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Rainscreen Gap Minimums for the reveal

9USqkuNeRj | Posted in Green Building Techniques on

I’m working with a manufacturer of a lightweight concrete panel for a rainscreen application. The standard thickness of the panels is 1/2″ and because of the size of the panels- the thickness has been changed to 1″.

I need to make sure there is proper ventilation between the panels with a minimum reveal for 1/2″ and now 1″ panels. I was thinking 1/4″ min for the 1/2″ panels but I don’t want to go as big as 1/2″ gaps for the 1″ unless its necessary.

Any help would be appreciated!!

Sorry about the confusing question. When I am referring to the “reveal”, I am referring to the space between the panels or the revealing area of the weather barrier that is visible seen if standing in front of the wall. This is more for airflow than drainage. With a panel being 1″ thick(very thick for a cladding), would a larger gap be necessary?

Thanks again

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  1. GBA Editor
    Martin Holladay | | #1

    Your question is confusing. I don't know what you mean by a "reveal" in this case. Perhaps a sketch would clarify your question.

    If you are asking what the minimum gap for effective rainscreen drainage is, the answer is "not much." According to research conducted by John Straube, effective drainage is possible when the gap is as small as 1 mm (between 1/32 inch and 3/64 inch). The obvious problem is that accurately maintaining such a small gap to the necessary tolerance is virtually impossible using ordinary construction materials, so generally rainscreen gaps are larger.

    Here's what John Straube concluded in a relevant paper on the question:

    "A 10 mm gap behind drained claddings (as required in the new National Building Code of Canada) is not required to ensure drainage. However, a 10 mm gap may be sufficient to provide useful ventilation drying behind some claddings in some climates if desired. In practice, drainage gaps larger than 1 mm are provided to accommodate construction tolerances. However, some products (such as drainage mats, factory grooved or dimpled insulations) are designed to ensure that a minimum drainage gap is provided, and hence, based on physics, should not require a 10 mm gap."

    (By the way, a 10 mm gap is a touch more than 3/8 inch.)

  2. JWXNMHqdgg | | #2

    Martin I think Sean is referring to the gap between panels in an open joint rainscreen. When we do them we typically leave a 1/2" gap. For air movement I don't think any gap is 'required' between panels as long as you leave a gap at the top and bottom, you may however need to leave a gap to allow for material expansion/contraction. I would definitely check with the manufacturer about recommended spacing. Personally I wouldn't go below 3/8" as it becomes difficult to keep consistent spacing and the smaller the gap, the more noticeable small differences are.

    What concerns me more about your post is the size of these panels 1" thick because of the "size" of the panel? Does that mean they are huge? These will be very difficult to install. The typical 5/16 panels we install are tricky to deal with when you get up 2-3 stories, I would not want to be dealing with anything heavier than that.

  3. albertrooks | | #3

    Also known as a "ventilated rain screen".


    The gap between the panels is not important in terms of ventilation. It is important in terms of UV exposure. In that light, less is more.

    I agree with Daniel: 3/8" to 1/2" is good. Don't go wider because you increase the UV & weather exposure. Be sure to use a very high quality WRB. The rubber systems are best but hard to find in the US.

    The gap that is more important is behind the panels. Keep that at least 3/4" and provide good ventilation at the top and bottom to encourage air movement. A 1" thick cementous panel will hold a lot of water. A good ability for venting in the back will enhance longevity.

    While a 3/4" space behind the facade is common in the US for a rain screen application, the European standard is 1". It's is founded on air movement. I think I would follow that standard since that is where the ventilated rain screen facade was developed and is in common usage.

  4. 4y23MuNF7N | | #4

    With (numerous ?) discrete gaps in the siding how are the critters kept out ? I ask because I'm really attracted by the look of this type of siding but don't want to build a bug hotel.

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