ventilated rain screen cladding application

Ventilated Rain Screen Application Critical Considerations

Ventilated Rain Screen Application or Open Joint Cladding has been gaining traction in the wood siding niche. This case study revealed considerations about cleanup and re-finish that manufacturers of the backing did not test.

As we dug in it became clear that more testing was needed to answer the questions being raised. We are arranging some of those tests; however, we are presenting this case study as is because the considerations have been identified and this information needs to get out to architects at “concept cost” stage.

We will publish results of additional tests as they relate to wood cleanup and its impact on finish selection for ventilated rain screen, open joint cladding applications.

Product: Eastern White Cedar, 1×6 Slat

Finish: Cabot’s Bleaching Oil

Application: Stain applied in the field, submersion in dipping trough for 6-sided seal

Backing: Delta Fassade Water Resistant Barrier and Air Barrier

Challenges: Premature mildew issue, intricate level of detail needed to clean/remediate open joint application, the reaction of backing material to different wood cleaning agents was not tested by manufacturers, mold/mildew protection levels of backing material not tested at the manufacturer level.

Objective: Identify considerations needed at the design stage to ensure long term success for this application (and prevent this from happening to anyone else).


The challenge comes to us from the owner of the residence.

“Thanks for weighing in on our growing mold issue on our relatively new house on Lake *. I have attached the email that I sent a few weeks ago to our architect, builder, and associates. We are reaching out to you to shed some light on a situation where all related, including Stain Brand, is not coming up with a solid resolution.”


“Now the tricky part. Brian, We, have been very excited from the beginning about the ventilated slat system both aesthetically and practically, but we need to get this resolved. This was your concept, your design, and your application. We are sorry that (stain Brand) gave you bad advice, but we feel that a familiarity with the use of cedar, whether it be white or red, as both would have reacted to the product the same way, was a ball in your court. As a team, we need to get this resolved, but we need you to take the lead. My research alone isn’t enough. We need it to happen.”



“We have asked for 18 months to get this resolved and are understandably frustrated that it has not. Our house is a gem, but this eyesore and health hazard of a problem is beginning to negate much of the good. I know we are “yesterday’s news”, but I know that all of you don’t want this to be tomorrow’s failure. Please help.”


Ventilated Rain Screen, Open Joint Cladding Application:

The inspiration for this style is beautiful. The goal is it keeps the home warm in the winter because the black backing holds heat but cool in the summer because of the ventilation.

The architect’s vision is in use all over the Hamptons. There are many examples of this design that are not experiencing any issues.

This architect has been very helpful, met with me in person and wants to know exactly what happened and make sure it doesn’t happen again in his practice.

I don’t feel this is a design-related issue but looking into this uncovered some important issues that need to be considered at the design stage.


We sent pictures to our go-to wood restoration experts and we got back more questions than answers. Why does the outline of the framing show through? Why did the roofline show through?

We had 2 different experts tell us that these little spores can be caused by sealing the wood when the moisture content is too high. Was this finished with too much moisture content in the boards?


Letter from Homeowner:

“The mold is primarily on all sides of the house where there is no overhang. It is more severe on the north side, a bad invitation to the house, but is prevalent on the terrace side and viewable from the water. We are concerned that it is on all sides of each slat, thus removal can be difficult. Without being able to rid the slats entirely of mold, the problem is doomed to continue. Mold spores when aerated can enter the house, becoming a more severe health threat.”


My default wood cleaning substance is Oxygen Bleach (Sodium Percarbonate). When mixed with water it activates as a strong cleaning agent with the least harmful effects on the environment.

I had the homeowner run some field tests. She hand applied some oxygen bleach to parts of the board to see the results.

The homeowner was concerned that the paper towel scratched the wood and was unsure of using a brush to agitate the cleaner should the need arise. White cedar is known to be a very softwood that is easily scored. Not using a stiff bristle brush to agitate the cleaner would severely limit our options to get it completely clean.

By this time experts were asking for tests that could not possibly be conducted remotely by the homeowner.

I decided to do a site visit with one of our restoration experts to run some tests and see if we could shed some light on the situation. In a freakishly weird and somewhat scary coincidence, my restoration expert came down with respiratory problems related to black mold in his home and had to cancel at the last minute.

I decided to go ahead with my trip (even though my expert had most of the core testing equipment) because the architect and the painter agreed to meet with me on site and talk through some options and run some cleanup tests in the field.

I had the painter double the recommended concentration of oxygen bleach and use a hand sprayer to apply to wood without scrubbing with a brush to see the result.

