Ceiling vent channels – homemade or buy
Looking to insulate the TJI ceiling joist in my new construction home. I seem to remember seeing a method for homemade vent channels using 3/4″ or 1/2″ foam board – does anyone regularly take the time to do this and does it substantially add any R-value for the cost?
Other alternative is the pink panther pre-fab and they seem a little pricey – is it worth it just for ease of installation?
I would probably fasten the foam board with a plasti-tacker nailer but I don’t want to take the chance of penetrating the steel roofing if the nail blows through the foam. What other methods hold the foam well?
Thanks for your input.
Replies
Measure the width between the webbing of your rafters (joists are something you stand on!)
Cut strips of you foam board about 1/4" less than that measurement.
Place a strip up into the rafter bay and take a measurement from the face of the foam to the top edge of the bottom flange of the rafter. Cut a bunch of scrap pieces this length, and use them to wedge up the foam board.
Get a foam gun ($25 to $75) and spray foam into the edges between the foam and the rafter webbing.
You can take out and reuse the pieces after the foam cures hard - a couple of minutes.
Remember, those vent chutes have to bring air in from somewhere, and expell it to somewhere.
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You're talking about channels for the roof rafters in an unfinished attic, right? Use whatever works. The molded plastic channels work OK, or you can simply staple old corrugated plastic political signs across the rafters, cutting a "tongue" at the eave end to be sure you completely enclose the space.
Insulation doesn't do much good here.
Let me clarify my initial question - sorry for the confusionCathedral ceilings, soffit and ridge venting -Plan on installing fiberglass batt insulation R-38 and plans call for 6 mil vapor barrier rather than kraft faced batts -I am most interested in determining if there is demonstrated benefit and added R-value by installing a vent channel such as was previously suggested.Thanks.
SIPS could have been a better approach than these joists.
I vote for calling in the foam insulation guy.
Why would someone want to vent an area where you're attempting to slow down or eliminate any air movement?
A little late to suggest SIP construction at this point I'd say...A sofitt and ridge vent system was spec'd in my plans and I am a firm believer that a roof needs to breathe. SIPS and super insulated construction allow for a "hot" roof design and I think that is allowed by code providing all criteria are met.Spray foam is not in my budget, the nearest installer is 250 miles away and was not interested in coming off of their exorbitant price. Basement is ICF construction and rim joist will be DIY foam sealed.I would rather let the house breathe on its own in the right places than seal it up tight everywhere and put it on a ventilator/life support system.
I'll tell you what I'm using on my two story gambrel roof....
I did buy some of those thin plastic vent-chutes that go between the rafters when Lowes had them at 10-cents apiece. They are flimsy...and worry me that they might squeeze-in when insulation goes over them... I'm insulating the roofwalls and adding collar ties/ceiling joists so I'll have an insulated upper floor.
Lowes also sells a wavy, corrugated, roofing material called "Ondura". Its a 6' x5' asphalt sheet with 1-1/2" corrugations that comes in colors. Since my rafters are on 16" centers, I ripped down the length and get four rippings. I turn the color toward the roof decking which gives me three deep corrugations that carry the soffit air up and over to a ridge vent system. I use aluminum duct tape to seal the transition point where the first angle roof hooks into the steeper angled second roof, then add a staple or two from a hand powered staple gun to keep the tape in place for when it gets old and the wet adhesive lets go... I also cut a piece of 2x4 to go between the rafters, push it tightly up against the Ondura, and airnail it in place to hold the material.
You can find some of those Ondura sheets either returned or store damaged and marked down to $5 or less. For what it now does, it's stout, easy to handle, and a heck-of-a-lot-cheaper than those plastic/foam vents that sell for around $2 a piece.
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Edited 11/15/2008 1:32 pm ET by BilljustBill
Edited 11/15/2008 1:37 pm ET by BilljustBill
That article was in FH by Mike Guertan.
Yes, I personally think they are worthwhile for the insulation value as where they are used is the place with the thinnest amount of insulation.
I dislike the cardboard vents, seem to disform and sag all by themserlves over time.
FWIW- one local inspector here wont allow foam ventchutes as he says they are inside the building envelope. He claims anything inside of the shingles or siding is interior. I think anything inside the drywall should be considered interior. Anyone here have an opinion on this?
Thanks - I just found the article this morning.We have no inspectors, codes or buiding department to satisfy out here. Just trying pay attention to detail when it comes to insulating.I think I will go the route of solid foamboard and try to glue it to the joist flanges with PL300 or get my foam gun back from my buddy's ICF project and go that route.Thanks for the help!
Hi,
I agree with you about inside the drywall is interior not just inside the shingles or siding, but what is his concern Fire hazard?w/ foam. I have a similar issue in a cathedral ceiling I want to insulate in a 3 season camp down east Maine. Thought I would put up say 1"or more blue foam board leaving an airspace w/ soffit vents and a ridge vent. seems easy enough with an air nailer to tack spacers rafters at the top and cutting the blue board to a close fit and sealing with spray foam.
It's just a camp and the finished ceiling would be pine T&G. I don't really want to drywall before the pine mostly because its almost impossible to get sheet goods up there as I want to leave the open joists (which really are just collar ties) which are 32" oc. The rafters are 16 oc.
Is foam inside the ceiling a fire hazard?
I'm not worried about code here either we just want to be safe and warm.
Bob
This inspectors concern was with the foam being exposed in an interior living space, in the event of fire could give off toxic vapors, he considers the attic to be interior space. I believe he is wrong, but easier to comply than to argue.
I just completed a small cathedral ceiling like yours but smaller. I first made an air dam on the outside of the top plates of each joist bay. I did this by nailing scrap furring strips to the sides of each roof rafter just to the outside of the top plate. Then I cut a piece of 1/2" Thermax the width of each bay and 3/4" shy of the height. I installed that leaving the 3/4 opening on the top. I caulked or foamed the sides and bottom of each dam. I installed some 3/4 X 3/4 scrap pieces 2" on center to the upper side of each roof rafter against the roof plywood. I then installed 1/2" Thermax continuously to the ridge to make a 3/4" air space. I caulked the sides and taped the joint between the dam and channel and at the ridge. Actually worked in the room in the winter and it stayed pretty warm. Then I installed my insulation tight to the Thermax. I used Thermax because in the summer I thought the alum. surface would reflect some heat and wanted it not to be a fire hazard. Had recessed lights and keept them below the Thermax and no air leakage.
Thanks for the input, that's about exactly what I plan on doing. I've got frieze blocks made from the rim joist with a 'V' notch at plate and ridge to butt the foamboard to, no can lights to worry about, just a couple light boxes and blocking for ceiling fans. Thanks!
Dumb Question, but did you vent the air space above the thermax or is the idea of the caulk and foam to keep the interior air from reaching it in the first place and venting not needed?
cheers, Bob
I held the Thermax 3/4" down from the roof plywood. this gave a channel for air to move from the soffet to the ridge. Yes caulking and taping kept the air from infiltrating into the room.
The way you did it is exactly what I'd envisioned. Glad to hear I was thinking right. Thanks and cheers. Bob