Critique my wall/roof assemblies

Started by Nate R, December 03, 2018, 10:17:28 AM

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Nate R

Critique my assemblies!
Narrowing down my materials list for a build, putting everythign through ResCheck again, etc. Before I get a permit next spring, just trying to hammer out more details.

PART TIME recreational cabin. Single story, no loft. Trying to find a balance between a functional wall, relatively energy efficient but not TOO much where the stove cooks us out of a small cabin, and not TOO expensive, but aesthetically tolerable to us. Northern WI climate. All 4 seasons, snow, but not TOO hot in the summer.   

Concerns: A lot of the stuff about air barriers, vapor drive, etc seems to be a bit different when you're not running air conditioning in summer, or only adding moisture inside in winter occasionally. This is an off grid place, no interior bathroom. Will be built on an insulated slab w/ vapor barrier underneath, so from what I've seen in another cabin in the area on an UNinsulated slab, it WILL stay higher than outside ambient in winter, maybe hovering around 30 degrees inside.
I plan to heat with wood primarily. No A/C. Only used a few times a winter (maybe a few weekends, or a week and a couple weekends), and then weeekends, etc throughout the rest of the year. Thus, it won't be heated full time or see full time moisture loading from interior occupants.

Walls, outside to inside:
Cement Fiber Panel Siding
WRB taped as air barrier (Benjamin Obdyke Hydrogap)
7/16 OSB
2x6 studs, 24 OC. Advanced framing headers (insulated.). 8 foot walls with double bottom plate (state code to overhang foundation insulation)
R23 Mineral Wool Batts
#15 Tarpaper, large laps. POSSIBLY taped? (moisture reservoir, and some areas may use barn boards or boards with gaps as interior finish, so this is also to keep the gaps from being too eye-catching.)
Wood interior, much of it T&G.

Roof:
6/12 Asphalt shingle roof, 5/8 OSB Sheathing.
Scissors trusses, with slightly raised heels. (~8" heels.) Ventilation channels over plates, possibly made from XPS. Soffit vents (continous) and a ridge vent.
R30 Mineral wool batts between the bottom chords. I'd go thicker, but can't find mineral wool batts with higher R Value. And I'm not a fan of FG. And, off grid, rural..blown in insulation would be a real hassle to get in there, power for the blower, etc)
R3 XPS sheets on the INTERIOR of the bottom chords. (Thermal break. Some of these trusses will be over a porch as well, so trying to keep them from bringing that cold right to the interior)
Thinking T&G right over the XPS on teh ceiling? Maybe a layer of #15 felt in between again for visual purposes. (XPS SHOULD keep vapor flow to a minimum. COULD tape seams on XPS.

Curious on thoughts with this. Suggestions on why certain things might not be a good idea, etc. Easier to debate now than when I have materials in hand waiting to be built!
Thanks for any input!

akwoodchuck

Looks good except for the tar paper deal...put it on the roof where it belongs, not inside the house!  :)
...find some black poly sheeting in 6-10 mil, it will look and work (and smell) way better than felt...
"The lyf so short, the craft so long to lerne."


NathanS

I think the felt would be fine, it is common practice to put it under wood flooring and I can't smell it.

Vapor barriers don't really do anything, it is the air sealing that stops vapor movement.

We used roxul in our house - walls and two layers of r-30 in the attic. The rigidity is nice to work with, I found it every bit as irritating to skin and throat as fiberglass.

I think you could save a lot of money on insulation if you used fiberglass, for not much performance penalty especially on something you aren't living in full time.

Only other thing is that the interior side foam on the ceiling means longer screws or nails and a harder time finding the the wood.

MountainDon

Can you do blown in cellulose for the attic?  We bought the insulation from Lowes and had a one day free use for the blower machine. Our cabin has something like R45 in the attic with that
Just because something has been done and has not failed, doesn't mean it is good design.

