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gfsdave 07-22-2008 01:33 PM

2 x 6 Cathedral Ceiling- options for insulation
Background: I have an 1.5 story cape that I am finishing the attic space. I have 2 x 6 ceiling joists 16 O.C. and live in a cold northern new england climate. Local building codes do not require minimum R-value for interior remodel of existing structure. I am trying to maximize r-value with least cost while maintaining max ceiling height. There was NO insulation in the attic prior to this work.

The local big box retailer suggested I order an owens corning high density product (R21 FastBats (no staples), 5.5 inches) and install foam rafter vents from the soffit to the ridge. I am also installing continuous soffit vents and ridge vents so each rafter bay is vented.

The questions:

Am I going to be okay putting the r21 product in the 2 x 6 ceiling joists with the foam rafter vents?

Specifically, after beginning the install, I have noticed the insulation still touches the roof deck on the edges because the rafter vents are only 12 inches across. I am concerned that these areas are not vented. Any tips and/or experiences with this scenario?

Also, the "fast batts" do not have overlapping edges to staple to the joists but are designed to be stuffed in the cavity. On a few of my joists the rafter cavity is anywhere from 16.25 to 16.5 OC, so the seal on the sides is not tight. However, I am concerned that 4 mil poly would create a double vapor barrier. Any tips and/or experiences with this type of scenario?

Any feedback is very much appreciated.


white29 07-22-2008 06:10 PM

Forget the rafter vents/baffles. No R-value,flimsy and as you've discovered they're limited to certain sizes. What has worked great for me on several vaulted ceilings is this; attach some 1x2 material(or rip to 1") in the corner where the joist and underside of the roof deck meet. these are your vent spacers. Next , fasten foam board insulationto these spacers in a thickness which will allow you to place your roll or batt insulation into the remaining space without cramming.Stuffing too much insulation into a given space defeats the purpose. I came up with this idea 20 years ago to achieve max R-value in changing a then flat ceiling to cathedral in our total kitchen gut job/remodel and it has worked well for me.Hope it helps,good luck.

white29 07-22-2008 06:16 PM

Oh forgot something, sorry. If you want to use the poly VP and are worried about double VP( a proper worry,indeed) just slash the vapor barrier on the batts with a utility knife along the length of the batt and you'll eliminate that as a second VP.At least thats what I've always done,no problems.

the roofing god 07-22-2008 09:10 PM

he makes sense,don`t use the plastic at all,and from underneath attach 2x2 `s to the bottom faces of the rafter tails so you can use more insulation---another option is install the batt as he desribed and then spray solid cell foam insulation to fill the cavity(up to R-7 PER INCH),THIS CAN BE DONE WITH A SPRAY MAX GUN,Check for the full scoop,the difference makes for great energy savings

gfsdave 07-23-2008 07:55 AM

Fugly work around-- Effective?
Thanks so much for the quick responses. I am far, far down the road of having this installed and do not want to go backwards too far, but will if I have to. One workaround that occurred to me was using a house wrap sealing tape along the entire length of each side of the joist to provide a vapor retarder (not barrier) in those situations where the joist cavity is slightly larger than 16 OC. This seems like it may solve the problem of the insulation not fitting tightly against the side in terms of moisture buildup.

Am thinking with a proper vapor retarder the silly "raft-r-mates" should work as designed, although the entire design does seem questionable. Thoughts?

Grumpy 07-23-2008 07:46 PM

Maximum r value for minimum cost? The two are mutually exclusive. Maximum r value would be a spray in place foam product and depending on your shingle manufacturer could probably be installed with the compelte lack of ventilation. You could completely fill those rafter pockets with spray foam!

Otherwise you will need to ensure positive air flow from the eaves to the peak using rafter baffles which are fiarly affordable and then installing some bat insulation which is also affordable but has a lower r value than spray foam. If it were my house, I'd spray foam.

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