*
Okay, I know you THINK you covered this in the thread below about dense pack cellulose, but I still have some questions. Remember, I have warned you all before, I am legally blonde. Dense Pack might also apply to my brain.
Okay, so, first can we test my knowledge. The way I understand this is that with a cathedral ceiling, we can fill the cavity with dense pack cellulose and forget about the venting. I like it. But, here are my questions. Our I-Joists are 11 7/8, so that would give us approximately an R-45 roof. Am I doing okay so far?
i Is this enough to keep the heat from going up through the roof, melting snow and creating ice dams?
Also, our roof pitch is 12/12. On the north and south sides, the roof meets up with a 3/12 porch. The porches seemed like a good idea to protect our logs, but now I’m not so sure. Are we doomed for ice dams, particularly on the north side, no matter what, when the roof meets the porch? Climate is Colorado Mountains at 9000 ft.
I think what we are leaning toward is venting it and then doing dense pack cellulose because it seems like the “safe” thing to do. And yet, those ridge vents are kind of pricey and soffit vents are kinda ugly and it would ultimately be more work. I’m sick of work. Anything to get out of it! 🙂
Oh, one more thing, it is really windy where we are. Will the wind whistle through those vents if we put them in?
Thanks for all your indulgence. You guys are the best. –Paula
Replies
*
Paula,
"Is this enough to keep the heat from going up through the roof, melting snow and creating ice dams?"
You could have ice dams even if you had a roof rated at R100.
You also have to make sure that the ceiling is sealed perfectly.
Is your roof metal, wood shingles, asphalt shingles or something else?
Gabe
*Dear Paula,First of all, ease off the dumb blonde jokes. It's going on 30 years since I met the most luscious blonde dudette on the planet. I don't think either of you qualify as "legal". I simply would not respond to your question if you were.Next, congratulations on your house project. You seem to be having a lot of fun. Then, check FHb issue #130 and search the Home Energy Archive the for more information on cellulose. Lots of info for brains not dense packed.Dense packed wood I-joists are a very effective thermal assembly. But your climate is extreme and I'm guessing that you get a LOT of snow? You may need additional detailing, but in any case, venting will cause more problems than it solves. It is not suitable for very cold, very snowy climates.Every insulation product carries a resistance rating based on lab testing. Your calculation of R-45 is correct. In real applications, all insulation performance is degraded by installation defects and thermal transfers in the framing. Wood-I's are great b/c they limit these effects. The rafters are fewer and farther apart than lumber framing. The webs are thinner and less conductive than solid lumber too. Wood I's are also very difficult to effectively insulate with batts. Insulation batts do not effectively conform to the structural flanges. This results in air channels that severely degrade performance. Blown fiber insulation fits closely to the flanges and everything else. These qualities mean that your real roof will effect a higher thermal resistance. But will it be enough? Sealing the ceiling is a key factor. If you plan to have boards for a ceiling, as many log home owners do, you must install a rigid, monolithic barrier behind the boards. Drywall, plywood, osb, thermoply for example. Foam boards are excellent too. And if you expect to have more than 24 inches of snow on your roof for extended periods, a layer of iso board in the ceiling would be a good idea. Even a half inch thickness of polyisocyanurate (the foil faced stuff) foam will provide a dramatic boost in performance.Your porch assemblies will need complete self-sealing underlayment regardless of the roofing product that goes over it. Install it with the pretense that it is the only roofing you have. Extend the goods up onto the steep roof a full 2 feet. Then lap the tar paper from the main roof down over it a foot. Use ring shank cap nails to secure all the underlayment instead of staples.Then you will have very little ice build up and no roof leaks at all.Regards, Fred
*Geez, Fred, it's good to have you back. Welcome.
