Supporting Cement-Fiber Siding Through 3" EPF Insulation

Started by MelFol, February 19, 2011, 12:50:12 PM

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MelFol

Next summer I hope to get the exterior of our SE AK 1.5 story sided.   I have 12' tall 2x6 walls 16" o.c. sheathed in 7/16 OSB with house wrap outside that.  I happened on a deal on extruded polystyrene foam (EPF),  2'x8'x3" sheets, with a psi rating of 40 psi, and one side foil faced. I purchased everything they had, which is enough to wrap three sides of the cabin.

Don gave me the link to a supplier of long wafer-head screws, ¼ inch diameter.  I plan to use 5 ½ inch long screws.  When I go through the ½ vent slats and 3" EPF  I should have ½ inch of screw in the OSB and 1.5" in the studs (only the bottom 2" of the screw has threads).  The siding is approximately 2.5 lb per square foot. So each 12' vent strip could be supporting as much as 60 pounds.

My questions:
-- How many pounds should I support with each screw? Keep in mind it will be supporting that weight about 3.5 inches out from the stud wall.  The psi rating of the rigid foam EPF is good. I can tighten the screws well.
-- Any problem with using wood for vent slats? I plan to attach the cement siding to them.  I see some of the design drawings show plastic slats.  I can see advantage to that but procuring them in remote AK might not be an option.
-- Any trick way to keep rodents and squirrels out of the top and bottom of the vent area. Our little squirrels love to get into insulation and nest. I have some ¼ hardware cloth but was hoping there is a neater more clever way. 
-- The screws are epoxy coated. Any chance of them loosening up over time?  They seem to be originally designed for landscaping applications.


glenn kangiser

I think 1/4 inch hardware cloth is about the only way to keep the rodents out if they decide they want to get in there.

What about fastening vertical 1x4 ribbons on the wall and fastening the siding to them?  That would give you a rain screen wall allowing drainage, which is even better than having the siding tight against the foam
"Always work from the general to the specific." J. Raabe

Glenn's Underground Cabin  http://countryplans.com/smf/index.php?topic=151.0

Please put your area in your sig line so we can assist with location specific answers.


MelFol

Glen:  I think we are on the same page. (Perhaps I didn't explain things well. I may also have included too many questions at once.)  1x3 or 1x4 slats (I called them vent strips) would be mounted vertically against the outside of the foam opposite each stud, providing air space between siding and foam, and the siding attached to those vertical slats/strips/ribbons.

My big concern is how much weight can I expect each screw going through slat, 3" foam, and into the framing to permanently support?   For instance, if I put ten screws per slat each screw must support about 6 lbs of siding.  Nine screws... 6.6 lbs.  Eight screws...  7.5 lbs.  The drawings of super-insulation I have seen, one showing eight inches of rigid foam wrap between siding and structure, haven't shown any detail on how they support the siding at that distance. 
P.S. My foam is Dow High Load 40. 

glenn kangiser

I got you now.

I am not an engineer or architect but I work with a lot of structural stuff.

The screws have great tensile strength - a guess - over 50000 psi - at 1/4 inch dia we are looking at a bit less than a 1/16 inch cross section so lets say it is good for about 2000 tensile strength. 

The wood will likely have a bit less than 1000 psi fiber stress allowed so 16 quarters (screw dia bearing on the wood) in a square inch - rounding that down to 20 I would say about 50 lbs per screw holding strength.  Multiply by .75 going through 3" of foam (Taking off 25% for leverage - allowing nothing for foam support) gives us 37.5 lbs per screw support strength.

A pretty convoluted way of figuring it I think, but I think it will be safe.   

Don_P is much better with this stuff and likely others too will give better input but that is a rule of thumb I might use. 
"Always work from the general to the specific." J. Raabe

Glenn's Underground Cabin  http://countryplans.com/smf/index.php?topic=151.0

Please put your area in your sig line so we can assist with location specific answers.

Shawn B

MelFol.....can you post the link for the long screws you are using? I will need to find some 5 inch metal roofing screws for my cabin, as going with exposed beams and t&g roof sheathing. I want to use 4" foam board on the top of the roof deck, then lay my roof metal on top of that.
"The natural liberty of man is to be free from any superior power on Earth, and not to be under the will or legislative authority of man, but only to have the law of nature for his rule." Samuel Adams


JRR

Congratulations on scooping up those foam panels!  I should be so lucky.  You are going through some of the same questions I have recently been through ... haven't quite resolved them but here are my thoughts:
... If the weight of the siding starts to bend the screws anchoring the foam, what takes place?  The bending moment of the screws is minor ... forget it, little resistance unless you install hundreds in each vertical row.  So the screws bend, what's next? .... when the 4" wide strips compress the foam, the foam easily resists a heck of a lot of weight.  I haven't actually done the trig, but feel that the screws will bend a small, almost imperceptable amount.

But I don't like it, and have decided to support the siding from below or above, or both; if I ever install a thick layer of foam outside the sheathing.

JRR

BTW, I know I'm repeating a question ... but still looking for good answer: 

As the exterior foam gets thick, how does one find the stud with the long screws? 

