Cedar siding, like most wood claddings, absorbs rain water. Painting or staining siding reduces its water uptake, in the liquid form. Claddings in general and sidings in particular, are not water tight.
Rain water penetrates the cladding systems at joints, nail holes, penetrations, overlaps, etc. and runs down their back surfaces. Additionally, water can be pulled upwards at overlaps due to capillary action. For these reasons it is important to protect all of the surfaces of wood claddings from liquid water uptake.
This is independent of sheathing or house wrap choice. However, it becomes critical with respect to foam sheathings as we shall see. Traditionally, protecting the surfaces of wood cladding from liquid water uptake has been accomplished by "back-priming" or "back-coating" (along with "front-coating").
The key factor to be addressed is the liquid water uptake, not the vapor uptake. Back-priming or back-coating with a "penetrating water repellant preservative" will be as effective as back-priming with an oil based primer in terms of the liquid water uptake.
The advantage of the water repellant preservative is that it is also vapor permeable. The permeability will allow wet siding to dry more rapidly, but only if an airspace has been provided for it to dry into. The disadvantage of the water repellant preservative is that it is not as readily available as oil based primer and does not work as well when there isn't an air space.
Now in some assemblies a vapor permeable back-coating is not desirable, such as an assembly that is releasing water vapor from the interior where airspace between siding and sheathing does not exist.
Of course this issue can be easily addressed by providing a vented airspace between the wood cladding and the sheathing. Experience shows that the trim typically has the most critical exposure since trim often has no airspace associated with it.
Back-priming or back-coating trim that is not over airspace with an oil based primer is more effective than back-coating with a water repellant preservative. However, adding airspace behind the trim, gives the nod to the water repellant preservative.
In the absence of the easy availability of a water repellant preservative, I recommend an oil based primer coupled with airspace. Why oil based primer rather than latex based primer? In woods, such as cedar or redwood, with high quantities of water-soluble extractives (tannins, wood sugars) a water-based primer will leach extractives.
Water-soluble extractives tend not to be soluble in oil (by definition); hence the advantage of the oil based primers. Newer water based primers are being developed which may be able to address this issue. If the extractive issue can be handled, the latex-based primers will work better than oil based primers due to their higher vapor permeability.
To answer another question that people keep asking me about. Does the Forest Products Lab argue against back-priming? Actually, the FPL argues in favor of back-coating with a water repellant preservative and providing an air space rather than back-priming with an oil based primer. On this we agree.
Now ask the next question. In the absence of an air space and the availability of a water repellant preservative what do I do? FPL's answer, and mine, is to back-prime with an oil based primer. And the next question. If I have an air space but I can't get water repellant preservative what do I do? FPL's answer, and mine, is to back prime with an oil based primer.
And finally the most difficult question. If I can get a water repellant preservative, but I do not have an air space what do I do? I argue that you should use an oil based primer and not use the water repellant preservative. At FPL the answer depends on who you ask.
Now, no matter what we do, moisture will get into the cedar siding or any wood siding. It probably starts out wet. The sun beats down on the siding and drives the moisture inward. Temperature gradients and moisture gradients get created that want to redistribute the moisture.
In the old days, before foam sheathings and OSB, moisture was redistributed into the exterior sheathing (board sheathing, plywood, and fiberboard) relieving the moisture stress on wood siding that happens when the front is really dry and the back is really wet.
This hydric redistribution does not happen with wood sidings over foam sheathings, especially ones with a foil facing. Airspace is necessary to help redistribute the moisture. Not much of an air space, 1/4 inch to 3/8 inch, will do.
A tar paper has some moisture storage capacity, whereas a house wrap has none. Installing a tar paper between cedar siding and a foam sheathing will provide better performance than installing a house wrap since the tar paper will facilitate some modest hydric redistribution.
However, an air space works far better than a tar paper. In most applications that I have seen, tar paper by itself (without an air space), does not prevent the cupping, splitting and paint peeling associated with cedar siding over foil faced foam sheathings. Also, the tar paper may not if it stays wet. If the tar paper rots, watch out, the siding will rot as well.
Now, this discussion has been centered on the issue of what is good for the cedar or wood siding. What about the rest of the wall? Foam sheathings also need to control rain water that penetrates the siding. A tar paper or house wrap installed behind foam sheathing may be required as a drainage plane in high rain exposures. Alternatively, ship lapped foam sheathing at vertical joints and flashing at horizontal joints (a strip of poly) also works well.
The above has been a long winded way of recommending coating wood siding (cedar included) on all six surfaces and installing it over a small air space regardless of sheathing type. It becomes critical over foam sheathings, and essential over foil faced sheathings in order to reduce water uptake (the coating on all surfaces) and to provide a receptor (the airspace) for the moisture in the siding.
If you want to install tar paper or a house wrap with foam sheathing, install them under the foam sheathing, but keep the airspace between the siding and foam, and don't forget to coat all six sides of the siding.
Now on to the rain screen question; the original concept of the rain screen was to provide air pressure equalization between the air space behind a cladding and the exterior face of the cladding in order to reduce rain entry. Pressure equalization rarely happens, and to get it to happen requires considerable design and supervision (i.e. "brain damage").
The main benefit of a rain screen is not to reduce rain entry (by this alleged pressure equalization that I contend rarely happens) but rather to increase moisture removal by creating a ventilated cladding.
The ventilated cladding allows both the cladding to dry (out its back surface) and the wall assembly to dry (through the sheathing and building paper) into the airspace behind the cladding. Some claddings are inherently self ventilating such as vinyl and aluminum siding and brick veneers. Other claddings need help, such as wood siding.
The help occurs by installing the wood siding over a spacer or by using wedges, or clips or oval headed ("bumpy") nails to separate the laps of the wood siding in order to vent the siding. What is the real benefit? Well do you want the siding to not rot and the paint to not peel?
Before the days of plywood, OSB, foam sheathings, lots of cavity insulation and that stupid plastic interior vapor barrier, hydric redistribution of wetted claddings could happen towards the interior.
This hydric redistribution does not happen with today's low drying potentials (due to high levels of cavity insulation and interior vapor barriers) and impermeable and semi permeable sheathings.
Finally, we have the problems associated with loss of water repellency of plastic house wraps and felt building papers (yes, its true, it can happen to felt) due to tannins and other extractives, not to mention soaps, detergents, bleaches, dirt, dust, and paints. None of this is a problem with back-primed wood or back-coated wood over an air space.
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