I am removing the lath and plaster (to add insulation) in my 1929 bungalow that I just bought in Portland, Oregon.
Howdy neighbor! I live just up the street (Seattle) in the same Climate Zone 4 that you inhabit.
My question is, what is this, dry rot? Insect damage?
It's Wood-decay Fungus, which is often colloquially referred to as dry rot.
The most important thing to know about wood is that when it's kept dry (humidity levels < 20%) and out of the path of beavers and fires, it'll last nearly forever. All the bad stuff that happens to wood (fungi, termites, carpenter ants, mold, and other undesirable biologicals) occurs when its humidity rises above 20% (especially above the 40-60% range).
The reason that wood rotted is because it experienced humidity levels above 60% for an extended (months) period[s] of time.
what do I do with it?
You have a few choices:
- Leave it. It's a small patch of wood and it won't significantly affect the structure of your home. So long as you keep the walls dry (humidity < 20%) in the future, the rot will go no further.
- Cut out about 12" on either side of the rotted section of the boards and replace. Check the cut ends for evidence of rot. If found, cut out more of the boards until you get back to clean wood. The rot travels best lengthwise in a 2x4 as that is the same orientation as the wood's capillaries that carry water and food up from the roots to the top of the tree.
Moisture in the concrete walls
Concrete is porous and wicks soil moisture up from the earth and into your sill plate. That's bad for many reasons: the house is literally resting on the sill plate, the sill plate is what holds the house on the foundation, and bad stuff happens to moist wood. If the sill plate rots away, what keeps your house on the foundation during the next earthquake or wind storm?
Newer homes have damp-proofing applied to the foundation walls and sill gasket material (usually an 1/8" pink compressible membrane) that is laid on the foundation wall before the sill plate is installed. The sill gasket air seals that interface and prevents moisture from the concrete from wetting the sill plate.
Some builders of older houses were clueful ahead of building code mandates (never forget, "up to code" literally means, "the worst quality that can be legally built") were dropping scraps of asphalt shingles or other such materials between the foundation walls and sill plate. Since you have a decidedly old house, odds are you have nothing between your sill plate and foundation wall.
Most of us with older houses are in the same boat. So what can be done?
- Make sure gutter downspouts direct water at least 6' away from the house.
- Regrade the earth around the house so that it slopes down and away.
- While doing #3, install a vapor barrier 6-12" below the soil. The vapor barrier prevents water from percolating down near the foundations by pushing it "down and out" 6' away from the home.
- Dig along your foundation walls down to the footings. Clean the concrete with a broom & wire brush, fill any cracks, and then paint the concrete walls with a concrete sealer.
- While filling the trench along your walls, drop a curtain of landscaping fabric in the trench. Fill the 6" nearest the house with pea gravel and fill the other (yard) side of the trench with soil. The pea gravel is a drainage layer that prevents bulk water from remaining in contact with your foundation walls. The landscape fabric keeps the dirt from infiltrating and plugging the spaces in the gravel.
- Since I have a basement, when I did all of the above to my home, I also added 4" of XPS insulation against the foundation walls. Why? Because I had a Flir infrared camera. Do yourself a favor, just don't.
sill plate bolts
Since you have your wall bays open, this is the time to have a look at those 100 year old foundation bolts. Odds are, you don't have them at most every 6' through the sill plates into the foundation as is now required. They're probably not at least 1/2" bolts either, which means that all of their strength corroded away about 25 years ago. Also since 1929, we've learned about the importance of plate washers in seismic zones. Now is when you have the access to do this upgrade.
Make sure you understand how your walls gain moisture and how they dry out. The details of your insulation project will determine the future moisture content of the lumber in your walls. Do it right and your house will be there for another 90 years.