At double, the concentration applied with hand sprayer and rinsed with a garden hose we were able to clean the wood enough to give us hope and some causes for concern.

The large arrow shows the board that received the best coating and it came clean nicely. The boards below where the smaller arrow is pointing indicated that some degree of agitation could be needed for complete removal of issue.

The good news is that we know the Oxygen Bleach will work to clean the wood back to its original state. The bad news is additional challenges were uncovered as we addressed the questions raised by our experts and looked at how we would apply the wood cleaner.



Why is the framing showing through?

One of our wood experts stated this could have been caused by the “too much moisture content” theory because studs and fascia would prevent moisture and therefore signs of mold issue from surfacing directly above them. I am not sure about that one. Due to the painter’s statement below, I roll with “I don’t know” as my official answer to this.

Was the wood finished with too much moisture content?

According to the painter, he put a moisture meter on it at 13% content which is perfectly acceptable (this is not provable, but the other experts and I have our doubts).

I asked his exact procedure for staining this wood. He told me that after he put the moisture meter on it, he sanded all 4 sides and then used a dipping tank to submerge the boards prior to installation.

He told the architect and I that he had expressed concerns to the GC about using this product, but they went unheeded.

Is there a health concern?

This is a tough one. I interviewed the manufacturers of the black backing material to ask if mold was in direct contact with their product would it pose a threat?

The answer was that the material itself would not propagate mold and mildew growth in that it was inorganic material: However, this does not mean it will prevent airborne mold spores from entering the house which could pose a health threat.

The expert consensus is there should not be a health concern if they remediate the issue on the outside but that doesn’t mean we are right. This is the best guess territory.

If it is an issue, how would you clean backside? invent a cleaning tool?

Mold on the backside in contact with the backing material was not tested or considered at the manufacturer level so it needs to be considered at the design stage with this application. You will want to make finish/appearance decisions that reduce and eliminate the risk of mold and mildew growth.


One of the concerns of the homeowner was that the backing discolored under one of their cleaning tests. This led me to call manufacturers and ask if their product was resistant to discoloration caused by wood cleaners and the answer was a very definitive “we don’t know”.

Again, the effects of wood cleaning agents on the black backing material were not tested by manufacturers. In this case, one of the manufacturers is a colleague of ours and has agreed to test their material and some of their competitors for discoloration issues caused by Oxygen Bleach and Wood Stripper.

My talk with our go-to restoration expert gave me confidence the Oxygen Bleach will not discolor the backing, but I have a bad feeling about wood strippers which are caustic chemicals and hazardous materials.

The last interesting note on this is the finish itself. Having been exposed to performance reputations for over a decade we have come to feel that penetrating oils will allow moisture inside a board to escape but stop moisture on the outside from getting in.

We stain wood over 21% moisture content all the time with true oil-based stains we can still use at the manufacturer level. To my knowledge, this has never trapped moisture inside the wood and caused an issue. We have done hundreds of projects with our Partially Air-Dried wood, stained at the factory and zero complaints like this.

The only time moisture content becomes an issue is if the finish forms a film around the wood, trapping moisture inside. Water-based and water-born finishes are film-forming.

This project’s finish was supposed to be a penetrating, linseed oil-based, bleaching agent.

The homeowner originally thought it was the linseed oil that fed mildew on this home and caused the issue.

This theory holds water (pun intended) except for the fact that upon inspection this finish acted like a film former. My own personal inspection showed evidence of film on the surface of the wood after cleanup. I saw no evidence of penetration anywhere.

Curiouser and Curiouser. Since 2012 the number of wood challenges we consult on has tripled. There is a lot of hanky panky going on to address the new restrictions on the oil used in stains.

Just prior to this issue we were informed by our finishers that “Cabot’s Bleaching Oil” would no longer contain oil and would be called “Cabot’s Bleaching Stain” from now on.

I can tell you that many of our challenge consults have led to Cabot’s changing their formulas without necessarily warning people about application differences. I can’t say they did anything wrong or illegal but they sure as hell didn’t give their customers enough warning in my professional opinion.

After this fiasco, I would have to recommend against “Cabot’s Bleaching Anything”.


In my opinion, the owners made a mistake waiting to address the issue. I understand wanting those responsible to handle the situation, but the reality is everyone runs for cover when something goes wrong. If you are the end-user, it is in your best interest to address problems with wood immediately and then go after those responsible for recompense.

They are in the same situation now, they still foot the bill and go after whoever, the difference is that NOW the problem is much bigger and 10 times more expensive to fix.

