Hi all. Question for venting exposed rafter tail roofs. We will be building an ICF home in a Craftsman style. The roof will be trusses 24″ o.c. with energy heels.We plan on blowing in cellulose to R-50. My question is, with exposed rafter tails ( truss tails) how to achieve adequate ventilation without a vented soffit. Remember ICF walls. Would gable vents and a full ridge be enough? We don’t want any ugly turbines or power vents standing off the roof. I’m somewhat aware of the sprayed in foam or SIPS but I think they will probably be cost prohibitive. Any ideas?
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I used Cor A Vent S400 strip above frieze blocks that ran between tails. The tails were lagged to 24-inch centered trusses with enough heel height to accommodate them.
You can see my detail, attached, which shows how we scabbed up some vents using scrap OSB, then urethane-foamed the eaves tight, before using blown cellulose in the attic.
Gene Davis, Davis Housewrights, Inc., Lake Placid, NY
Ima, thanks for the drawing. Couple more questions. Did you make your own osb chutes instead of using the foam units in order to have firmer backing for the spray in urethane? I'm asuming the trusses are set flush with exterior wall with no factory overhang. How did you detail the decorative tails at the bolt joints? Were they"sistered" to the trusses and if so how did you deal with the spacing issue at the gable ends where the truss sits flush with the wall? Or did you somehow bolt the tails end to end with the trusses? Also what does H.A.P. mean in regard to rafter plumb dimension?
Hi Scamp, FWIW, truss supplier told me that if I cut the loose 2x6 tails to shape, they would incorporate them into trusses...basically scab it to 2x4 further up roof. I imagine a diagonal brace ajacent to splice somehow, but haven't seen actual design.
Boss Hog could comment more about it.PJ
Everything will be okay in the end. If it's not okay, it's not the end.
Peter, Never crossed my mind that a truss company would (or could) do the scab on work . I'll have to check into that.
Yes on the OSB chutes. They are shown in the print. We had more than enough 7/16 OSB to make them, after cutting out all the windows and doors. A chute of 1 x 24 is more venting than what you get with the staple up foam things like Prop R Vent.
We stickframed the gable ends, and let the gableface tail into the framing. You can specify to your truss plant that the gable trusses are to be shortened so you can stickframe the ends where you want to have a deco tail let in. Either that, or the gable ends can be little separate trusses that lap the larger gable truss, and sit back 1-1/2 on the plates, so you can lap your end tail to it.
The reason we stickframed the gables was that a.) it was a little less expensive, and b.) we dropped the gable to ladderframe across with lookouts to support barge rafters with ends detailed like all the tails. I know, we could have dropped the tops on a gable truss also, but I wanted the lookouts to bear on a plated stickframed gable, rather than a flat truss.
Deco tails were sistered to the trusses. Truss ends were flush to wallfaces.
H.A.P. is "height above plate," more commonly a term used by framers when cutting rafters. Truss guys call it heel height. I wanted the HAP of the not-birdsmouthed tail to be 1/8 or so shorter than that of the truss, so my 3/4 boarded overhang would plane out to my 5/8 roof sheathing. It was folly to try to get that precise. We used architectural shingles, which cover a little bump like that without telegraphing it.
Gene Davis, Davis Housewrights, Inc., Lake Placid, NY
Edited 6/6/2005 6:29 am ET by Ima Wannabe
Thanks Gene. I think I've got it. How has the Cor-a-vent worked out for you?
We went through an ice cold winter, and everything vented fine. No ice dams anywhere.Gene Davis, Davis Housewrights, Inc., Lake Placid, NY
Hi Gene, I will need to do something similar on exposed tails. How did the Cor-A-Vent look after install? Pics by any chance? Trim will be white in what I'm doing...vent is black, right?
There was an article in a recent JLC about concealing vents behind a secondary frieze layer with space behind. Thus the venting went vertical between blocks and over the top of rear block cut short like you've shown. I'm sure it looks nice, but it's gotta be a fussy thing to build, and perhaps not worth the hassle.PJ
Everything will be okay in the end. If it's not okay, it's not the end.
Cor A Vent is black and my color is dark green, so with the black strip way in the shadows buried in a surround of dark green, it is almost invisible.
I am sure you can find a way to face out with a 5/4 frieze block with the Cor A Vent strip behind it, to achieve the same thing. Just start doodling.
Gene Davis, Davis Housewrights, Inc., Lake Placid, NY
with r-50 cells and a good tight vapour bar, I don't believe that you would need an attic vent other than to satisfy shingle warrantees or roof inspectors
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Piff, the vapour barrier would prevent moisture buildup in the conditioned space below, but I'm also concerned with heat buildup from above via the roof deck and wondered if ridge and gable vents alone could adequately handle that. Not worried about shingle warranty as it seems most warranties are so limited as to be useless anyway. Inspector may be an issue, I'll have to talk about that at permit time.
Re your Q about using gable end vents in combination with ridge vents - well, yea, that would vent the part of the attic from the ridge vents up... Really, from a functional standpoint, gable end vents are out of "vogue" and if anything they would be used in combination with eave venting. Use eave venting - Gene's method would be good - in combination with ridge vents to get even & overall venting of the attic space.
FYI - if you are going to use blown in insulation, my insulation contractor uses heavy cardboard insulation baffles that I believe give better performance than the foam ones without the hassle and expense of making something out of OSB.
BTW - what state, providence, etc do you live in? Why not fill out your profile with a little info?Matt
I'm in Michigan.If your looking at your left hand flat on a table I'm about a 1/2" below the bottom knuckle on your pinckie. Near the big lake, as we call Lake Michigan.
Be real careful of the amount of free unrestricted air space you have under your sheathing and above your insulation. Don't use gable vents in combination with eave vents as your gables will become intakes for the ridge vent and suck snow in during cold weather and will only vent the ridge. I got some really good info from Air Vent (http://www.airvent.com) "Principles of Attic Ventilation" and have had great success with some of their products. Also WSRCA in combination with National Tile Mfg Assoc wrote a manual called "Design Criteria Manual for Cold and Snow Regions" which is excellent. It is written for the tile inducstry but it is completely applicable to any roofing product. I got a free copy from "http://www.trasnowbrackets.com" about a year ago.
Good Luck!
Lazera, thanks for the links, I will check them out.
I agree with the above thread that you shouldn't need ventilation with R-50 in your ceilings. We've done several dense-pack cellulose held in with 1" Thermax retrofits on old houses, and no problems yet.
Dan, any details on your method of dense packing? What is the Thermax- spray on or rigid foam or something else?
Thermax is foil-faced rigid. My installers typically put up 1" and blow the dense pack behind it.
i just finishes a job where 2 houses were built conected by a 12' breezeway and archy spected exposed ragter tails. so i drilled a 3" hole in each one(about 150 or so) and got some 3" round plug vents from my supplier. they fit in snug and i also used 5200 to make sure they didnt come out. the blocks were painted white and the pugs were alreay white. tried to talk the ho out of exposed rafter tails here but they were adamant.
oh yeah pitch was 8:12 so i drilled holes as high as possible and also centerd each hole from side to side.
these archys spec this look but not how to vent sheeesh. do i gotta do everthing?
tyke
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