Fastening Strapping in a Bonfiglioli-Inspired Ceiling
humm9er
| Posted in General Questions on
Porch conversion to year round room.
Sprayed 4-5” of CCSF to underside of roof deck. Rafters are 2×8.
Plan to add 1.5-2” XPS strips to underside of rafters, then strapping, then 5.5” rockwool in the bays.
Should I use a framing nailer and 3.25” ring shanks to affix the strapping? Can the strapping stay parallel (on ends) of rafters or do I have to affix perpendicular to the rafters for the drywall install?
And can the drywall be fastened to just the strapping or should fasteners penetrate through the strapping and foam into the rafters?
thx
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Replies
I've always done strapping as 1x3's perpendicular to the joists. It makes for a more level ceiling. Single ring shank in each crossing point. Drywall direct to strapping.
I'd put the batt insulation in first and then the strapping so the strapping holds it in.
For this kind of work a couple sawhorses and a plank is great so you don't have to keep moving the stepladder around.
Solid advice for standard strapping, but this is a Bonfiglioli wall so there is foam between the strapping and the joist. Better to run it parallel in this case.
Except it's not a Bonfiglioli wall, it's a ceiling.
Ceiling joists are notoriously uneven. Running the strapping perpendicular evens out the unevenness, running them parallel accentuates it. Sometimes you can't avoid running it parallel -- like in a room where the joists change direction -- but if you can, run them perpendicular. I don't see anything in Justin's design that can't be accomplished with perpendicular strapping.
In the Bonfiglioli wall they put R-30 insulation -- meant for a 2x8 -- into 2x6 framing. A 2x8 is 1.75 inches wider than a 2x6, and that difference is made up by using 1" foam insulation and a 1x3 which is 0.75 inches thick. The 1x3 is part of the thickness and the batts are stapled onto it. So it has to go parallel to the studs. Plus people are going to be expecting vertical studs for things like outlet boxes and hanging cabinets.
Justin,
I don't think it makes any difference either way. Do whatever strikes your fancy when you get up that morning.
The drywall can just be screwed to the strapping.
Thank you both
I will actually plan to use 3.5” rings if I use 1.5” foam…if I use 2” foam I think I have to go to 4”+ nails.
Actually guess this leads to a question: how much penetration into framing should the strapping fasteners have? 1”+?
And I assume 1x3 strapping is preferred? I see sometimes on walls people use 1x2…
Thank you!
Justin,
1"x3"s are a good choice. Less chance of splitting.
If you are into 4' nails, you might want to consider using screws instead.
Malcolm,
I’m hand nailing 4” nails into zip and 2”foam on a 3/12 schedule…I would never do it again!
Screws it is 😎
Justin,
The very first house we built over three decades ago we nailed down the sub-floor with 3 1/2" spirals. I think my wife has forgiven me.
I'm from Massachusetts, where strapped ceilings like this are customary (they're actually code in MA but I have no idea what the logic is behind that.)
For a ceiling you want 1x3 or 1x4. As Malcolm notes 1x2 is more prone to splitting. You also want you strapping to be quite strong, you should be able to grab on in the middle and do a chin-up without it feeling at all unsteady. You're going to be hanging 70-lb sheets of drywall, light fixtures and whatever from the strapping, you want it solid. The other thing is when you're putting up the drywall, and you're holding 70-lb sheets over your head, it's a lot easier to get those first few screws into a wider piece of wood. Plus you don't have to put the screws as close to the edge.
If you run the strapping perpendicular it smooths out any variation in the joists. If you run it parallel it amplifies the variation.
I use a siding nailer and 2" ring-shank siding nails. 1-5/8" drywall screws also work. With 2" of foam the fasteners have to be 2" longer. I'm not aware of a nailer that shoots 4" nails. Working overhead there's no way I'd hand drive them so you're looking at 4" drywall screws.
Here's how I do it: get a helper, some sawhorses and a plank. Set the sawhorses at a height where you can comfortably reach the ceiling. Run the plank parallel to the first joist in the ceiling and walk along it as you mark it every 16". Do the same with the last joist. Cut all the strapping to approximate length. Put the plank perpendicular to the joists. Have your helper hold one end while you start at the other, fasten it and then "walk the plank" toward the helper putting a fastener in each joist. Move the sawhorses and repeat. Do a whole room in ten minutes.
Great advice I’m also in MA will do as you say with 4” drywall screws
Justin,
Don't use drywall screws to secure the strapping. They have too low a tensile strength. Use deck screws.
Malcolm,
Roger that I will use 4” deck screws. Many thanks — you had me laughing on the 3.5” spirals and your wife.
Have a nice weekend,
Justin
There is a couple of items here.
If you are installing the foam strips to increase your rafter depth for extra insulation, the strapping and foam needs to be along the rafter. The nails holding in the strapping should have similar penetration as strapping (in my area code for that is 2 1/4" nails). Screws are definitely better but more work. Once the strapping is up, you can than use regular length drywall screws.
You can also skip the wood strapping and use foam only.
Staple the rigid up to the rafters than hang your drywall with long drywall screws. I've done this for ceiling with continuous rigid over the rafters and works pretty well. Make sure to mark out your rafters accurately so you can see the center line when the ceiling is up, snapping a couple of guide chalk lines once the ceiling is tacked up will make installing the rest of the screws easier. You can compress the foam at panel edges if you tighten the screws too much, so a higher density foam (ie polyiso) is a better option.
Having done a Bonfig wall, I'm hesitant to recommend it, or at least do so without a few warnings to anyone attempting it.
