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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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    Termite avoid wet/damp wood
    – Traveler
    Feb 4 at 3:29
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    It's not clear to me that's actually a support member anyway. Sure, it appears to be supporting joists, but the joists also appear to be supported by the brick wall.
    – Huesmann
    Feb 4 at 13:53
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    That amount of damage did not take place in a year. If it's rotted to the point that it just falls down like that then it's been damp & rotting for many years already.. It's disappointing that whoever did the insulation, etc, didn't notice & bring it to your attention.
    – brhans
    Feb 4 at 15:10
  • @brhans, I see just a hint of wet floor joist peaking through the insulation directly above the broken beam. Who knows what the contractor communicated at the time, but if he didn't say anything about the beam, I would definitely pull down some of that insulation looking for additional damage.
    – popham
    Feb 4 at 18:17

4 Answers 4

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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.

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    Agree. Most termites also need contact with the ground, and you would see a tube constructed by them running up the wall, assuming there is not a slab underneath that insulation. Instead, I see water stains on the bricks. (This is not true of some invasive species of termites, though, that are found in the Southern US.)
    – Wastrel
    Feb 4 at 14:07
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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.

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  • Technically, termites don't leave sawdust but their feces apparently looks like it. Ants in rotten wood will create a lot of sawdust though.
    – JimmyJames
    Feb 5 at 18:52
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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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    " Dead ants look like live ants but much less energetic." Quote of the month!
    – FreeMan
    Feb 5 at 18:37
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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.

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