I've installed 4" foam, and it was not a pretty process!  If the foam were thicker, I would consider installing stud "tees" made of 2x4's...these instead of 2x6 studs. This would provide a wide target for the screw.

glenn kangiser

Not easy or great, JRR but I would measure the inside studs writing down a running measurement and layout lines on the outside.  Even that would not be fun, easy or totally successful.

I like your idea of the built up T stud.
"Always work from the general to the specific." J. Raabe

Glenn's Underground Cabin  http://countryplans.com/smf/index.php?topic=151.0

Please put your area in your sig line so we can assist with location specific answers.

MelFol

Glenn: THANX MUCH!   I don't want it to sag over time but also didn't want to buy $250 worth of extra screws.  I'll put in six as a test and put some weight on it and see if it holds me. That will be the 37.5 per screw equivalent. If it does well I'll probably settle on 4-5 per slat. Without your info I would have gone with a lot more screws than that for safety factor.  At 50¢ a screw I probably owe you lunch  ;D

Shawn:  Don's link is about the only place I've found that sells these screws.   http://www.mcfeelys.com/product/TMR-4055/14quot-x-5-12quot-Extra-Long-Wafer-Head-Timber-Screws--Spider-Drive

JRR:  About a year ago I walked into a Habitat-for-Humanity Restore. Someone had donated two pallets of unused NOS EPF. The Restore was short on room and gave me a super deal.  $200 plus tax for all 64 sheets. At the time I had not heard of super-insulation. I figured on putting it under the floor joists. Eventually I realized it had a better application in a cold climate. The sheets are 40psi.  When I walked on one I felt a slight mush, but my 220 lbs left only slight indentation.  Hopefully the siding can't move if the insulation won't crush under 60 lbs of siding. I may try to support or hang the slats some other way as well if practical.

My layout is 16" OC and I figured I'd just run a tape to find the studs.  I suspect I may miss a stud now and then and have to re-set the screw. I imagine I'll know when screwing one in whether I hit stud or just OSB.  If I can get top and bottom the middle screws  should be easy (said before experience).


glenn kangiser

There will also be tension on the screws pulling the board tight to the foam which will in effect put the brakes on it to keep it from sliding down and bending the screws which we really did not even take into account.

That safety factor and a bit of experimenting should give you a good wall.
"Always work from the general to the specific." J. Raabe

Glenn's Underground Cabin  http://countryplans.com/smf/index.php?topic=151.0

Please put your area in your sig line so we can assist with location specific answers.

Don_P

Can you nail a ledger of 2-2x6's around the bottom? Then stand the foam on top of that and nail the vertical strapping to the ledger before screwing through the strapping and XPS above. If you're still concerned wrap a metal strap around the ledger from the face, down under the bottom and up the backside against the wall and fasten the metal strap to the wall. I'm thinking 2x6's for the bottom ledge so there is no possibility of it wanting to roll over and pull the nails but make it slide and put the nails attaching it to the wall in shear. Then the number of nails is right off the AWC connections calc and the oly screws are just holding the strapping against the wall.

There is a company that makes a vent that looks like plastic corrugated cardboard that could be used at the top and bottom, no idea if rodents would have the desire to go through it though.

firefox

#11
Here is an idea for a screw guide:
Screw one of the wafer screws in at the top center of the stud
and the bottom center of the stud so that when you place the foam
the top and bottom of the foam will just touch the screw shank.
Don't screw them in all the way though, leave 1/2 inch of shank
showing when the foam is placed.
Now go get two strips of 1/2" x 1/2" angle iron 1 inch longer than the foam. Drill a hole in the top of each length so that when you put a bolt through the two pieces, the form an inverted T. When you put the bolt through put an oversized nut in between (the thickness of your screws shank) to act as a spacer.
You can now hang the jig on the top screw like a coat hanger, and sandwitch the bottom screw with the tail of the jig.
Take a spring clam and snap it on the bottom to keep it in place.
Take a marking pen and mark on the jig the spots where you need to screw.
Now you can screw at those locations using the sides of the angle iron to keep your screw at right angles to the foam and on target.
Once  you get the screws all in up to the jig, remove the jig and hang it on the next stud over. Then finish screwing in the screws.
A side benefit is that the initial marker screws will help support the foam in position.
I hope this was clear enough.
Note: You may need to keep the bolt holding the two jigs together loose,
or use a wing nut. Otherwise it might be awkward removing the jig each time. depends on the size of the heads and spacing of the screws.

Bruce
Bruce & Robbie
MVPA 23824

MushCreek

I asked about this on another forum, and never got a satisfactory answer. I do like the idea of a support underneath, but it's important to not block the lower vent opening. The whole idea of the furring strips (vents) is to allow the siding to properly dry when moisture gets in there ( and it will). The problem I worry about is creep over a long period of time, not right away. I would think the foam should overlap the top of the foundation, otherwise the wall might look funny with a 4"+ overhang. Also, lots of critters will tunnel into foam if the bottom edge is open to the environment.
Jay

I'm not poor- I'm financially underpowered.

