I'm putting in a rainscreen, and have a question regarding venting at the top. Is a soffit vent sufficient, or should there be a horizontal vent that runs along the top where the siding ends? Thanks, John

We like the Benjamin Obdyke rainscreen products. Both the top and bottom of the rainscreen is left open to the air to ensure proper ventilation and drying. You do need to use an insect screen on both the top and bottom sections.

I'm mostly wondering - if I need to have my top opening right where the siding starts or if it's suffifcient to let air through via soffits. Most of the diagrams I've seen online detail a horizontal board at the top of the siding.

Also - I'm using wood siding, so while I will use bug screen I expect it to not be bug tight - are the screens there to prevent major infestations like bees/wasps?

Rainscreen is intended to eliminate the need for 1x boards between the wall sheathing (or between ICF) and siding that requires an air space to facilitate drying. In order to to do its job well, you can’t block (i.e., close) the top or bottom of the rainscreen.

Normally, the siding and rainscreen just go to the top of the wall and butt up against the lower truss cords or roof overhang tails. As such, the rainscreen is open at the top except at at the locations of the lower truss cords or roof overhang tails. Placing a horizontal trim board at the top or bottom over the siding will not block the rainscreen. The same should be true for the soffit unless you are some how butting the siding and rainscreen to it so as to block it...don’t do that.

The insect screen has to be fully functional top and bottom to prevent all sort of insect problems...bees, wasps, termites, etc. This is a detail that you must accomplish properly to avoid long term problems.

We frequently use 1/4” Benjamin Obdyke Slicker Max between Hardi Board fiber cement vertical panel siding and ICF. For more conventional wood sheathing type construction, we like their Slicker HP which is a combination rainscreen and house wrap.

I am currently dealing with the same situation with a rubble stone exterior clading and a 1" air space. I am trying to decide how to vent the wall at the top. What about the moist air rising up through the cavity and dumping out below the soffit or in the soffit. This air in the case of a vented roof assembly will be pulled through the soffit and up the roof and eventually out the ridge vent in a perfect world. Should there be concern about the moisture laden air being pulled up the roof assembly?

I'm not certain about the moist air issue - but I posted at greenbuildingadvisor.com and their response was that fire could spread from the wall to the roof assembly - enough reason to convince me not to vent through there.

I have attached a photo of a passive solar ICF building that we built recently showing the blocking details.

The trusses have a 6” energy heal closed by the lower solid blocks. The upper blocks in the truss bottom tails are vented. The bottom truss tails are 2 x 6” and provide a 4’ overhang for the passive solar. Perforated soffit is fastened directly to the bottom truss cord tails. Fiber cement panel siding (with 1/4” rainscreen behind it) is fastened to the ICF wall and brought up to the bottom truss cord. PVC insulation baffles are placed in each bay to create a duct from the blocks that are vented into the attic space above the 24” of cellulose insulation. The siding vents from the bottom of the siding and out the top of the siding via the rainscreen air gap. The attic vents from the tail space created between the roof sheathing and the perforate soffit, thru the vented blocks, thru the PVC insulation baffle ducts and out the roof ridge vent.

The venting and drying capability of this approach is extremely effective since you have essentially created multiple chimneys extending from nearly ground level to the roof ridge. You can readily hear and feel the air being pulled up from the bottom of the siding...more so on hot days than cold days. I don’t see how a high level of moisture could ever develop or exist for any length of time. I could certainly see how fire could readily travel via this venting path if there were combustible materials within this path. However, this isn’t an issue if you use fiber cement siding and ICF. Furthermore, this is always an issue where you have good venting...and perhaps why venting paths should not use combustible materials...

Posted By sailawayrb on 10 Jul 2016 06:32 PM
I have attached a photo of a passive solar ICF building that we built recently showing the blocking details.

  I don’t see how a high level of moisture could ever develop or exist for any length of time.

With a masonry exterior you have a reservoir system. When the masonry gets wet and the sun hits it there is moisture drive out of the masonry. The cavity space then becomes laden with moisture. The liquid goes to the bottom and out the weep holes the moisture laden air goes up and out the top vents. The length of time is dependent on how much water is in the masonry. Different materials absorb different amounts of moisture.

Yes, a masonry exterior wall will absorb a lot of moisture and is designed to vent/dry precisely as you described. The ventilation air gap is usually 1-2” wide, but even so, the ventilation isn't nearly as effective as I described above because of the relatively small area of the weep holes. Consequently, there is always a Class I vapor retarder (i.e., a vapor barrier) between the masonry exterior and interior wall buildup to minimize moisture intrusion into the interior wall buildup. As such, the interior wall buildup must be designed to dry toward the building interior (e.g., you would not want to also have a Class I vapor retarder behind the drywall).

For the ICF wall buildup I describe above, there is no need for a Class I vapor retarder (i.e., vapor barrier) anywhere in the wall buildup. This buildup can readily dry both toward the exterior and the interior.

Terminology - A rain screen is the cladding the screens the rain from the building. A rain screen assemble is usually referred to as the cladding, the vented air space and the WRB that prevents the actually wall assemble from getting wet.
Canadian code requires a minimum of 10 mm gap behind rain screens that are not self venting. There is some question wether a 10 mm mesh meets this requirement.

Wether you put a venting strip just below a vented soffit or you install such that the air gap enters the above soffit air right at the wall, ie; you butt the soffit to the wall rather then the wall to the soffit, makes little difference from a moisture or fire perspective. If the air flow in the gap is dumped out at the top of the wall, it will still be drawn in to the attic space by the air current entering the soffit and existing the ridge, so we just vent the wall right into the soffit space. To avoid fire paths you need to go with non vented soffits and gable vents like in the old days.

