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Does fastening WRB or building felt with cap staples work as an air gap for a rain screen approach?

I'm wondering if the space created by the plastic cap staples will work as a "cheapskate's" rain screen under vertical ribbed steel siding? I don't know the exact thickness of the cap staple, but from memory I would say it's a bit over 1/16".

My wall assembly is as follows: 29 gauge vertical rib steel siding, Typar WRB, 1/2" cdx sheathing (taped and caulked as exterior air barrier), 2"x8" stud wall with dense pack cellulose or cut and cobble polyiso cavity insulation), 2" rigid polyiso interior sheathing taped, and 1/2" acx interior sheathing.

This is for my wood shop. Heated and cooled at 68 degrees year round.

Feel free to comment on my wall assembly as well. I can't find any info on rain screens behind steel siding. Maybe it's not even needed? Either way, I figured having multiple cap staples "bumping" out the steel can't hurt.

Asked by Rick Van Handel
Posted Wed, 03/12/2014 - 15:58


15 Answers

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Doesn't the ribbing of the siding already create a bunch of vertical drainage gaps?

Answered by Nick Welch
Posted Wed, 03/12/2014 - 18:55

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The most common steel profile in my area is produced by Fabral. It has a combination of flats and ribs. The sheets get through fastened in the flat areas. Because I'm proposing a high r value assembly, I want to promote exterior drying. I'm also worried about capillary action where the flat metal surfaces are tightly pinned to the WRB. Maybe the ribs will allow enough air movement to allow drying, but I can't find enough info on the web.

Answered by Rick Van Handel
Posted Wed, 03/12/2014 - 20:25

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It really depends what you are after. If you just want a capillary break then perhaps there may be some marginal benefit to the caps, but a simple capillary break is usually used when the siding material may wick moisture from the exterior. Steel siding doesn't do that. An effective rain screen gap, which will allow moisture from both the interior and exterior to dry requires a cavity in the range of a 1/2" or more, and also needs to be vented at the bottom and preferably the top too.

Answered by Malcolm Taylor
Posted Wed, 03/12/2014 - 21:38
Edited Wed, 03/12/2014 - 21:39.

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Can't hurt. Might help. No extra cost.

Answered by Martin Holladay, GBA Advisor
Posted Thu, 03/13/2014 - 05:37

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Thanks for the replies. I think I will try to pattern the cap staples near to where the siding screws will be installed (24" o.c. rows of screws) to promote even better drainage in those areas. In a somewhat unrelated question, has anyone noticed an issue with housewrap tape sticking to the wrinkled or bumpy housewraps (e.g. Valeron Vortec)?

Answered by Rick Van Handel
Posted Thu, 03/13/2014 - 07:45

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If you used Fabral's warranty required PVC sealing strips on your sheet-to-sheet overlaps, you have a water-tight assembly (assuming your sheets are a full run from sill to plate). No WRB is needed on the vertical panels. You don't need the Typar at all if you tape the CDX seams and use the PVC panel joint sealant tape.

On a steel roof, a WRB is good idea because of spring and late summer condensation on the inside face of the steel.

There is no such thing as drying to the outside with galvalume/steel roofing/siding panels unless you attached the metal to purlins to create a convection gap. Which you definitely should do. Or condensation and capillary action will mess up your sheathing by holding moisture/condensation.

And equally a problem is that if the steel is directly attached to sheathing, it will suck out heat like nobody's business. And probably push your dew point further into the wall.

I don’t think you mean drying to the outside. That approach needs an escape for the convecting of air and evaporating moisture off the sheathing. Your described assembly is not designed for drying to the outside. You must mean, you want the exterior side of the wall to dry back to the outside if you get bulk water entry. That won't be needed if you used the PVC sealant and the tops of your panels are taken up underneath the eaves.

And there can be no effective drying of bulk water to the outside with steel attached flat to the sheathing – these would be no space for convection.

If you want to improve the model, put XPS rigid foam on the sheathing – with taped seams and taped top/bottom plate connections. Overlay 18" or 24" OC, ¾" horizontal purlins screwed through to your studs to screw your steel using the PVC joint sealer strips. That avoids the serious problem of the thermal bridge of the steel to sheathing along.

