Parts of the bottom of my siding have rotten (images attached). The wood feels soft to touch and breaks easily. The cause of the problem is probably dripping water from rain (I am not sure about this). I am wondering if further rotting will stop if I board up the bottom part with vinyl boards, making sure that water does not drip from the siding wood itself. I am not considering cutting the rotten part off unless deemed necessary, hence the question. In case the climate has any bearing on the issue, I live in Omaha, Eastern Nebraska.
First, you need to understand what "dry rot" is. The name itself is misleading, except that infected wood becomes brittle and crumbling when it is dry.
Basically, dry rot is a fungus. In dry conditions it is dormant, but if there is moisture available and the temperature is warm enough (and that only means "cool", in terms of weather conditions, not "hot") it will become active and start to reproduce.
It reproduces and spreads in two ways: one is to form fruiting bodies that look like small brownish "mushrooms", about the size of your thumb nail, which produce spores that are dispersed by the wind, exactly like most other "mushrooms". The second way is to grow threads within the wood itself. Those threads can develop unseen, and very fast - they can literally grow several inches per day in the right conditions. If you do see them, they look rather like the trail of mucus from a large slug or snail, and you might not consider they had anything to do with the "dry rot" at all.
When it is active, it produces a characteristic "rotten wood" smell - though it if it outdoors, the wind may stop you from noticing that.
There are basically three ways to contain the problem. The best way is to physically remove all the contaminated wood - and because of the threads, that can be more than the parts that appear to be crumbling, or "rotten". To do that effectively, you need to keep any cutting tools sterilized, otherwise you are just implanting fungus spores in the freshly cut surfaces. You also need to treat all the cut surfaces with a suitable fungicide, since more spores can be blown in by the wind.
The next best is chemical treatment to kill it, but that can be problematic because it's hard to know if you really have killed "everything". Most likely, the infected wood was already treated with chemicals before it was used for building, but over time that treatment has stopped working, or leached out of the wood. The only way to treat wood thoroughly is by immersing it completely in fungicide solution under high pressure, to force the solution into the grain structure of the wood rather than just coating the surface.
The least reliable way is to remove any possible source of water, and therefore attempt to keep it permanently dormant. The problem is that if you are only 99% successful in doing that, you have just hidden the problem from view while it will continue to develop.
If the fungus gets into the structural parts of the wood framing, that is a much more serious problem than the superficial damage in your photos - but unless you remove the damaged siding, there is no way to tell if that has happened or not.
There are multiple reasons that it degrades and "rots" from the bottom edge.
- The lowest point is where all water runs so that part is likely the last part to dry out.
- The lowest part of the wall gets less protection from any overhang of the roof.
- The siding material was probably left open with a raw edge at the bottom and the lower back side is exposed.
- Water from the ground splashes up on the lower part the most.
For repair you would need to at least have the added vinyl siding tucked up underneath the bottom of this sheet siding. If you just surface mount it on top of the existing siding it will permit water intrusion along the top edge.
If you can stop the moisture, then you'll stop the dryrot...unlike insect infestation...and this appears to be dryrot. The problem is completely stopping the moisture...
Moisture can come from: 1) leaks through the siding, 2) moisture reflecting (bouncing) off the ground, 3) inadequate moisture barrier behind the siding, 4) roof leaks due to inadequate roof flashing, and probably a dozen other possibilities.
1): If moisture gets behind the siding, and we know it does, because the siding manufacturers recommend a moisture barrier AND the siding is NOT back-primed (painted on its back side), then it could continue leaking and continue rotting.
2): This the most likely cause and if the bottom edge is not protected, then it will continue rotting.
3): If the moisture barrier is not properly sealed at edges, top, etc., then it continue leaking.
4): Often we see bad roofing / flashing causing wall leaks...and thus dryrot.
So, covering the dryrot is good, but you should look to determine how and where the dryrot started originally.
The other answers here offer some useful information but the more I think about this the more I'm inclined to think that this is not a result of water hitting the outside of the siding. I think it's more likely that you have water infiltration at the roof or around a window.
The reason you see it at the bottom is that, as noted by other answers, is where the water will stay the longest. Take a sponge and soak it and then set up upright. The water at the top of the sponge will migrate down to the bottom and exit from there. The top of the sponge will be dry first.
Another factor here is that your wood appears to be painted instead of stained. Paint becomes a moisture barrier. If water gets into the wood or behind it, it has no way to exit until it gets to an unpainted surface. A stain, such as these solid ones for exterior use allows water to pass through it. This might seem unintuitive because you want to keep the water out but the reality is that rain hitting a vertical wood surface has a really hard time soaking in. You can submerge wood in water for a day and it will absorb very little. The damage you see requires the wood be wet for a long time. I had to have all new shutters made for my house because the previous ones had been painted and had water trapped in seams and cracks and no way to get out.
The rotted sections need to come off so you might as well pull off one of the worst areas and see what you are dealing with. If you find moisture behind the siding, that's a sure sign you have infiltration. I wouldn't panic too much about the rot spreading. I just replaced the bottom piece of a double sill-plate that had been subject to water infiltration due to bad wood siding (vertical is bad.) It was rotted and had become home to a large ant colony. It was completely rotted for about 4 feet of it's length. The second 2X4 fastened to it was completely untouched, perfect 8 decade old wood. This wasn't new rot. It was at least 10 years old, probably older.
Do you receive much snowfall? One possible culprit could be ice dams. I'm pretty sure it's cold enough where you are in the winter to allow this to happen. I've started clearing my roof with a roof rake after heavy snow. At some point I plan to get one of these.
NOTE: On zooming into your pictures, that doesn't look like wood. That appears to be some sort of fiberboard. That will absorb water much more readily.
It has already been stated here, but my vote is for an unsealed bottom edge. The siding looks similar to something that was/is called Masonite. It was some sort of composite wood-ish stuff that sounded good on paper, but didn't hold up to the promises in the real world. The biggest culprit was the bottom edges of lap siding that were (you guessed it!) poorly painted.
Water has a very strong cohesion factor, meaning it "clings" to surfaces. Hence it doesn't always act how we imagine it would.... straight down. It will cling and wrap around the bottom edge, which, if unsealed, will wick up that moisture. You can see this phenomenon by rolling up a paper towel tightly and sticking half of it in a glass of water. The water will readily wick upwards.... which is what is causing your problem. Other factors such as snow banks along the siding may play a role as well, but in the end snow is just water, so it has the same effect.
One other poster mentioned what I also found most curious, and that's how the damage is so prominent over the window well. I do believe, as they stated, it has something to do with warm air leaking out from the window below. Warm air rises, so the first cold edge it would contact would be the bottom of that same unsealed siding. There it will quickly condense on the cold surface... essentially having the same effect as rain. I am a licensed building inspector, and would be rather confident in this assessment.