I've only done it on a wall where the fastener retention strength isn't as critical as overhead. That said, I still stressed out over just how strong it would be in the end. Get the longest ring shank nails you can and don't make it too thick.
The foam doesn't need to be very thick to achieve the thermal break. You're not looking to fully insulate the wood as much as just disrupt the ready flow of heat. (Yes dear engineers, I know that's the same thing but on a practical level it helps if you think of the objective of just disrupting the full effect of heat transfer)
Getting high density foam is key. Anything the big box store stocks has a good chance of compressing from the nails. Ask me how I know. :( If the foam compresses you'll wind up with a lot of dips in your wall/ceiling.
Strapping will split. You can lessen this by either ripping strips of 3/4 plywood or by using siding nails that have a blunter tip.
A better question may be, what's the best way to pad a ceiling to add a couple extra inches of insulation?
Hi Andy,
How long were the nails you used?
What foam did you use and what was the thickness?
Did you feel like you wall ended up being strong enough?
I'm looking at doing 1600 sq ft and don't want to work out kinks during the process.
Thanks!
It would seem that a calculation on the foam strip vs the perpendicular strapping would help give a direction. Verifying the the ceiling R value with flat 2x4's or on edge 2x4's, So a 1.5" gap or 3.5" gap, plus fluffy insulation.
Right, would it work better to fill the rafter bays with R-13 3-1/2" unfaced, then run 2x4's perpendicular on edge and fill them with r-13 faced? The only question is how to attach the 2x4's. I'd go with 7" vertical pieces of 2x4. That way you can just use a regular framing nailer throughout.
With 4-5" of CCSF. That leaves about 3.25" of rafter space for fluffy.
My fluffy will be rockwool.
1.5" XPS and .75" strapping gets me to 5.5" of rafter space for fluffy, which means I can use r-23 rockwool.
XPS has a density/compressive strength similar to polyiso at 25psi. I have used foam under Zip on walls and am well aware of foam compression challenges.
As Malcolm notes, the screws will be under tension, so I would sink them just enough to hold the strapping tight to the foam, avoiding foam compression.
Are people concerned a 4" deck screw can't hold the strapping to the rafters with the foam break?
I'd be just fine ditching the foam (or using .5" foam etc) and using 1x3's or similar but I thought the foam is critical for the thermal break...
Thanks
> using .5" foam etc
If you are going to the effort to make a thermal break, make it a good one.
I don't know, but seems like perpendicular (with no foam) is less work and slightly higher R value.
That's what I thought!
Do we think I can use 1.5-2" XPS in this arrangement "safely"?
I dont want the drywall falling on my wife's head 6 months later haha!
I'm very curious about the 1.5 inch thickness. Two inches seems like a bit of stretch without using long screws and a ton more work than just 1 inch thick foam. I'd personally like to use 1.5 inch thick (on walls, not a ceiling), but would like to hear other people's thoughts.
Justin,
There is absolutely no chance of the screws securing the strapping failing. They are holding up about 5 lbs each. Exponentially less than they are rated for. If it's going to keep you awake at night, buy some GRK R4s. I'd just use deck screws.
Thanks Malcom I appreciate your reassurance!
I'm thinking Boni wall is the wrong model. Dropped ceiling is what we should be thinking about.
Tell me more? Thx
I think page 6 from this link is an example. I plan to do it with my house to get to ~R56 whole roof in climate zone 7.
https://www.jlconline.com/_view-object?id=0000014d-ba87-d635-a35f-fb97cac70000
IMO, it only makes sense with dense pack. Pictures suggest it also has perpendicular strapping, which makes it more complex.
Would be interesting to know how the unvented, cellulose filled, poly VB, cathedral roof is doing. Maybe the underlayment was vapor permeable.
As I recall Dan Kolbert had moisture sensors and checked it a couple of years later and it was fine.
What would your code say about that assembly?
In such a roof, IECC code doesn't allow cellulose only or a Class I vapor barrier on the interior side. I expect the double vapor barrier design would perform poorly if moisture ever gets in.
From the article:
"We used 2x10 rafters and
created a deep insulation cavity by nailing
14-inch-wide 1/2-inch-thick plywood
gussets to 2x3s about every 3 feet, then
nailed these assemblies to the sides of the
rafters to create a new lower ceiling plane
(Figure 7). The reduced contact between
framing members — along with the 21/4-
inch gap between the members — further
minimizes thermal bridging. We opted
for an unvented roof and installed solid
blocking at the eaves between rafters,
caulking all the edges for airtightness."
I think they used blown insulation rather than batts.
I'll pass on the dropped ceiling, stick with the foam and strapping.
Thanks all for your input.
Circling back
Someone offline raised the concern that 2” foam and 1x3 strapping on a sloped/cathedral ceiling could cause the drywall to sag/walk/creep over time.
What does everyone think? If that’s possible, how much benefit of the thermal break would I loose moving to 1” XPS and 2x3 for strapping? Doesn’t really change my costs or labor…
Thx all
"raised the concern that 2” foam and 1x3 strapping on a sloped/cathedral ceiling could cause the drywall to sag/walk/creep over time."
Doubtful. 5/8" drywall and a healthy number of fasteners and the assembly will be fine. The weight of the strapping and foam is negligible on the 2x8 rafters so long as they were sized properly for the span(s). If you're really concerned about it, paralleling the strapping with the foam and rafters will increase the surface friction for added assurance.
Thanks so no one sounds concerned
Running the strapping parallel to the rafters rather than perpendicular makes hanging the sheet rock more challenging yes?
Justin,
Six of one, half a dozen of the other.