Insulation baffles for dense-pack
I want to maintain an air channel in my joist bays of a cathedral ceiling that I intend to insulate with dense-pack cellulose. What sort of baffles do people use that will maintain this airspace without being crushed by the dense-pack. I have seen the flimsy styrofoam and thin plastic channels available at HD, etc. which seem like they would work fine for loose-fill or batts, but don’t stand a chance against dense-pack (I wouldn’t think).
One thought I had was running 1″ rips along each joist and using 1/4″ ply to create the airspace. My codes require this airspace, so any suggestions of forgetting the airspace won’t work. I have heard concerns about using certain materials that don’t breathe to create this space, creating a double vapor barrier that won’t let moisture escape into the airspace. I guess CDX is considered a vapor barrier (for a good chart of perm ratings see: http://www.dyplastproducts.com/water_vapor_permeance.htm ) but 1/8 hardboard isn’t (what about 1/4? it would bend less).
Are there good pre-fab channels out there for this?
Thanks!
-Rich
Replies
How about perfboard?
happy?
I'm not familiar with it. A google search came up with all sorts of computer circuit board materials and some kind of plastic wall hook material (picture expensive plastic hard-board peg-board).Are you just talking about what I would refer to as pegboard: 1/4 inch hardboard with holes drilled every inch or so?That would work but would likely be expensive. Since 1/4 inch hardboard isn't likely a vapor barrier (VB), I would probably just use that. I don't know, but would assume that the same material with all the holes drilled in it would cost more.I noticed on that perm chart I referenced that 1" expanded polystyrene isn't a VB. Would that hold up to the pressure of densepack? I doubt it would.-Rich-Rich
Yeah, pegboard. No direct experience, but I'm a bit skeptical of how well foam can hold up to cells, given that cells can blow out a weak plaster wall.You'd think they'd make something specifically for this. Even if you used foam it would be a PITA to cut and fit it all. It would make more sense to have something like the regular plastic/foam baffels, only made of a slightly stiffer plastic with dimples at intervals in the middle to keep it from compressing. And of course perforated. Or something along the lines of the ridge vent mesh that comes in rolls. In practice running a couple of perforated drain pipes up each bay would probably work fine, but likely would not pass inspection.
If ignorance is bliss why aren't more people
happy?
We do this all the time. Rip 1 1/2 inch expanded polystyrene foam into 1" strips,
fasten these to the sides of the rafters with 2" vinyl coated DW nails.
Rip more foam to fit between joist and rest on your spacers, "toe nail into place. We also use dots of hilti spray foam to hold them in place.
If your spacing is wider than 16" oc you can also use a center strip "nailed to the foam".
Seal the top and bottom well with more foam, cardboard or reinforced plastic sheeting.
Great, thanks! Just to clarify, you are doing this for Dense-pack right?Any tricks for cutting EPS into 1 inch strips. I would think it would just crumble. When you say "toe-nail", are you just "nailing" the two pieces of foam together (I assume you can just push these nail in by hand), or are you trying to get the nail into the joist?-Rich
Yep, it's for dense pack. We use 1 1/2" foam that is 2lbs. per cubic foot, it's a little stronger than the stuff you would buy at a home center or lumber yard. We buy it direct from the manufacturer.
We use a table saw for the rips and a hot knife for irregular shapes. A little silicone spray on the saw and the blade helps a lot, just remember to turn off the saw before spraying, silicone is a conductor.
Also, go easy in the silicone if you are going to use the saw for finish work because you will want to clean if off before doing any "nice" work that is getting a finish coat. The silicone residue will keep the finish from bonding to the surface.
Cutting EPS be carful the stuff grabs and binds if you don't have a steady hand.
I have never had or seen a problem with this method, we did however "tear" up some of the pre made foam vents with the tube we use on the end of the blower hose in a project prepped for us by the GC's crew.
I don't understand the pegboard idea, why the perforations? Are you not separating the insulation from the air space? We try to stop the movement of air in our insulation.
Garett
The pegboard holes would allow moisture migration, but would (I think) be small enough to keep the cells from blowing through very much. With dense-pack you don't get much air movement anyway, so I don't see how that's a concern.
If ignorance is bliss why aren't more people
happy?
I plan on doing the same think. My thoughts are 1" panel blocked with cubes cut from said panels. The blocks will be placed along the edge and in the field to prevent the 1" from collapsing and pressing against the sheathing. I dont think the dense pack is going exert much upward pressure on the foam board,could be wrong though.
I think it can exert a pretty significant force, as DanH says above. I've never seen it go in, so I don't know, but would hate to put all that work into building foam baffles to have them collapse when the insulation goes in. The worst part would be that I wouldn't know they failed, but my roof just wouldn't vent well.-Rich
we always blow denspak .. in this case it will be into 2x10 bays @ 19.2" spacing
we form a dam at the plate line
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and pass the Propa-vents thru,
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then gun foam the voids..
