kneewall/ratfer insulation question
Ok guys I have searched the posts for a while and cant come up with a conclusion to my question so here goes. I am finishing a bonus room above a 3 car garage in michigan. I just built the house and went all out with nuwool, low-e windows,superior walls bsmnt, geothermal heat/cooling and want to insulate the bonus space correct. My kneewalls are 2x4s and the rafters in the bonus space are 2×6. I have a continous ridge vent and am going to continue from the soffets up to the ridge vent on each rafter bay, because I believe in the vented sytem saving shingle life. From what I have read the best method to insulate this space involves putting poly-iso or foam board on the inside, after blowing cellulose or even using fiberglass then foaming the seams of the poly sheets to make it air tight. This all sounds doable to me but hear lies my question……….Why do you suggest furring over the poly sheets and then putting up the Xrated drywall? Why cant you just insulate, the apply 4×8 poly sheets with capped nails, foam seams and then apply drywall directly to the poly board, by screwing it into the rafters with longer screws. This furring out after the foam board seams like a waisted step to me, what am I missing. Also if the poly is so flammable why create an air space between it and the drywall??? If there ever is a fire wont this help spread it faster yet with the air space from furring it out?
Replies
Three reasons I can think of
One - the space is a place you can run wiring so as to avoid penetrations in the foam.
Two - The blown insulation will create a bulge in the containment fabric, making it hard to force the insulation up to the rafters to nail it in place, so we put it up first and then the strapping to help hold in place straight and cut holes to blow which are sealed later
Three - part of two maybe, but the strapping give better hold up than the cap nails when the insulation is blown putting pressure behind it, but also, the strapping makes it far easier to hit solid wood with te DW screws, both because it is visible and because it is wider
bonus reason - we use stqrapping on everything around here anyway and it helps reduce sound transmissions, adds to structural strength, helps prevent seams in SR from openning cracks, etc
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Here in Raleigh code requires R30 for ceiling/roof insulation, can't see how you would get that in a 2x6 rafter.
Let's not confuse the issue with facts!
you could put in r19 batt and then nail up 2in poly board on the interior=R34 and then screw drywall right on top of poly foam board through to rafters is what I am thinking.
Have a look at this Q+A and look farther into the website.
http://www.uwex.edu/ces/house/dynamic/subdetails.cfm?topicid=349&name=Attic%20Ventilation%20
Bill Rose has spent a lot of time researching venting- it's not near as important as we think!!!
but still nobody has said why you CANT screw drywall directly to foam board. If you used fiberglass batts, there would be no builging in any of the bays. Then on top of that you nail up the foam board seal all the seams with foam. and then screw drywall on top of the foam directly into the rafters and kneewall studs, this should work, correct? I know that for sound absorbtion the strapping would help.
Johnny, got the second email and found the thread. Thanks for the link.Here's the deal...my opinion...I'll detail my own attic. I used 2" polyiao on the rafter faces with furring strips over that. I used 1" polyiso on the gable-end walls with no furring.The strapping really wasn't a big deal to install. I ripped my own from partial full length sheets of scrap 3/4" ply. If I didn't have the scrap I would have gone out and bought a sheet or two to rip to width.The strapping goes up extremely fast. Remember, the ends don't have to break over the rafters. The added bonus of the strapping is that you can place it wherever you need for blocking. Going to hang a pendent light? Add a 12" square piece of 3/4" ply about where you want it and adhere it in place right on the polyiso with some Liquid Nails or something similar. When the 12" square of ply is sammiched between the drywall and the polyiso, it's now a secure backer for whatever you want to use it for...screwing faux beams up to break up the plane of the cathedral ceiling...adding a smoke detector...screwing in eye hooks to hold up a screen for your projection TV...or most importantly..as a nailing backer for your trim.It's either that or nailing your trim up through a couple inches of foam.Now I'm going to skip ahead. The gable-end walls. No strapping. Drywall directly over 1" foam. When I drove in the screws, sometimes the underlying foam would give a bit and compress, sucking in the drywall in that spot. Not a big deal at that moment, but over time the foam can kick back a little, and I ended up with a few screws that blew through the gypsum core after the foam "decompressed" and pushed the drywall out a bit. It was hard at times to gauge when to stop driving the drywall screws. With a solid wood backing, setting drywall screws at the proper depth is a cinch and you can fly through the sheets. However, I found that going over the foam required me to really go slowly, I even had to back some screws out a bit. Back to the ceiling. Adding the strapping allowed me to do a couple of things. My attic space was 46' long, so there was a lot of roofline up there. Any waviness in the drywall would have looked cheap and flimsy. With the strapping, I could tighten some screws down a bit more and back off on others, all to bring the strapping into plane. Flat strapping means flat drywall over the strapping.I hate hanging drywall. I hate hanging it when I can see the framing member that I'm trying to screw into right on the other side of the drywall. I'd hate it even more if I was searching through 2" of foam for the screws to find purchase. To then overdrive a screw and have the foam pop the head the next day...or two weeks later after it was taped and mudded...and painted. Ugh.Screws? This way you use the long screws to attach the strapping. Easy. You use 1-1/4" screws to attach the drywall. Easy.Lastly...While it took me a fraction of a day to cut and install the strapping, I know that it saved me a helluva lot of hours when hanging the rock. The 3/4" space can be used for fishing wires.The 3/4" space also adds a bit more R-value to the roof assembly.The 3/4" space allows the foil face of the polyiso to act as a radiant barrier. It'll keep your attic warmer in the winter.So...you DON'T have to use strapping, but by using it for me made the job easier, faster, of better quality, and actually saved me money.You can take the attic from naked studs to drywall in one day.
