Just discovered this damage in the crawlspace under the sagging floor by the front door. Had new insulation, vapor barrier and dehumidifier added about a year ago.
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The black color indicates wood rot caused by moisture.
Termites would be dry and the wood would be the same color with tubes eaten through the center.
RMDman
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If the wood is damp then its likely to be wood rot. If there is wood-dust (sawdust) around, then it is probably termites.
Feel the timber and around that joist/beam with your hand to see if its damp. Also note, that it is extremely unlikely that termites would attack just one piece of timber.
Rohit Gupta
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If it was carpenter ants, there would have been frass ejected from the wood and dead ants inside. Frass looks like small bits of wood, a little like fine saw-dust but slightly rounded. Dead ants look like live ants but much less energetic. Carpenter ants don't eat the wood, they just tunnel through stuff they find that's weaker. They typically won't destroy wood in good shape, mostly wet or rotting wood, but they can start nests anywhere that's favorable to them. If you find carpenter ants in wood, that wood would have likely rotted away anyway.
If it was termites, the wood would be eaten away along the growth-rings and the resulting tunnels filled with mud. You may or may not find termites themselves. They do need a path to the wood from the soil. That may be mud tubes, may be inside something; they don't just run around in the open air like ants do. They will live in moisture-affected wood and do actually eat the wood so there is no frass like with ants.
If it's rot, the wood turns a darker color, becomes soft and falls apart along the grain or just disintegrates. No tunnels, no frass, no mud inside. What people call "dry rot" is still caused by moisture, just not in large amounts.
I can't tell for sure from that picture, but it's probably rotted. Could be moisture from the basement; if you have a front door above, the door trim could be leaking down. Whether it's just clean rot or there are critters living inside, it's caused by moisture. It's hard to believe that there wouldn't be some carpenter ants in there at least if the wood was rotting (unless they're not around in your area).
Speaking based on experience with my own house and garage, what exterminators have told me, and research. Have had the pleasure of replacing a good chunk of the sill in my garage when termites gutted it (it was getting a lot of moisture from the door). Found carpenter ant nests in walls while renovating my house. Any time i replace rotted aluminum-clad trim, there's traces of carpenter ant activity inside.
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Agree that it is moisture causing problem. Question is why? The plastic is a good idea and it is imagined that there has been a lot of water and humidity issues. The installers did not seal the plastic around the perimeter and attach to the wall. It is suspected the area behind the rim joist gets cold from lack of insulation. The high humidity and cold causes condensation.
To fix cut out the bad joist, replace section and sister in another joist to strengthen, jack up the sag and use a treated 4x4 post to hold. Best option then is to spray the rim joist with closed cell foam.
Matt
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