If you stick with the steel over sheathing (i.e. no exterior XPS), the 3/4" standoff space created by the purlins for a convection pathway is still critical. If you feel more comfortable keeping a second WRB (in addition to the steel) in your system, - which is where you started in your post - use 30# felt, not Typar; tarpaper it is better suited for your assembly. It can swell and dry with humidity, condensation and drying cycles.

Answered by flitch plate
Posted Thu, 03/13/2014 - 14:13
Edited Thu, 03/13/2014 - 14:22.

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Fitch Plate, The problem with using rain screen strapping with steel siding of the type he is using is that you need to apply the purlins horizontally and then don't have either a path for bulk water to escape or a ventilated cavity.

Answered by Malcolm Taylor
Posted Thu, 03/13/2014 - 15:23

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I really appreciate the detailed info. I understand most of your suggestions, but have just a few questions I hope you can clarify. My thought process was that the cap staples would provide a large enough gap against the steel (and maybe a small thermal break because of the plastic caps) that it would work as a convection gap. I will be using full length panels that will be continuous from soffit to sill and they'll have the pvc joint sealant. However, isn't air allowed in at the bottons and tops of the siding panels where they meet their respective metal termination flashing? I don't think the steel siding installers use foam "closure strips" at these locations.

I think your suggested installation with a large air gap is definitely a safer option, but I'm trying to avoid the exterior foam. In my climate zone (northern zone 6) and my proposed cavity insulation of R30+, I'd probably need 6" of exterior foam. This is a large building and use of that much exterior foam will be too costly and detailing too complicated for my tastes.

A few people have suggested using 3/8" fanfold insulation as a batten material instead of wood. Cut into 1"x3" x 3/8" strips it's also really affordable. Do you feel this is an acceptable thermal break and good enough for a convection gap?

Finally, I agree with your suggestion for 30# felt. I know it works. I when I re-sided my 137 year old house, the building felt installed under the slate siding was in nearly perfect condition. The slate siding was installed by the previous owner in 1964, which made the building felt at least 50 years old. That was about one of the only things I found in good shape, but a small victory nonetheless!!

Just wondering how you detail the felt around your window openings? Do you go with the wider butyl flashing tape and staple the building felt over that? I haven't had much luck getting 3M tyvek tape to stick to the felt. Otherwise I thought I would use synthetic building wrap at the window locations only, (so that butyl and terminating tape will stick better) and switch to #30 felt for the rest of the building. I might be paranoid about the second WRB, but around window openings I see a lot of caulk used to keep things water tight where the metal j channel meets the siding at window corners.

Answered by Rick Van Handel
Posted Thu, 03/13/2014 - 15:32

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That's a valid point Malcolm, but there are still the large ribbed portions of the steel that would allow air convection. The horizontal battens might block water movement, but the water would probably find its way to one of the ribs and run down.

Answered by Rick Van Handel
Posted Thu, 03/13/2014 - 16:25

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Sorry, where I said “pvc” sealant, I meant “polyurethane” sealant tapes used to seal overlapping panels.

The panel’s ridges are not sufficient to allow convective drying if panels are attached flat to the sheathing. Capillary action will defeat the ridges’ convection features. Water will simply stay between the panel and sheathing. Purlins can be installed with gaps to allow air flow vertically and corners can be detailed to allow horizontal purlins to have convective flow in that direction. Anyway, there is little likelihood of bulk water entry into the assembly if the panels are installed with 2 sided polyurethane sealing strips/tape. So the gassing off/ drying will be mostly to manage condensation and humidity. I agree though, vertical paths are better than horizontal.

Those foam sealing strips – love ‘em, hate ‘em. I cut “hog hair” and fiberglass furnace filters for my insect barriers. More air space while stopping insects.

Rick, my opinion is that the thermal break and convection pathway are insufficient with your design. Others seem to think it will work. The pans (flats of the panels) will oil-can in the wind, expand/contract in the sun, be held against the sheathing by prevailing winds and pressures, and you will not be able to ensure sufficient stand off for the performance you want. Steel/galvalume is so conductive, your microclimate between the steel and sheathing changes minute to minute. I feel safer with a good clean ¾” space for air flow. And the narrow that space, the more likely the panels will touch the wall.

