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I want to start a discussion of venting of vaulted ceilings.In my shop I want to pack the 2×8 rafter bays with R-25 insulation,then add foil face rigid insulation on the face of the rafters ,tape the seams and drywall. The inspector in my area is demanding that I provide a 1” vented air space above the insulation in each bay. I say that allowing cold air into my insulation just moves the dew- point inside of my insulation.He says I need the venting for dissipating moisture. I say my foil faced insulation provides an excellent vapor barrier.I think packing the rafter bays full of dry insulation is better than allowing cold wet air to circulate above and around the insulation. What do you think ?
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I agree and disagree with your building inspector. I agree with him on the need for vented air space. I disagree with him on the depth of the airspace. In Minnesota, the minimum distance for any kind of airspace above insulation is 2". Otherwise, the air flow is impeded. Here in the north country, the only reason you'd consider packing your insulation bays full is if you have such a steep roof pitch that you're not worried about ice dams. The rest of your plan sounds great! It's a good idea to use rigid insulation across your rafters with builder's tape sealing the seams. I know it seems counter-intuitive to have a cold air flow into your attic, but it's important to keep your roof absolutely COLD to prevent ice dams, and you will have some heat escape, no matter how much insultation and vapor retarders you install. There's a couple of builders in our area who pack the bays "full", and there beautiful vaulted ceiings have mega-ice dams on the roofs. Good luck!!
*Paul, the inspector knows the code, but I think you've got the physics closer to right. Air wash over FG insulation can significantly reduce its R-value. I'd worry about the foil-facing, or about any VB, if you ever plan to cool the space, however. Air seal, be sure your ceiling is a continuous air barrier (drywall, for example, without can lights). I'm a cellulose fan, but if I had to use FG in your circumstance, here's how I'd do it. Build vent channels the full width of your rafter bays from PIC foam such as Ultra-R, and seal them to the rafters with non-expanding urethane foam. Air seal, air seal, air seal. Add kraft faced FG and drywall.Andy
*If you do run foil-faced RFBI on the face of the rafters, gap the sheets by about 3/8ths of an inch. The sheets don't have to break over the rafters. Foam the gaps. Then tape the seams.Nice 'n tight.Non-venting of cathedral ceilings can be accepted by the inspector under the alternative methods section of the code...but it may require a combination of you educating him and him being open-minded enough to accept alternative ideas. If you're going to do it differently, do it well, and do it right.
*Vaulted ceilings - I've seen much moisture damage over the years when rafters are packed full of insulation - these "hot roofs" have no venting thus moisture control, dew points, and condensing surfaces must be closely coordinated - obviously they are not in many situations and rotten sheathing and sagging rafters are the result. Cold Roofs allow venting and experience fewer problems but 1-1/2" and smaller channels have proven to be inadequate to properly remove moisture and heat in winter. A 3-1/2" space works very well. 2" will probably do the job.
*Paul... dense pack with cellulose after the inspector is done.More info... Stray "As such, energy conservation has the potential to destroy more buildings than architects." 1/31/02 9:42amnear the stream,aj
*Andy... your idea is good and even better if the foam board is put on the roof prior to roof... less seams to seal.
*That's what we're considering doing for an attic finishing--but this dense-pack stuff . . . How does one know if a contractor's equipment is really capable of the necessary force? They all say (in effect), "Sure, little lady," but how, er, does one know short of snarling "Lemme see the documentation on that blower motor?" We're in Salem, VA, where the insulators don't read Fine Homebuilding much.(I meant to be responding to the advice to "wait until the inspector is gone and fill the rafter spaces with dense-packed cellulose."
*pal h, et al. You have the right to use the alternative methods and materials section of the code, and the inspector has the right to ask you to document your alternative to ventilation.Ventilation is the code's method of getting rid of warm moist air from leaky ceilings--attic floor. Filling the rafters with--preferably cellulose, adding RFBI then GWB, all foamed and taped is another, and better way to do it. Good luck. GeneL