JRR

That would probably work on the walls OK, Bruce.

Let me draw a more challenging scenario.  The roof rafters and sheathing are in place.  A crew has come through and applied a 4", or more, thick layer of foam.  You are on top of the roof looking at a sea of foam ...all the way out to the edge of the 16" roof overhangs.  The next step is to put 1x4's down on the foam, aligned with rafters ... and attached to rafters. It's a good climb down just to look at the rafters.

My sloppy method:  The rafters are precisely predrilled through with vertical 1/8" pilot holes ... spaced to suit the need, approx 12" - 16"(more on this later).  All rafters are (jig) drilled, dead center of width, so the hole locations match .. rafter-to-rafter.  Using the longest 1/8" pilot drills you can buy ... 18" or so ... drill up thru the pilot holes, up thru the sheathing and the foam.  Leave the drill in place.  You will need a few long drills.  I ended up making my own out of 1/8" drill-stock.  A flat screwdriver shaped end works pretty well.

Now things get a little indefinite.  You can go back up on the roof and, using a tee-square, drill holes in the 1x4's to match the drills sticking thru the foam.  Or, if you have worked closely enough, you may be able to drill the 1x4's by measurement.  If you drill a large hole, say 3/8", and use a washer under the head of the long screw ... you may be OK.  At some point the drills have to be pulled back down ... and, with luck, the 1/4" long screw will follow the hole down thru the foam and find the pilot hole in the rafter.

... and BTW, the original pilot holes are located to fall between the purlins.... the purlings being the next step.

I have tried it both ways, transferring and measuring, to locate the holes in the 1x4's .... the Jury's still out!   Ain't nice ... but it works somewhat.






 



MelFol

THANK YOU ALL for your GOOD input.  All your ideas help me think the problems through.
 
I like the idea of putting a ledger around the bottom. It will have to be 2x3 to allow vent space.  I will probably use pre-drilled holes and long screws to attach it.
Due to your suggestions I am thinking I can put a triangulation strap (simpson or plumbers tape, outside of ledger up to sheathing or stud) so it can't possibly roll. The ledger will be placed low outside the rim board just above foundation. Therefore I can notch insulation 3+ inches without compromising the wrap around the actual living space.  If I don't like how far it extends up I can spray some foam over the notch/strap.  I would probably space these triangulation straps every 3-4 feet. With something like that in place and adequate screws through the slats I will feel a lot better about stopping all possibility of sag when I am pushing up daisies.  It may be possible to attach slats to every other overhanging roof rafter with strap too. I'll have to see how they situate. In addition I framed every window and door opening with 2x10 (3.5 inch protrusion beyond the 2x6 wall surface). I did that so siding ends will be solidly into the structure. I'm now feeling good about this.

Good idea to use a long straightedge as a guide for screws.  I like the idea of the hole at the top hanging on a screw and letting the straightedge hang, thus holding the insulation to the building while I place slats (I'm working alone).  Finding the top stud with a screw first is a good idea. Perhaps I can do that with a stud finder to find center. Bottom area is floor joists and 2x rim-board so a bottom screw will be difficult to place on-stud. A drywall Tee square across the top of the insulation should help me hit the stud too, if I get the top edge of the insulation level. Perhaps a level to set the tee square leg vertical?  (The insulation sheets are 8' long).  Ultimately I suspect it's going to be trial and error at times to find the stud behind 3" of insulation. Plus the sheathing is covered by house wrap. Using a tape measure to find layout down low should be another option.

My rim board edge is flush with the concrete stem wall of the foundation.  I will have approx 4" overhang of insulation and siding.  Without a wide stem wall there isn't much alternative if one wants the floor joists adequately sitting on the foundation. I suspect I'll notice it at first, but with time and once I backfill to about 8" I won't even notice. Hopefully other detail will draw the attention more than that overhang.

Don_P

2-2x's stacked is 3" thick, the vent space would be intact, you'd lose a band of R value but I'm assuming this is down low on the sill and floor rim.

I did do a SIPs job where the stone below was to be 4" thick. The archies detail was to use a 2x12 sill that was slid ot beyond the foundation wall that amount. I didn't like it because the sill could split in that direction fairly easily and it forms a level entrance back into the basement for water running down the foam in your case, but there's another thought that came to mind.

MelFol

Don_P:  
Now I see what you meant by 2 2x6.   I'm out of 2x6 (54 miles away by water to get more doug fir 2x6) but do have 2x4s on hand to do it. If I don't like how they hold I can get the 2x6.  I will combine ledger and the slat screws to support the 60 lbs..  Right, ledger would be down low on the rim board and not affect the thermal barrier much if at all.  I'm liking these ideas and feeling much better about siding support.  Whoever said all of us are better than one of us had it right. Thanks All.  Mel

MelFol

I got an answer to one of my other questions from Kim at McFeely's technical department.  He listened to our application and my question about whether these screws will tend to loosen or back out on their own over time.  He called the manufacturer, Fastenmaster, and got back to me.  Fastenmaster said the chance of these screws backing out is "virtually nil."   Kudo's to McFeely's for researching that for me.