I plan on having the exterior sheathing be a combination of wood plank, cement fiberboard (actual sheets and not just specific Hardi planks and such), and sheet metal (thin and some thick panels (Corten)).

I like the concept of rainscreens for their breathability, but how do I fasten the furring strips to the OSB exactly (How many screws? Do I screw from interior-out (and how difficult would that be?)? etc.)... without fear that the weight of the system will rip the OSB skin right off the EPS?

Are there any charts on what weights the OSB can handle? The vast majority of my panels are 8' wide.

Thanks.

How you attach furring strips to SIPs depends on the weight of the materials and the thickness of the furring strips. The thicker the furring strip the more moment at the SIP wall. For lightweight siding or thin wood planks on 1” furring, it is possible to use #10 wood screws to hold the furring from the outside and, when attaching the siding, use longer screws that pass through the furring and into the OSB skin. Your SIP supplier should be able to provide you with pullout and shear values for their skins. For heavier siding, such as the Corten, I would use 2x furring and attach it with SIP screws through the wall and into the back of the furring strip. A typical SIP screw has an allowable shear of around 250 lbs per screw. Make sure you protect the OSB skin with felt paper and run your furring vertical.

Brian

You shouldn't need furring strips if you put a wrap on the house. Felt paper has worked for years. The rainscreen does not have to have depth, it needs to provide a plane between the OSB and outside finish.

If your cement panels are very heavy, you would want to get SIPs with thicker OSB. It will add some expense.

You should be able to go to an OSB manufacuturing site and get info on the strength of the OSB. If your exterior finish is on your architecturals, your engineered plans should include connection details including spacing.

You might check the acrchive posts for half log sididing. There were some discussions on this topic.

I have to disagree with mmacgowa concerning the rainscreen. A wrap by itself, will offer protection to the SIPS panels and OSB, but the function of a rainscreen is to allow the SIDING to dry completely. If wood siding is applied directly to felt or housewrap, moisture will build up and remain on the back side of the siding. Not a problem for cement board or vinyl, but wood siding will rot from the back side outward. I always use either 1x or 2x furring with wood siding on any type of wall construction.

Thanks all.

I will go do the math and ask the SIP retailer for product capabilities. I didn't know you could get thicker OSB. I guess it never dawned on me. It would be stronger, but also less insulation, correct?

I won't use the virtual gap method as I desire real drying immmediately... especially for the wood, but also the OSB. I don't think there is enough perfect product and installation that would make me 100% confident.

My big question is how hard is it to aim a screw (especially for 8" SIPs) from the interior and still hit the furring strip halfway centrally on the outside? What is the technique, because I'll be doing the labor myself for the finishing like that.

Has anyone had experience with VAPROSHIELD's building wrap product line? I've read that building paper is not good (for long term durability) when you have a rainscreen... especially one with gaps between the sheathing that will allow direct rain and sun to get through (i.e. Water + UV + heat = slow degradation of tar in paper). I can't use most building wraps, because they are either white or blue or green and have all manner of writing on them, which would easily be seen if my gaps at the sheathing component's butt joints are ~1/8-1/4".

Just say NO! Just screw it from the outside!


I guess that I don't understand. 'Direct Rain and Direct Sun' should never contact the rainscreen. The rainscreen should be completely devoid of direct light, and should never be directly exposed to rain. Your siding material should eliminate direct sunlight and shed 99.9% of the rain. The siding joints must be made over the furring, and should be sealed with high quality caulk.

Tar paper(roofing felt) will work just fine. You seem to have the wrong conception of rain screens.

What I am trying to explain, is roughly demonstrated by this picture:


Notice that the rain screen(in Orange) is not exposed to the sun or the rain. It does not show the siding joints directly over the furring, but they should be.

Good Luck!

It's not that I have the wrong concept of rainscreens per se. It's that I like a modern interpretation of siding style over the traditional lap norm.

