Hi y’all, I’m in the late planning stages of “The Dream Garageâ€. I’ve been wanting to do this for years and it looks like it’s gonna be a reality. I’m jazzed!
Anyway, I’ve hit a design snag I thought I’d bounce off of the fine minds here. 6.5†SIPS walls, 3/12 pitch (matches the house) raised heel trusses, shooting for R50 cels over walls. The eves will be exposed 2×6 rafters with a flat 2×4 let in at roof edge (also like house). House has 2x blocking between rafter tails at wall, also gable vents.
I’m wanting to install an inconspicuous eave vent along with ridge vent, but the cels will theoretically go right against the inside of the 2x blocking. I’ll install foam channels in rafter bays, but the detail for eve venting has me stymied.
I imagine I could build in a passage of a couple of inches behind blocking and use traditional louvered and screened vent, but I’d like it more stealthy. Another idea is to leave a space at the top of blocking, but how to keep the critters out?
What say you? Am I missing the obvious? Ideas?
Many thanks, PJ
Replies
PJ:
I don't have an answer, only exactly the same dream garage and exactly the same rafter tails/eave venting dilemma. So if any genii are out there, mine is being framed next week and I need to be able to state a preference! You will be helping 2 people.
Carol
PJ, Have you thought of stapling screen wire between the 2x block and underside of roof deck? You want max air flow into the attic and between the baffles and decking. If in an A/C region, I would use a continuous radiant barrier instead of the baffles to channel the out door air all the way to the ridge vent. With a low slope roof, you will get a better convective air flow with all the hot air trapped in the challels until roof vent exit. Paul
Thanks Paul.
Yep, some kind of screen is in the cards. If I vent at the top of blocking, I suppose the screen could be installed inside prior to rocking the ceiling...no soffits to cover the ugly on the outside. I'd end up with a 1 1/2" "shelf at top of blocking...not ideal. I've also thought about routing a slot through blocking, rabbeting the inside and screening the backside before installation,(can you tell I'm a cabinetmaker?). Hoping for an off the shelf solution, I guess.
Tell me more about the radiant barrier you suggest...material? Attach to rafters? Space between deck and barrier? It is a "garage"(the city's requirement) and won't be air conditioned, but I hope to spend countless hours out there, so reducing heat gain would be a good thing.
PJ
Peter, a good system with rad barrier is to staple the material across the underside of the roof rafters(aluminum side down). However, with your low pitch, I would drape it over the rafters and let it sag into the space between the rafters, prior to decking the roof.
The barrier is made of kraft paper foil one side, and a scrim of string to give it extra handling strength. Very easy to install. Cost of material is about 10 cents a sq ft. The beauty is the hot air is channeled between the roof deck and the barrier, thus travels to the ridge vent, and that pulls in more outdoor air from soffit, which sets up an ongoing river of fresh air between the barrier and the roof deck. The foil needs 3/4" space without contact. The shiney side goes down so that it does not get a layer of dust over time. The aluminum is not reflecting heat, it is not re- emitting the heat.
Emissivity of aluminum foil is very low, like 3%, so 97% does not get re-emitted or passed on to the air beyond the barrier.
I can keep my attic to 100 deg, black roof, on a 100 deg day, thanks to the barrier system I installed. The "system" is important, as you want more than just a barrie. You want to channel the hot air as well, with a steady stream of fresh outdoor air flowing into the chamnnels rapidly. Is all that clearer now? PaulEnergy Consultant and author of Practical Energy Cost Reduction for the Home
Thanks Paul, that's super. One question. Is there any reason to "seal" the channel from the rest of attic...like at the eave and peak? I assume this barrier comes in a roll sorta like felt, so I imagine the droop may be difficult to keep consistent if it installs perpendicular to the trusses. Maybe easier to roll top to bottom one rafter bay at a time? Guess I need to do some research on the stuff, any place on the web you'd reccommend?
Shalom, PJ
Peter, where do you live? I will be able to direct you to best source.
With your slope, I would drape the barrier across the rafters from one end of building to the other starting at the low end of rafter. I would let it create a tent in the attic space.
The material comes in 4' x250' rolls, and has pinho;es so that it does not trap moisture. With continuous soffit intake air, you can drape the first run right at the lowest point of the rafter. You understand the air flow that you are creating. Perfect alignment from one horizontal row to the next does not have to be perfect, but the more consistent will produce a better air draw. PaulEnergy Consultant and author of Practical Energy Cost Reduction for the Home
Humble abode in Beautiful Boise, Idaho. Thanks again for all your help.
PJ
Have you considered Cor-A-Vent eave vents?
I've put this on my own new addition - check it out at http://www.cor-a-vent.com/s400.htm.
-TJ
Oooopppssss! My mistake!! It's NOT 'ADS' metal roofing. I got my vendors mixed up; I'm drawing up my shop plans now and have too many things on my mind.
It's 'cor-a-vent.com'. They make ridge vent stuff - you leave an opening at the rop of your roofing and then nail in their pre-made ridge vent. They also have the venting for the lower blocking.
I decided to go with the metal vent provided with my metal roofing so had put the Cor-a-Vent catalog aside but remember their vent at the bottom.
Sorry for the confusion.
J-J
Yes J J! This would do it, offered in white even, looks better than anything else I've seen. Could install on top of blocking. Anybody used this stuff... what's it look like from the ground? I actually perused Cor-A-Vent site before and somehow missed it.
Is there a screen over that stuff somehow? Hard to tell how big the openings are, size of critter that could crawl through.... any thoughts?
http://www.cor-a-vent.com/soffit.htm
Best so far, any other posibilities?
Thanks, PJ
I'd rethink your rofing plans, why not use SIPs on the roof as well? Great heat preservation, and realtively simple construction on a roof that shallow..
On my house I don't need to vent it. I used cedar breather under the shakes and that provides a good heat escape path..
If you don't plan on using shakes, you could build a cold roof and vent that thru the facia..
I've run that by the SIPs guys. They advised against it for a couple of reasons.
1. Complexity. The city has to have "architectural continuity" between house and shop...errr, oops, I mean "garage". The house is brick, and I don't have the $ to do that on garage (3000 sq ft), so I'm convincing them that the rafter and roof (dutch hip) detail is "continuity" enough. The exposed rafter tails have no fascia save the flat 2x4. Anyway probably could be done in SIPs, but then there's...
2. There's a short second story on part of the building for storage. The lid of the insulating envelope is the floor of the second story to reduce heated area. One of the goals is to heat it cheap, so I don't really want to add additional heated area. That area is better than 12,000 cubic feet...no small potatos...or is that potatoes...
So there is a method in the madness...
Thanks,PJ
Edited 5/12/2004 4:56 pm ET by Peter J
Take a look at 39444.1 in the archiweves.. those are SIP roofs. see if you approve of their look..
Frenchy, I think your house is spectacular. I've followed your posts about building it with much interest.
I'll ask the Precision Panel folks to take another look, and perhaps compare $ both ways. The panels would certainly make the finishing of the roof go a lot quicker. The roof will be composition shingles to match the house.
PJ
On my house I don't need to vent it. I used cedar breather under the shakes and that provides a good heat escape path
Frenchy
Ya know.....I must have had tunnel vision or brain lock or both because I was going to use soffet vents on my additions where I used CB under the new striated shingles I installed.
Guess all that'd vent would be the plywood sheathing huh? hmmmm.
Now ya got me thinking (for a change).
I think I may still add the soffet vents along with a gable vent but...ya got me thinking.
Interesting topic.
Be well
andyMy life is my passion!
http://CLIFFORDRENOVATIONS.COM