*Dear J,Good to be with you too. I need to hear more about your drug that erases memory. I have this client who wants to change color half way through a siding job...........Best, Fred
*Thanks for the responses everyone. In defense of the dumb blonde thing, I have to say that I am probably one of the smartest dumb people, or dumbest smart people you'd ever care to meet. I can easily score 100% on a calculus test, but it took me over an hour to figure out how to use a weed sprayer. I guess you could say I'm mechanically challenged, maybe not a dumb blonde :-)Gabe, roof will be dimensional shingles. What is the best way to seal off the roof completely. Will the dense pack alone do it with say a vapor barrier, or do we need to consider other things as well.FredL, your picture was frightening. EEEK. We will consider adding a layer between the i-joists and the tongue and groove. Also, our porches and the bottom 6 ft of the roof will be covered with Ice and Water shield, so it looks like we are in good shape there.Paula
*fred.. i wouldn't think of talking for gabe.. but why don't you lay off...now that you have the whole scene to yourself.. you could be a little gracious.. you could also try talking in plain talk instead of the gobbledy-gook you are pushing... ..i have certainly tested a few ideas against your theories...but you always fail to say that that's just what they are... theories...want a little more discussion ? then don't pretend you are the font of all knowledge.. are you going to take up right where you left off .. or are you going to start off with a new discourse?
*wrong time, wrong place mike. i'll be happy to get into it later if you like
*Hi Fred,Good to have your thoughts and suggestions back here again.Thorny issue ahead:I note in your prescription the added detail of the monolithic air barrier under the T & G. Has your thinking changed in the last couple of years about the viability of the DP cells alone to be a good enough air barier behind a t&g ceiling?This is something I have wrestled with in my own mind, particularly when it comes to a retrofit situation. How many roofs were built in the seventies with 2x6 rafters and a t&g board ceiling? Are we doomed to have to rip out the ceiling or build up from the outside? Is it safe in such a situation to simply retrofit DP cells and call it a day? I know it'll depend on climate as to whether it will stop ice dams, but would it be safe in terms of moisture control?Steve
*fredl: u _____ ______ deleter... are u going to do the hit , delete , & run thing again.. or is this your admission that your cut at gabe was uncalled for?if it's the latter, then my apologies ...if it's the former..then your up to your old anti-social behavior again.... get counseling
*Dear Steve,No, no, no. I'm still doing it the easy way on retrofits. I looked at a 10 year old installation today. 2x12 low slope roof with 4 inch t&g ceilings from the 1930's. Damp slab floor. You can see my cels peaking through the knot holes and gaps inside. Absolutely no trouble. I have a chance to reroof the place and I'm bidding low so I can cut the bays open and take photos/samples. They're heating 3600 square feet of space with a theoretical woodstove. In Paula's case, I would slab the ceiling with an air barrier before the boards. Bad climate, bad roof geometry, easy measure to install on a new house. I think R-45 is minimal for that house and if the air barrier were to be 1" foam, I think it would pay. But I wouldn't fret the moisture control issue regardless.Dusty, Fred
*mike i tried to email you, but it bounced. this is over your head. please chill and X all g/j stuff.
*fred.. the only thing over my head is my vented roof with 20" of cellulose in it since 1984... don't be condescending .... you ain't the only game in town... just the one with the biggest horn
*Dear Mike,Lets check your scorecard:Posts on topic 0Personal attacks 3Discussions disrupted 1Later, Fred
*"In real applications, all insulation performance is degraded by installation defects and thermal transfers in the framing. Wood-I's are great b/c they limit these effects. The rafters are fewer and farther apart than lumber framing. The webs are thinner and less conductive than solid lumber too."11 7/8 TJIs will conduct through the web in a solid filled unvented roof. I thought I had it covered with 1" foam strips on the top flange, 4" spray polyurathane, and 10" fiberglass. We see light frost melt in line with the TJIs on 20-30 degree days. I didn't think that would happen. If it's colder or covered with snow we see nothing. It got much better when we finally got the extra fiberglass in the rafter bays and sheetrock on. But still, we can't be loosing much heat as we are heating (4500 sq ft) for about $80/month with electricity. It seems there needs to be more board insulation on top and/or the bottom of the TJIs than just one inch in North Dakota anyway.If I were to do it again I would still use the poly spray + dense pack (I learned about dense pack too late) and an inch of foam board on both sides of the TJIs.