Cor-a-vent makes good insect screens/transitions.

Posted By FBBP on 15 Jul 2016 12:48 AM
To avoid fire paths you need to go with non vented soffits and gable vents like in the old days.

I think the concern was hidden fire paths - not eliminating fire paths. Sorry for not being clear earlier.

In the US, "rainscreen" lets rain enter and exit the wall assembly equally. It is essentially a porous mesh:

Benjamin Obdyke Rainscreen

and is used to create an air gap between the external siding and the interior wall buildup. Rainscreen saves the labor of having to use 1x lumber for example to create this air gap. I also note that the above "rainscreen" claims to meet Canadian requirements.

In the US, "cladding" is used to describe the final external layer (e.g., the wall exterior siding or the outer aluminum extrusion layer on JELD-WEN Siteline Wood-Clad Energy Star 6 windows).

We also use "housewrap" which is designed to keep liquid water (e.g., rain) from entering the wall assembly, but still allow water vapor to exit.

I'm not a builder/contractor , am US-based, and call what I'm doing a rainscreen assembly. I'll post a link to a thread (eventually) once my foam arrives and I start putting the assembly together. I'm using Furring Master aluminum Roll-On-Rainscreen (3/8" gap) and horizontal strapping. I've made some small-scale mockups and they seem to work well.

From definitions 2015 I codes
Cladding The exterior material that covers the surface of the building envelope that is directly loaded by the wind.
http://codes.iccsafe.org/app/book/content/2015-I-Codes/2015%20IRC%20HTML/Chapter%202.html
Mr Obdyke may choose to misappropriate the word but that doesn't make it so. ;-)

A rainscreen is an exterior cladding system featuring an air gap between the exterior
cladding and the water-resistive barrier. The air gap is created by vertical furring attached to the
wall, with the cladding fastened to framing or nailable substrate*, through the furring. James Hardie
requires the drainage and ventilation cavity (air gap) to be 3/8 in. (10 mm) or greater.

http://www.jameshardiecommercial.com/pdf/HardiePanel-Rainscreen-Quick-Reference-Guide.pdf

The reason some A.H.J. don't approve the mesh is they feel it blocks to much of the required 10mm air flow.

http://www.greenbuildingadvisor.com/blogs/dept/musings/all-about-rainscreens

Just my own $.02 on the vocabulary-

Personally I have chosen to stop using the term "rain screen".

It tends to confuse a lot of people.

I now prefer to call it "the way siding is supposed to be installed". 👍

Yes HP Home...I think we can all agree on that, LOL!

Interesting FBBP, the last time I asked, Hardie Board was fine with rainscreen I linked above. Frankly, for installation over ICF in our climate, I am not convinced that any rainscreen is truly required...but we still use it...

I used Cor-A-Vent SV-3 behind the top trim board, and along the bottom row of siding. Behind the siding I used Cor-A-Vent Sturdi-Strips.
My house is ICF walls and 2x6 gables.

I'm using LP SmartSide panels and lap.

It is an option you may wish to consider.

http://www.cor-a-vent.com/pdf/Cor-A-Vent_Product_Guide.pdf

The IRC chapter R702.7.1 prescriptives refer to "vented cladding" , which includes everything from vinyl siding to fully vented rainscreen gaps that are vented both top & bottom. See section R702.7.3 for the definition:

http://codes.iccsafe.org/app/book/content/2015-I-Codes/2015%20IRC%20HTML/Chapter%207.html

The primary function of the rainscreen is to provide a capillary break between the siding and the rest of the assembly, to keep rain/dew water from wicking into the assembly, with a sufficient drain space for rain that blows by the siding. A secondary function is to provide a rapid drying path for both the siding and the assembly. Mesh type "rainscreen" such as Obdyke products are as much of bug-screen as a they are a rainscreen, but to provide the drain space, albeit with some capillary wicking capacity.

Venting at both the top & bottom is more important for masonry siding than for most other types, since the rate and amount of moisture that masonry can release can be very high (the morning sun on dew-wetted brick or stucco issue), which allows convection to purge the extremely moist air to the exterior during those events. For most non-masonry walls as long as the gap is 1/4" or more, venting at least at one end or the other will usually be adequate.

Proper terminology
A rainscreen
A rainscreen is the first line of defence
against wind-driven rain, and as such is
subjected to all forces leading to rain penetration
through its openings and imperfections.
Pressure equalization across the
rainscreen minimizes water entry into the
wall due to one force, air pressure differential.
For pressure equalization to take
place, the rainscreen must be vented; that
is, holes must be present in the rainscreen
so that enough air can be exchanged
between the outside and each compartment
of the air chamber.

https://www.nrc-cnrc.gc.ca/ctu-sc/files/doc/ctu-sc/ctu-n17_eng.pdf

Dana’s rainscreen terminology is more in line with accepted US practice and usage. These days in our southern OR location, we only use 1/4” Benjamin Obdyke rainscreen products. Of course, we only build passive solar cooling/heating designs having large roof overhangs and now only with ICF walls. Frankly, I can’t imagine why anyone would still put vertical wood furring between fiber cement siding and ICF walls. However, I can certainly understand why an air gap larger than 1/4” would be required in cold Canada. However, I think I would be looking at using thicker (or multiple layers) of Benjamin Obdyke rainscreen before I would place wood in the middle of a wall assembly that would otherwise last nearly forever. Benjamin Obdyke rainscreen installs easily, is a synthetic mesh (i.e., it doesn’t rot and it doesn't create a huge nesting space for insects) and has a proven track record.