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we don't have any problem with the cells crushing the PropaVent.. it's a pretty uniform pressure, and the PropaVents are designed with a reinforced profile to resist that pressure
Mike Smith Rhode Island : Design / Build / Repair / Restore
Edited 3/22/2006 11:28 pm ET by MikeSmith
Edited 3/22/2006 11:28 pm ET by MikeSmith
Edited 3/22/2006 11:29 pm ET by MikeSmith
Edited 3/22/2006 11:29 pm ET by MikeSmith
Nice work Mike, I really like the dams cut to the vent profile.
How do you handle T&G ceilings? And how do you attach your fabric to the joist?
Garett
grott, T&G ceilings would be no problem for us , we'd put up our insulmesh, then furr the ceiling with 1x3
then install the T&G ..if there is a n attci space above we'd loose-blow the attic
if no attic, we'd dens-pak thru the insul-mesh before we install the T&G
on ceilings we staple the insulmesh, pulling it tite in both planes..then the furring finishes it
on walls we staple, pull tite, and then glue with Elmer's white glueMike Smith Rhode Island : Design / Build / Repair / Restore
Thanks for the photos. That's a nice system with the cutouts in the blocking. Unfortunately, I couldn't do that. My engineer wouldn't let me notch the top of the blocking. I have 2x12 joists with rimboard blocking that I drilled a few 1" holes in to allow air in. Not ideal, but it will work. I didn't see the issue with notching the blocking (not a big notch, just something like yours), but maybe that's why he makes the big bucks.On a 19.2 roof like the one in the pictures, the vent channel is only covering about half of the roof sheathing. Do you find that is enough to keep the whole thing cool? I am shooting for a vent channel that is the full width of the joist bay (or at least as much as possible). The previously suggested EPS systems seems like it will work well for me.-Rich
well, it doesn't add up....
you want to have full coverage of your vent channel, but the inlet air is limited to a few 1" holes.. no way will you have a balanced soffit/ridge ventilation system
what is the objection of your engineer to cutting a channel.... and why does he want solid blocking for your 2x12 rafters ?
the rafters are nailed to the plate, and the sheathing nails the tops, the only purpose i have with the blocking is to form a dam to keep the insulation from filling the soffits
what design purpose does your engineer want from the blocking ?
what did you use for your fascia ? do you have a sub-fascia of 2x material ?
if you do , then the sub-fascia has to get some credit as blocking also..
i'm curious as to the reasoning behind your engineers requirements
Mike Smith Rhode Island : Design / Build / Repair / Restore
Edited 3/26/2006 7:38 am ET by MikeSmith
You got me on that one. I am stuck with some weird rules around here (“Boulder, Colorado. 25 square miles surround by reality”). Originally I was being told that I live in a “wildfire danger zone” and therefore was subject to all sorts of contradictory rules that I won’t bore you with. One of these rules was that I was NOT allowed to have ANY soffit ventilation. You would think that would mean I could just insulate right up under the decking (no air channel), but you would be wrong. I am required to have the vent channel and ridge vents. In a long conversation with the plans examiner he agreed that the vent channel wouldn’t do anything without soffit vents, but there was nothing he could do. The rules had come down from the County Commissioners who wanted to look like they were doing something about all the wildfires everyone was hearing about in the news (my interpretation, not his).I was finally (months) able to convince the County that there isn’t a tree for hundreds of yards, but there are TWO fire hydrants closer than that. They decided that the rules didn’t apply to me and I could have my soffit vents. In the meantime I found out my engineer didn’t want me to notch the plates.Here is my recollection of the engineer's reasoning (its been a while): The roof joists actually sit on top of the first floor wall, next to the second story floor joists (11 7/8 TJI’s). I decided not to sheathe the second floor all the way to the edge and have the floor joists sit on the floor, the way you did in your pictures, because I wanted to keep the exterior wall shorter. I think the engineer wanted the solid blocking for the floor joists more than the roof joists, but I don’t know for sure. My sub-fascia is 2x, and might have counted for something.Basically I didn’t fight the notched blocking thing because it didn’t look like I was going to get soffit vents anyway. When the county finally changed their mind, the holes was the best I could do. It’s not ideal, but I think it will work.Does that explanation make sense?-Rich
absolutely.... although, notching the blocking ( the substiture for our band joist ).. ..
well, i'm not an engineer, but i don't see where it would alter the performance of that section..
your rule writers do ask for some strange things ..
kind of like in RI, wehre they allow exterior foundtion insulation, but you can cut it off at the finish grade line... so everything from finish grade to the sheathing is nakedMike Smith Rhode Island : Design / Build / Repair / Restore