*Jessie, have your shingles curled up and blown away yet? Joe H
*No, but I kinda wish they would so I could put another inch of foamboard on the roof. Did you move yet?
*A question on the idea of the sealed monolithic layer (ala sheetrock or something) above the T&G ceiling...Won't the hundreds of nail holes that hold the T&G up, poke 100's of holes in this layer? I'm thinking especially with T&G moving with seasonal moisture variations, the holes in the sheetrock or foam around these nails would widen over seasons.just asking.
Fred,
In a milder climate (Fayetteville, Arkansas), the same 12/12 roof decked with Solarboard, and 2x8 rafters, the ceiling is sloped from the 4 ft. knee walls up to the 9 ft. flat ceiling (with a small attic space above). Roof has continuous ridge-vents and soffitt vents. Insulation contractors suggest venting every other or every third rafter space but I want to vent each rafter space with 1x1 furring strips and 1" foam board because of the the Solar board decking. Doesn't the SolarBoard require airspace to be effective? And is 1" enough space for venting as well as enough to reduce radiant heat transfer?
We are using cellulose but were not planning on any monolithic barrier between the rafters and the t&g wood ceiling. Do I need to install drywall under the t&g?
Will appreciate advice,
GWILESKI
Dear G,
Let's see if I can pick out all your points.
Nix on your contractor's idea of skip venting. I think he's sending you a coded message that venting won't do a thing for you. But if you want it, do the whole roof deck, every bay.
Solar reflective surfaces take a bit of explanation. Yes, the mirror effect can't work without the air space. However, after a while, even the most perfect installation degrades. As dust clings to the shiny surface, it becomes more emissive. After a few years, the effect is gone.
No, one inch of venting will not cool your shingles. There are 14 miles of air on top of the roof, so another inch on the bottom has no effect.
I highly recommend a monolithic substrate for T&G ceilings. When clients get my proposal to retrofit this essential building component into a finished, occupied house, the cost makes their knees buckle. Lack of a solid pressure barrier is a huge house defect. It makes people sick, wastes their money, ruins their dreams, and wrecks marriages. I can't overstate the human tragedy and can't understand why building codes continue to allow this.
My best advice would be to skip all the work involved with the foam board inserts, use the [styro]foam to sheath the underside of the rafters, blow the rafters full, and live happily ever after.
Congratulations on your new home, Fred
[email protected]
I'm in the process of dense packing cellulose between 16" I-joists. I still don't know what dense pack means. I'm trying to get 3.5 lbs/cu. ft. I saw that figure in some literature, but I'm not sure. What is the definition of dense packed cellulose?
arvid... settled density is typically 1.4 lb/cf
standard blown in-wall is 2.2 lb/cf
i personally firgure anything over 3.0 lb/cf is dens-pak..
we can usually blow between 3.0 & 4.0 lb/cf.... especially in a "Regal Wall installation ( insul mesh stapled & glued to the studs )Mike Smith Rhode Island : Design / Build / Repair / Restore
Mike,
Thanks for the information. I guess I'm getting the right density. For what its worth, I found that the blowers at Home Depot won't get you up to 3.5 #/cu. ft. I measured the cavity between the I-joists and calculated the volume and then weighed the bags of cellulose. I found I had to manually pack the stuff with a ram after every foot or so of blown cellouse. I slide four foot lenghts of 3/8" OSB on the tops of the lower flanges to support the cellulose and to withstand the pounding. This left a space the thickness of the flange (in this case 1-1/2") under the OSB. I ripped Tuff-R (1" and 1/2") to squeese in between the joists (foil face down) which fills this space to the bottoms of the flanges. As a precuation I caulk around any penetrants and noticable gaps. When we're done with this we'll attach dry wall for the ceiling. The I-joists directly support a nearly flat roof covered with a PVC Duralast roofing which we walk on. Venting each bay would not be practicable so we are hoping the cellouse manufacture's claims that there is no venting needed with dense pack turn out to be true.