The house is very modern in style and the planks of wood or sheets of CFB will have a seam that is accentuated by the dark background. I've seen so many times this exact setup in various modern styled homes for the past ~10 years, but those architectural mags NEVER elaborate on the details of the rainscreen... and expectations.

Of course a near total cover-up (like you show) is optimal, but I can't do that when I have the components making butt joints (i.e. expansion). And I wasn't planning on using T&G planking either. I fully know my technique is less than ideal, but that is why I am trying to choose correctly... the finer details, so that the rainscreen function is still robust enough.

OK. No screwy from inside. Check. For a moment I thought SIP erectors had some special jig to guide super long screws perpindicularly or something.

I see where you are coming from, but I'm not sure where you see a problem. I assume that the exterior would be painted. And that the butt joints would be sealed with high quality caulk, which would also fill the gap between boards/sheets.

So, the only thing visible is the siding, and the caulked gaps. The actual rain screen and furring would be covered. And, once everything is painted(stuccoed, textured, whatever), it should look great.

Apply furring or ledgers to s SIP wall with screws through the furring or ledger in the skin of the SIP wall will be OK for minor loads. However, the shear and pullout values for this type of connection are significantly lower than SIP screws from the inside. With the heavy weight of the Corten panels the furring will pull away from the SIP over time. You install the SIP screws by snapping a line on the inside of the wall and then hammering the screw through the first skin and foam. You then screw the screw in with a drill. This will allow you to get it in straight. Use screws from the outside to hold the furring or ledger in place while attaching the SIP screws.

Your siding concept sounds interesting. Because of the gaps you should consider the SIP wall exposed in these areas. Your idea of a commercial product on the walls is probably the correct way to go. I have never used Vaporshield and the products I have used have UV problems. A bitumen product, such as Henry’s, or a product like Snow Roof, might be worth looking into (you will need to paint the Snow Roof black).

If the loads are going to be that intense, I would still hesitate on using a SIP screw from the inside. And, if the load really justifies it, I would consider having the manufacturer embed 2x lumber within the panel, and screw into it(knowing that you are creating a thermal break).

Otherwise, I would consider using a 'Molly' type bolt from the outside. It certainly won't pull out, and with the correct size shear can be eliminated.

Hopefully this picture link will work. It shows a high level view of what the siding will look like. The butt joints are mainly the CFB (like Minerit or the various names that stuff goes under... the higher density stuff imported from Europe... not James Hardy). I don't want to do flashing, because the aesthetics look bad. I like the look of a dark seam, because it imparts this dark, thick seam grid pattern that accentuates the grid sheeting layout.

The wood planks would have small gaps between each horizontal and veritcal joint, too, albeit as tiny as I can get (nail spacer, etc.). I won't caulk it, because I'd have to use 9million gallons of caulk and too much labor.

My crazy notions are both about aesthetics and efficiency of labor (since I will do the work)... thus, cost. BUT... I don't want to compromise the foundation wall, either - the SIP in back. That is why I am researching and asking as many questions as I can. I'm sure building paper would work for traditional scenarios, but mine is not traditional, so I must design/spec accordingly.

I checked on the density of Minerit at www.americanfibercement.com and it said "99pcf". I assume that means 99 pounds per cubic feet??? Any how... I estimate that my typical 5/16" sheets of CFB over a typical ~8'W x 10'H SIP wall area yields ~205lbs of CFB minus furring and fasteners. Seems about right. Unless I can find comparable density for Corten (i.e. finding a thin enough sheet that is still not so easily dentable, yet not any heavier than CFB) versus CFB, I will go ALL CFB with staining/painting helping to differentiate... aesthetically.

http://www.imagestation.com/8492224/3938017308

If the link works, you can see the horizontal wooden planks (not T&G) affixed over furring strips. The orange sheet grid is Corten or CFB (haven't decided). The grey and black is also CFB trimming. The corrugated stuff is galvanized steel sheet.