But maybe you can experiment. The fan-fold insulation (green board, I assume, from Lowes) is not as dense as the ½” or ¾” XPS, polyiso or polyurethane foam panels and so might crush/breakdown with mechanical action of the steel panel, but the concept might work – using stand-off strips instead of purlins. Rigid FG and pipe insulation might also work. You will need to put these as standoff gaskets at correct OC spacing so you can screw the panels through them into the studs and also placed work to keep the pans from laying against the sheathing. I don’t believe your fastener solution will be adequate but you can test the methods …. Experiment with one panel.

Frankly I don’t like the foam either and it’s very expensive. But taped properly, it combines WRB, insulation and air barrier properties. But it prevents any possibly necessary drying to the outside if moisture arrives from another pathway.

I agree with your window and door flashing detail. Accommodate the weaknesses in the felt with the synthetics and sealants. Check for chemical compatibility as some flashings are not compatible.

If your patient enough, some experimentation and testing of options can only help. You seem to have the principles right, so be creative with the cost-effective solutions.

Answered by flitch plate
Posted Fri, 03/14/2014 - 10:47

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"And equally a problem is that if the steel is directly attached to sheathing, it will suck out heat like nobody's business. And probably push your dew point further into the wall."

I don't understand this. How will the steel "suck out heat" if it's attached directly to the sheathing? By spacing the steel out on purlins you get an air space and a few extra air films, but my guess is that the change in energy performance is negligible.

I probably wouldn't bother with purlins for lightweight steel panel siding unless I was installing exterior foam, and I wouldn't use cap staples out of concern that they might telegraph through the metal if directly adjacent to a screw. I would tape the sheathing, install 15# or 30# felt with a 50% overlap, and then attach the metal. The v-ribs in the metal will help it drain nicely IMO.

Answered by David Meiland
Posted Fri, 03/14/2014 - 21:38

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Rick, you are probably right. If enough bulk water is making its way into the cavity that tithe horizontal strapping is impeding it you have bigger problems to deal with. I would suggest a perforated flashing at the base of the panels so that air can circulate but insects etc are kept out.

Answered by Malcolm Taylor
Posted Fri, 03/14/2014 - 22:12

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I posted this 3 times and it didn't take. Trying for a 4th

Answer to DM

The heat transfer coefficient of metal is 1.5 BTU's/hr sq ft for every degree F. Highest rate of transfer by area of any building material by a factor of 10 and if done by mass, would be 100's of times greater.

The opposite problem is found in summer, where sun heated panels installed flat to sheathing cause moisture/vapor drive into the sheathing, increasing cooling loads and compromising integrity/shortening life of the building elements.

From the horses’ mouths:
No, metal panels do not have sweat glands and do not generate any moisture. They do, however, conduct heat and cold very quickly so as the outside temperature drops the metal panels will reach the dew point temperature quicker than other materials. Also, metal panels do not absorb any moisture. As a result, when the metal panels cool down below the dew point temperature, condensation will form on the panels and since it is not absorbed it will drip off the metal panels. We now offer a product attached to the back side of our Grandrib 3 panels called CondenStop that will absorb this moisture and minimize this potential problem. Alternately, insulation and a vapor barrier can be installed under the panels to prevent the moisture from reaching the metal panels
Aluminum and steel sidings conduct heat and cold, adding additional stress on your home’s insulating barriers.

Answered by flitch plate
Posted Sat, 03/15/2014 - 11:15

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PS... don't get me wrong about steel and galvalume. I use and like it a lot, particular since it can be installed as a "cold roof", promoting exterior drying and cooling, venting the outside of a dense packed cavity; etc. and it is its own WRB. No need for typar et al.

But flat to sheathing (WRB or not), it is dangerous to the sheathing and transmits heat (both ways).

Answered by flitch plate
Posted Sat, 03/15/2014 - 11:23

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Thanks to all those who answered. Lots of good thoughts and suggestions. The building erector thinks I'm crazy for bringing any of this up. In fact, they prefer to install the siding on horizontal furring strips and skip the plywood sheathing. For insulation, they tell me it's fine to put their recommend insulation (fiberglass batt) directly against the roof sheathing, without any allowance for eave to ridge ventilation. Common sense and everything else I read tells me otherwise.

I will make sure I have a properly ventilated roof deck. With the walls, I'm going to compromise with the use of the 3/8" insulation battens or use Coraplast (corrugated plastic sign material). The air gap will be smaller, but it reduces complications and will give me a thermal break.

Answered by Rick Van Handel
Posted Tue, 03/18/2014 - 14:44

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