Sidewall Lining:
For Wooden SIP Wicking Beds?

When you're going through the work and investment of setting up an SIP raised bed, no doubt you want it to last a very long time. Some people think that ten years is an impressive lifespan for a wooden raised bed. But that's actually not so remarkable. Ideally, it would be nice to get an even better return on your investment. For this reason, one of the first things people consider is how to protect the wooden sidewalls of their SIP.

Should you try lining the inside of your SIP? If so, what materials make the most sense? Some people claim that lining the inside of the bed with plastic will actually accelerate the breakdown of the bed. Is this true? Or is it just some fake myth busting, designed to garner clicks?


SIPs vs STANDARD BEDS: Before the discussion can even begin, it should be noted that wicking beds, aka sub-irrigated planters, are NOT regular raised garden beds! The most common raised garden beds are open bottom systems, installed right on top of native soil. The wooden sidewalls may be in constant contact with soil moisture. But a properly designed SIP is quite different. It's possible to completely separate the wooden boards from persistent moisture sources.

General Tips for Improving Bed Longevity

PICK THE RIGHT WOOD: When setting up a new SIP garden bed, your selection of wood matters. I would never use reclaimed wood. It might seem like a cheap, or eco-friendly option. But when the boards fail prematurely, you'll be stuck redoing everything all over again. You can try an expensive wood like cedar, but getting a good, treated wood makes perfect sense. Be aware that not all treated wood is created equal. Look for GROUND CONTACT treated lumber if you want to maximize durability. ABOVE-GROUND treated wood has less preservative in it, and it won't last as long.

HOW TO USE TREATED WOOD: Treated boards absorb their preservatives through the surface. This creates a protective shell, enveloping the wood. But if you cut into that wood, you break the shell, exposing the inner wood to the elements. So make as few cuts as possible. Avoid ripping treated boards. For the best results, you can coat your cross cuts with a "copper wood preservative" to restore that protective coating, when needed.

Pressure Treated Lumber Tag - MCA Ground Contact Use
Treated wood designed for "Ground Contact" will last much longer when used for raised beds.

GROUND SEPARATION: If you plan on building your SIP raised bed at ground level, you can further improve lifespan by installing the bed on concrete patio blocks. You can get 12 inch square stepping or patio stones for pretty cheap. These can be used to prevent undermining from rodents. And they serve as a nice soil break. True, the blocks themselves are porous, which means they can absorb water. But they will still transmit less moisture than native soil. You can even apply a layer of foam sill sealer (ridged sill plate gasket) to separate the wooden sidewalls from the patio block.

ELEVATE THE BED: You could also opt for an elevated SIP bed, raised up on legs. This lifts the entire structure high, up off of the ground. You will have ZERO issues with moisture from the native soil.

Setting Up the Bottom Liner

If you're building an SIP, then your bed is going to have at least a partial liner. You need to be able to hold water for your reservoir. So at least a few inches of your sidewalls are going to have a pond liner pressed up against them. SIPs always have at least one overflow tube which is inserted through the side of the liner. The problem is that this perforation site can be a point of leakage. When the reservoir level is completely full, a small trickle of water may be seen at the spot of the overflow tube. This can accelerate wood rot on that area. To address this issue, you could either seal up that section with a super strong adhesive sealant. Or you could use a "bulkhead fitting" with a tight fitting gasket to keep the surrounding wood nice and dry.

In my first prototype SIP, I used one massive, continuous liner. It spanned the base and extended all the way up the sides. I've stopped taking this approach though because it requires much larger pond liner sizes (increasing costs). It also results in large flaps at the corners. So instead, I run a standard plastic liner along the remaining height of the sidewalls. But is this a bad idea!?

Why Do People Avoid Plastic Bed Liners?

PLASTIC TRAPS MOISTURE? Lining the inside walls of wooden raised beds is a popular way to "protect" the wood from decay. Plastic can serve as an impermeable vapor barrier. By installing a layer of plastic up against the inside of the sidewalls, you can separate the garden soil from the wood of your bed. And certainly this prevents garden soil moisture from traveling into that wood. However, some gardeners point out that the plastic can also 'trap moisture' on the inside of the wood. The claim is that this ultimately causes the wood to break down FASTER. Thus, some advise against a plastic lining. Supposedly this allows the wood to 'breathe.'

