Conflicting information
dwbrowny
| Posted in Mechanicals on
I have had 2 insulation contractors review a rehab I am working on. It’s a small cape, 2nd floor, it has been gutted. There is an existing ridge vent and gable end vents, minimal soffit venting. Walls are the slope of the roof.
Both are saying dense pack cellulose. One says install proper vents “rafter vents” the other says no venting is required. One will spray floor to peak, the other floor to flat part of ceiling, and fill above.
Who is right, do I install proper vents and open up soffit vents??
or just sprayy floor to peak and close up vents??
thanks
David
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Replies
First, where are you located? (US climate zone: https://www.greenbuildingadvisor.com/sites/all/assets/htzones_with_cities_abc.jpg )
Combining gable venting with ridge venting short-circuits the air flows. Gable venting (without the ridge vent) is fine, soffit-to-ridge venting is fine- provided that the free area of the soffit vents is at least a bit larger than the free area of the ridge vent (which you don't have.)
Code requires a 1" minimum air space between the insulation & roof deck for vented assembly, independent of the venting approach. If unvented some fraction of the total R (varying by climate zone) needs to be air-impermeable (basically foam) in direct contact with the roof deck, or a comparable amount of insulation value above the roof deck. Dense pack cellulose is air permeable, and would not qualify.
Where do you live? Are you replacing the roof? What type of HVAC system? What's your objective? That is, are you trying to make the space habitable or very energy efficient?
David,
The contractor who says that it's possible to install dense-packed cellulose in your rafter bays without ventilation baffles is mistaken. He is suggesting an insulation method that violates the building code.
For more information on code-compliant ways to insulate sloped roof assemblies, see How to Build an Insulated Cathedral Ceiling.
Sorry, zone 5a, No not replacing the roof, was done just prior to us purchasing this year. Forced hot air,Propane. Trying to make re-habitable, had to rip everything down due to water damage. We will open soffits and close gable ends.
In zone 5A at least 40% of the total R needs to be air-impermeable insulation exterior to the fiber layer in an unvented roof assembly to fully comply with the letter of code. You can cheat that a bit if you use a Class-II vapor retarder or "smart" vapor retarder on the interior side (either half-perm paint,on gypsum board tight ot the insulation, or a smart membrane tight to the insulation), but it has to be air-tight to the interior too.
Current code-min for zone 5 is R49 total, which you probably won't be able to do everywhere. The 1" minimum air gap gives you less space for insulation in the sloped ceiling section. How deep are the rafters?
Even above the flat part of the ceiling, there is barely enough room to get R49. rafters are going to be padded out to 10" above the proper vents, that gives us about R36, that's about all the room we can spare. this will still be a heck of allot better than the 4" of fiberglass that was hanging there.
If it's 2x8 rafters and you're adding 2x4s, you can get a decent amount of thermal break by running the 2x4s perpendicular to the original rafters rather than simply extending them. If it's 2x6 rafters you can built up truss-like assemblies with 2x4s and 10" x 10" sections of half-inch OSB to connect them every 3-4', with far less thermal bridging than you would get by extending them with an other set of 2x6". Either approach goes a long way toward bringing the total performance closer to that of R49 between joists or rafters.
Thanks Dana, sounds like a plan! Should I be thinking of some sort of vapor barrier between the sheet rock and cellulose?
David,
In your climate zone, the building code requires an interior vapor retarder (not a vapor barrier). Vapor retarder paint (primer) on the drywall works fine; you don't need interior polyethylene.
Remember, it's always a good idea to have an interior air barrier -- in your case, that means making sure that the drywall layer is relatively airtight.