We just finished a house with bonus room and did basically as you said.
1/2" Tuff-R cap nailed to trusses then dense packed blown cellouse insulation.
Hung drywall directly over Tuff-R with longer screws. Worked great.
Roger
Thanks very much for the well thought out advice and time to write it all down for me. If you do use 1 or 2 inch foam board you are saying were ever you have a light fixture or electrical box that you are gluing up some 3/4 plywood for a mounting point for the box or recessed light or whatever? are you guys using batts for the ceiling then or putting up some type of mesh and then blowing cellulose? How would you know how much cellulose you were blowing up there depth wise if you cant see above it? last question, does any one use 6 mil vapor barrier any more if you were to just go with batts or blown in?
Personally I use 2" on the ceilings, but 1" could suffice. Depends on your climate and how deep the rafters are. Yours are 2x6 aren't they? In that case I'd encourage you to use 2" foam.I don't want ANY penetrations of the polyiso. Zero. Okay, so there are a few cap nails and the screws for the strapping.<g> But I don't want the polyiso cut or notched out to embed j-boxes or can lights either in or through it. Any holes will degrade and defeat the performance of the ceiling.Yes, I use either the strapping itself (if you rip it from 3/4 ply you can make it whatver width you need) or pieces of 3/4 ply as backers for attaching mounting blocks for anything that will be "hung" from the ceiling. Faux beams, etc. I also use the beams as wiring chases. these 3/4" backers are right behind the drywall and in front of the polyiso. They are, in essence, "in plane" with the 3/4" strapping.Cells: If I'm going to blow cells in rafter bays, I want them to be dense packed. There are plenty of threads on dense packing that are failry explicit on how it is accomplished. Dense packing means you are installing the cellulose at a density greater than the density that it will naturally achieve if it is blown loose and allowed to settle on its own. Thus, there is no settling. If you don't dense pack you will get settling.If you're going to dense pack cells in a rafter bay, the dense packing will likely crush those wimpy foam vent chutes that mount on the underside of the sheathing. So you'd need something more substantial. If you want to vent. Which I believe is what you want to do.If you want to use FG batts, then install the vent chutes, the unfaced friction fit batts, then the polyiso, then the strapping and drywall.If you want to dense pack cells, install your fabricated rigid vent chutes from soffit to vent, then the polyiso and the strapping. Cut a section out of the polyiso to use as an access hole so you can get the feed tube into each rafter bay. You'll likely need a2 r 3 holes per bay, depends on the layout. Dense pack, then reinsert the cutout and secure it with canned foam.If you're going to dense pack cells...several inches of cells...2x10 rafter, for example...my opinion...but you could forgo the polyiso. The dense packed cells will provide an outstanding air infiltration barrier, an outstanding barrier against radiant gain in the summertime and decent R-value to boot. If I recall you have shallow depth (were they 2x6?) rafters. And you want a vented roof, which will take another 1-1/2 to 2". In that case I'd install the flimsy vent chutes, then friction fit FG batts, and for added R- I'd use 2" foil-faced polyiso.While code-mandated roof R-values are a nice guideline, on a year-round basis I'd bet that an "under-insulated" roof (less FG but with polyiso) will out perform an "overinsulated" roof (lots of FG) on a year-round basis. In both winter, mostly due to zero air infiltration and the foil face keeping heat IN the house, and in summer, due to the polyiso keeping radiant gain from the roof OUT of the house.For what it's worth. My house originally had FG batts with a poly vapor barrier in the attic rafter bays, as well as on the gable end walls. Unfinised, no drywall. Winter temps were quite cold up there. Can't tell you how cold, but we used it for storage (full set of walk-up stairs) and it was pretty darn cold. Summertime it was an oven. I only measured the summer temp one time, when I was going to install the polyiso, and it was 127 degrees at the peak.The day after installing the polyiso it was 77 ddegrees. Over the next few days as the attic gave up residual heat, it settled in to the low 70s. No air condirioning.Winter temps it's in the high 60s. No additional heat.With the space finished off, the unoccupied winter temp is in the high 60s and as soon as lights of warm bodies get up there it gets to the low 70s. In the summer it maintains in the mid 70s during a hot spell. I do have a couple of A/C vents up there to help move air around to prevent stagnation.Your last question: If I was just installing batts between the rafters...nothing else...I'd use unfaced batts and then 6-mil poly. If just blowing in cells, then I'd use netting and strapping to help contain the netting, because were still dense-packing to eliminate settling. While celluolose has wonderful water capabilities in terms of holding and drying, your local code will still probably require you to install a VB, and again, I'd use 6-mil poly.If using foil-faced polyiso that would be my VB so poly wouldn't be required.Dang that's a long post. Too long to proofread. Hope it makes sense!
thanks for taking the time to share, much appreciated.