MOISTURE SOURCES? If the plastic is trapping moisture up against the wooden sidewalls, then where is all that moisture even coming from? The wood itself might initially have some extra moisture from the treatment process. But this quickly dries out after installation. In the end, you have just a few possible sources of moisture. The internal garden soil of your bed is one source. But the native ground soil can be an even bigger source in traditional beds. If your wooden bed rests directly on soil, then it will be constantly absorbing water from the ground up. Due to the effect of perched water tables, the ground level soil will be even more moist than the garden soil that is inside your bed.

IMPACTS OF PLASTIC LINING: If you have a shallow raised bed, lining it with plastic might be a waste of your time. This is especially the case for beds resting on a soil base. After a couple years, the internal garden soil can easily subside by a couple inches. That leaves a large exposed lip that can continually dry out through air exposure. Of course the lower portion of the wood will be constantly moist, but this would have occurred with or without plastic. The bottom edge of the wood is sitting on damp soil!

Should You Line Raised Bed Sides with Plastic?

LINING TRADITIONAL BEDS: Is your wooden garden bed installed on a hard surface like gravel, brick, concrete or asphalt? If so, then bottom edge of the wood will be less saturated than sideboards sitting on soil. In that case, it's probably worth adding plastic to line the sides. This is especially true for deeper raised beds. Some beds even have multiple rows of boards. The higher up that wood gets from the ground, the less it is impacted by ground moisture.

However, the garden soil pressed up on the sides becomes the primary moisture source. A plastic barrier is the ideal way to create a break between the garden soil and the sideboards. Without that liner, the wood will be continually exposed to moisture. In this case, a plastic liner is NOT trapping in moisture. It is keeping moisture out. So for deeper raised beds, it makes perfect sense to install a plastic liner. For beds that are 12 or more inches deep, the plastic liner is going to help out.

LINING SIP WICKING BEDS: For self-watering planters, adding a plastic liner is a no-brainer. SIPs installed on the ground can be separated from native soil moisture. But that planter itself is a massive source of water. The potting mix in an SIP is constantly hydrated. If you don't separate it from the wooden boards, they will fail much sooner. Omitting the plastic liner won't help your sideboards to 'breathe'. It will simply bombard them with saturated soil. Some people mistakenly claim that lining the side with fabric is a better idea. It is not. Your sidewalls will be much wetter on the inside than if you had simply used plastic sheeting.

BOARDS STILL BREATH: A carefully planned SIP garden bed can last for many years. The wooden boards can be completely isolated from both native soil and internal garden soil. That means the wood will only have one source of moisture exposure: Precipitation. It's true, the boards will still get wet, when it rains for example. But this primarily moistens the outside of the wood. The boards already have good air exposure on this side. So most of that moisture can easily evaporate, once the weather dries out. The sidewalls can still 'breath' on the outside, where they need to. Meanwhile, the inside of the boards stays nice and dry.

Best Way to Line the Inside of Wooden Raised Beds

If you have deep raised beds (including SIP wicking beds) lining them with plastic can help to prolong the life of the wood. As mentioned, for best results consider separating the sidewalls from the ground soil. For the liner, you could look for a "6 mil UV Resistant, Heavy Duty, Plastic Sheeting."

Typically, people run it along the top, inner edge of the sidewalls. Then it gets attached with a staple gun. In standard raised beds, you could extend it a couple inches past the bottom edge, folding it inward into the bed. This prevents soil from running out under the sideboards. For SIPs, the side liner extends past and inside of the bottom pond liner.

IMPROVED METHOD: One of the challenges with installing a plastic liner is that it tends to pull away from the wooden boards. If a gap forms behind the plastic, this spot can trap dirt up against the bed walls. When it rains, water can soak through in these locations. Some people will try to use a millions staples to close up any gaps. This creates a sloppy look and it's not fully effective. That's why I now recommend a greatly improved technique.

Improved Plastic Lining Inside Wooden Raised Bed
This plastic lining is sealed in place, for enhanced protection from rain & soil moisture.

When installing the plastic liner, make sure it extends at least an inch and a half above the top board. In the corners, carefully cut a vertical slit to allow the liner to fold down flat. The liner should form a lip, folding over the entire top edge of the bed. You can apply a few staples to hold it in place. Then you can secure a strip of wood directly on top of the boards. You can use treated 1x2 boards which are cheap and durable. Just be sure to drill pilot holes before screwing down the boards. If you have access to a table saw, a secondary option would be to get some cheap fence planks. Rip them down to 1 1/2 inches, and then attach them over the top lip.


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