MountainDon

Quote from: MountainDon on December 03, 2018, 02:58:14 PM
Think about blown in cellulose for the attic; a generator can power the blower.  We bought the insulation from Lowes and had a one day free use for the blower machine. Our cabin has something like R45 in the attic with that.
Just because something has been done and has not failed, doesn't mean it is good design.


Nate R

Hmm, been thinking harder, maybe R49 FG batts would work in the ceiling. That'd be better than R30, and about the same price as R30 mineral wool.

Came back to thinking about why I'd ruled out blown in, and while you're right that I could find a way to make it work with a generator and such, making a 30 foot journey to blow it in through these trusses seems pretty un-doable....There's just not much space to move around. I've blown in insulation in a larger attic, but something this tiny seems impractical? Or am I missing some other way to do it, as I often do?  ( :D )


MountainDon

Yes, it would be a difficult crawl into there... being a scissors truss didn't sink in when I first read this.  It was crowded enough in our cabin attic with 5:12 rafters, regular ceiling joists on a 16 foot wide. I laid 2 foot wide  x 8 foot sheathing strips down the length and blew in under and then over that.

If batts do you intend to insert then from below and then sheet the ceiling?

Because it is a scissors truss it does appear that you cannot obtain a full depth of insulation out to the side walls. Compressing batts reduces the R-value.  Or maybe there is enough height and it just seems like too skinny a space out there near the walls ??? 
Just because something has been done and has not failed, doesn't mean it is good design.

Nate R

Quote from: MountainDon on December 03, 2018, 08:22:06 PM
If batts do you intend to insert then from below and then sheet the ceiling?

Because it is a scissors truss it does appear that you cannot obtain a full depth of insulation out to the side walls. Compressing batts reduces the R-value.  Or maybe there is enough height and it just seems like too skinny a space out there near the walls ???

That's my intention, yes. And I intend to avoid any recessed can lights as well. Only penetrations into the ceiling will be overhead light electrical boxes, and I'll probably make sure those are air sealed.

You're correct that there's not enough space for full R-Value. Heel height is about 8". If I leave a 2" airspace for soffit/ridge venting (Code is 1", IIRC, but I've seen several recommendations to go with 2"?) and create that airspace with say, 1/2" XPS, that leaves me 5 1/2" above the top plates for insulation at the lowest clearance. I haven't completely thought out how to handle that yet.
Since these heels are already 4" higher than standard, I'm already doing "better" than the minimum clearance there, so I see it as an improvement from worst-case.  I may decide to insulate over the plates and a bit beyond them with solid layers of XPS foam. If I filled that 5 1/2" space with XPS and made the vent channels from 1/2" XPS, you're talking R30 above the top plates. Not too bad. I see there's formulas for finding R-value of compressed batts, but compressing 13 3/4" R49 Batt to 5 1/2" seems to be extreme.

azgreg

Have you considered a raised heel truss?



Nate R

Quote from: azgreg on December 04, 2018, 11:30:49 AM
Have you considered a raised heel truss?



Yep, going somewhere in the middle.  Not a 4 3/8" heel, but not a full 15+" Heel.  If I was living here full time, heating all winter, I'd probably push harder for that. But to keep costs and complexity of ordering trusses and such lower, as well as help stick to an aesthetic, this is where I've landed. I'm using dropped end trusses on each end to support 2x4 lookouts so I can run a 2' gable overhang, and the field trusses are raised up enough to accommodate that. This already pushes me into running 9' sheathing and siding, full raised heels would push above 9' (With double bottom plates.)

NathanS

For our eaves the rigidity of the roxul was really nice to work with. You can cut/form it to get a tight fit without compressing it. One option to consider is to buy just enough for the eaves, then fill it up with fiberglass everywhere else.

Another thing that was really nice about roxul is that at 16" OC it will friction fit and hold indefinitely in ceiling joists. 24" you would need those long metal pin things, I had trouble with those on our one 24" wide bay.

I think kraft faced batts would be easiest so you could just staple them up.