My daughter bought a house in the Nashville, TN area about 2 years ago. (I live about 25 miles away.) House is brick, built in 1946 and was structurally sound. There is about 950 sq ft in the downstairs of the house. There is an attic space with about a 7’ center height. Since she bought it, we have replaced most of the plumbing, 95% of the electric and refinished the oak floors.
Now comes the good part, the roof is next. The roof is gabled, about 8 in 12 pitch, sheeted with 1x’s and is shingled. Some time ago someone had put kneewalls in the attic and drywalled the walls and the ceiling. They attached the drywall to the bottom of the 2 x 6 rafters and put batt fiberglass insulation between the rafters. This of course left no space for air to travel through.
I am considering stripping the shingles (it’s been shingled twice, so by code we have to strip), putting 1 x 3’s running from the eave to the ridge on top of the existing sheeting and putting 5/8 or ¾ plywood down with felt/shingles for a finished roof. My idea is I would in this way create an air space for ventilation, with a ridge vent and a gap in the plywood at the ridge. I would make sure that the plywood was nailed through the 1 x 3, the sheeting and into the rafters.
Any thoughts would be appreciated.
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In this area, you can really get by with 1/2" cdx or 7/16" osb. I say that because all the 40's houses I've been in have marginal roof rafters/ceiling joists and the extra load of the 5/8 MAY be too heavy. You could possibly make the ceilings sag due to the kneewalls transfering the weight from the roof.
Otherwise, the devils will be the facia/drip edge/gable vents/gutters/vent screens/red wasps/mud daubers. Let us know how it goes. I have a potential in Donelson that is almost exactly the same project if they decide to finish the attic.
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What you're proposing is called a "cold roof". It will keep the roof shingles a little cooler, and possibly lower the AC load a little, but will not remove moisture from below the existing sheathing. I'm not sure what you think you will accomplish.
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My thought is it won't really make any real difference in the life of the shingles. And it probably won't make any real difference it the attic temperature.
All that you are venting is the back of the shingles, a waste of time in my opinion.
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I'll beg to differ with several other respondents. I'm a firm believer in roof venting, and with a finished attic, this will significantly reduce summertime heat build-up, extend the life of your shingles, evacuate moisture that duffuses or leaks through the kneewall/ceiling/roof assembly, and prevent ice dam formation.
I will agree, however, that with 2x6 rafters that are now carrying a drywall load, I would not overburden them with unnecessarily heavy sheathing. 1/2" CDX should be more than adequate. If the rafter spacing is 24" oc, then use ply-clips (H-clips) between strapping.
While you're at it, I would suggest laying 1" of rigid XPS foam board over the existing roof deck before strapping and re-sheathing. This will stop the thermal bridging through the rafters and will upgrade the roof/ceiling assembly from R-16.9 to R-22.3 (assuming 24" oc framing) - a 32% increase.
In addition to the ridge vent, use a starter vent (combination drip edge and vent) at the eaves because an exit vent alone is nearly worthless.
http://www.airvent.com/homeowner/products/intake-ventedDE.shtml
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Edited 2/23/2008 6:20 pm ET by Riversong
Riversong, thanks to you and the others who responded. Always great to get different views on these subjects. As we get into the work I'll keep you all posted on the progress.
Good link there. Bob's next test date: 12/10/07
Good link there.
I've been using Airvent products for many years. Even back in the days of aluminum ridge vents, they were the only manufacturer making one with a wind baffle. They're useless without it, as they'll allow wind-driven rain and snow in and reverse the airflow whenever there is wind pressure, while the baffled ones actually increase their outflow when the wind is blowing.
Now Airvent and Lomanco are the only two companies I know of that make a shingle-over plastic ridge vent with wind baffles (the Lomanco has nice internal baffles as well).
http://www.airvent.com/homeowner/products/ridgeVents.shtml
http://www.lomanco.com/ProductPAGES/OR4.html
And Airvent also makes a Flashvent for where a shed roof meets a vertical wall and Peakvent (one-sided ridge vent) for shed roof terminations.
http://www.airvent.com/homeowner/products/ridgeVents-specialtyFilter.shtml
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
How have you found the Airvent ridges for clogging? Around here all the ridge vents I've tried end up either clogged with coniferous needles or they allow them into the roof's airspace. Any luck with the Airvents?
Can't say I've noticed that problem.
I would suspect that the wind baffle would also keep out a good deal of wind-blown debris. The Airvent also has an internal mesh filter, while the Lomanco has internal U-shaped baffles. I believe they both have a finely slotted entry point which would also keep out most debris.
Hadn't thought of that, but another reason for the external baffle.
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
Edited 2/25/2008 7:09 pm ET by Riversong