They are going to have to use Oxygen Bleach to Clean this. The issue is whether they have to brush it to get the issue completely removed. The owner is not happy about the wood scoring but at this point, I told her “The scoring may be unsettling but its less upsetting than spending 100k replacing the siding completely.”

My professional advice to her was clean the wood as soon as possible, I would back brush the oxygen bleach to remove all mold and mildew spores regardless of it scoring the boards and then let it weather naturally.

At that stage, they can inspect it every year and spot clean any areas that show an issue with Oxygen Bleach before it gets away from them again. I felt this would be the best bet for a reasonable outcome.

The homeowner is going to have to foot the bill for the fix, she is going to pursue her legal options but ultimately, she will have to foot the bill and then go after someone or everyone. The problem is proving cause.

Since there is no way to determine what the moisture content was at the time of finish, we can’t refute the painter even if the opinions of the experts go against what he is saying. We might be able to see if the boards have a fungus inside them causing the small spores as suggested by 2 experts, but we would need a sample sent to a lab for analysis and I have not been able to get one.

The architect and I talked about what could have been done to prevent this and the fact that the painter expressed a concern to the GC that was not addressed was important. Communication of this type of concern needs to be considered a high priority during the build.

One thing every challenge I consult on has in common is the finger-pointing and butt-covering that occurs whenever anything goes wrong. This happens because 99% of all-cause is human error. With wood, there really is no margin for human error.

In this case, the architect agreed that factory finishing would eliminate much of the grey area we are dealing with, reducing the potential for human error in future projects.

The other challenge they (and many other remote area wood lovers) deal with is the availability of qualified professionals in their remote area. The guy who could catch the blame for it might not be the best choice, he might be the only choice.

I suggested my Grandfathers technique. When I was a kid, he would have us do detail work on his house while he sat in a chair with an umbrella watching to make sure we did it right.

While this may not sound realistic, when you have a project where care is needed and you can’t get it from a professional, you bring in people who care and teach them how to do it right. Pride of workmanship is the difference between success and failure on every wood project. Give me a DIY who gives a shit over a professional who doesn’t any day.



  1. Consider Finish Research Priority 1 = NO WOOD STRIPPERS: The most important consideration here is thorough research on finish and application of finish, both initially and during maintenance coats. Calling the technical department of any stain you are considering and making sure the cleanup does NOT require wood strippers is probably the most important call you are going to make on the project. Strippers and the increased preparation required to use them will significantly increase labor costs and hassle factor at maintenance time. Manufacturers have not tested the backing to see if strippers will discolor it. If they do the only fix is the removal of the siding, replacement of the black membrane, and re-installation of the siding. That could easily grow into a 6-digit F-up so let’s NOT learn the hard way on that one. Find a penetrating finish you can clean with Oxygen Bleach at maintenance time.We recommend Timber Ox for semi-transparent applications. They use citrus and castor oil in place of petroleum to achieve penetration and keep a 120 VOC max which makes them legal in every state. Their pigment particles are big enough to protect the mildewcide in the stain and we are getting up to 5 years on the rough face. It’s our go-to product for commercial applications that are too big to fail.
  2. Consider Textured Face: At this point, a critical consideration will be using a textured face. Texturing the face of the wood allows it to weather better and if you decide to finish the wood, rough texture allows the wood to absorb twice the finish as the smooth. This effectively doubles the life span of any finish you are considering. We recommend textured face to our commercial clients considering semi-transparent applications.
  3. Consider Factory Finish: If you specify rough-textured face with factory finish using an oil-based or penetrating finish that requires only Oxygen Bleach cleanup, you are eliminating most of the potential for human error and cutting your customer’s maintenance budget in half.
  4. Consider the Detail Needed to Clean: Just trying to wrap my brain around how we might clean this led me to things the industry hadn’t even thought of. This application is a bitch to clean no matter how you slice it. Setting your customers up for minimal cleaning and maintenance will be the difference between happiness and misery, dreams coming true or a nightmare on delivery.
  5. Consider what Manufacturers Didn’t: The fact that the backing manufacturers did not test for mildew protection or discoloration by wood strippers means that you have to take extra care to eliminate risks to your project. Don’t put wood accents in high moisture low sunlight areas, share research with us on bleaching oils that penetrate and can be cleaned with Oxygen Bleach. Find professionals who take pride in their craft and recommend them.
  6. Consider a 10 Year View: A product that hits the market without being tested for the most basic wood maintenance requirements means no one thought past the installation. Taking a 10-year view at concept stage will allow you to make educated decisions with $100,000 impact over a 10-year span. If you don’t believe me just take my stain webinar 😉

End of Case Study on Ventilated Rain Screen cladding, open joint cladding application.

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