensure that it can breathe, or just make sure that I don't do anything to prevent it? Boss- your suggestion makes sense too, what I am not sure of is how much good the holes will do with the whole cavity stuffed with insulation (I wonder about the effectiveness of the vents in the first place, for the same reason). Sometimes I see why a peaked roof is 'normal'!
Jeff
jlb... i'd take a trip to the building dept and see what they are going to allow / require..
with a flat roof/ parapet.. a zillion mushroom vents are asking for trouble... if you have to vent i'd get the vents up onto the back vertical wall of the parapet and keep them off the roof deck.Mike Smith
Rhode Island : Design / Build / Repair / Restore
Mike-
I think that talking to the county will be the ultimate answer, I am looking now for info to decide how hard (or if) to push. The plans as they have them now have the vents. They should be rejecting them this week (I don't know what for, but they need to keep their jobs, after all). When we are working out what to do to make them happy, I think we will bring up the vent question. There are lots of houses like this here, the builder we talked to says they have never made him put in vents, so I am hoping it is really a non problem.
Thanks,
Jeff
Go to http://www.buildingscience.com
You will find some research papers on why non vented is best for your climate. They treat the attic as conditioned space.
Though I have not looked into it yet there is a product from GAF called strata vent for this problem . You could as Mike said let it vent back through the parrapett wall caps.