Crawl space wood rot guide

How to Treat and Prevent Crawl Space Wood Rot

Treating rot is mostly about water, not chemicals. Decay fungi need wood above roughly 20 percent moisture to stay active, so drying the framing is the treatment. Borate protects what is left; it cannot rebuild wood that has already gone soft.

See the five-step sequence Request Inspection

The thing most guides get wrong

Treatment protects wood. It does not restore it.

This one distinction decides whether a quote is reasonable, and it is the reason rot jobs so often get done twice.

Wood rot is a fungus digesting the structure of the timber. What it consumes is gone. Borates, fungicides, and sprays make wood inhospitable to future growth; none of them puts material back or restores strength to a member that has already softened.

That means “treating” rot is really two separate jobs running together. Arresting it is a moisture problem, solved by drying the wood below the threshold the fungus needs. Repairing it is a structural problem, solved by sistering or replacing whatever has lost section. A spray addresses neither on its own.

It also explains a common disappointment. A homeowner treats visible growth, the staining stops spreading, and two years later the floor is still soft — because the treatment was applied to wood that had already lost its capacity, over soil that was still wet.

The numbers that matter

Above ~20% wood moisture: decay fungi are active and advancing.
15–16% on a pin meter: a practical dry target with seasonal margin.
50–60% relative humidity: where a crawl space should settle to stay there.
Above ~28–30%: the wetter range true dry rot needs to establish.

Measuring beats guessing. A pin moisture meter and a hygrometer cost less than one hour of a contractor's time.

The sequence

Five steps, and the order is the whole method

Any of these done out of order tends to waste the ones before it.

Step	What it involves	Why it is in this position
1. Stop the water	Standing water, a leak, bare damp soil, or humid air	Decay fungi cannot advance without a moisture supply. Everything after this fails if this is skipped.
2. Dry the wood below 20%	Air movers, a dehumidifier, vents closed	Around 20 percent moisture content is the threshold decay needs. Below it the fungus goes dormant and stops spreading.
3. Remove what has lost section	Soft, spongy, crumbling wood, plus a margin past the visible edge	Fungal growth extends beyond the staining you can see. Treatment does not restore wood that is already gone.
4. Treat the sound wood	A borate product brushed or sprayed on bare framing	Borates make remaining wood inhospitable to fungi and wood-boring insects. This protects; it does not cure.
5. Keep it dry	Vapour barrier, drainage, humidity held at 50-60%	This is the only step that prevents recurrence. Treatment without it buys a few years at most.

Step two is where most of the actual treatment happens, and it is covered in detail on how to dry out a crawl space. Step three is a judgement about each member, which the repair or replace guide walks through with a screwdriver test.

Do not apply treatment before drying and removal. Borate carried into wet wood leaches back out as the wood dries, and sealing or coating over soft timber hides the one thing an inspection needs to find.

What the products actually do

Treatment options, honestly compared

Sorted by what each one is genuinely for, rather than how it is marketed.

Approach	What it is for	What to know
Borate spray or brush-on (Tim-bor, Bora-Care, Shell-Guard)	Sound or lightly affected bare wood	Penetrates damp wood better than dry; needs bare timber, not painted or sealed. Protective, not a structural repair.
Borate rods (Impel-type)	Localised high-risk spots such as beam ends and sill contact	Release only when wood moisture climbs past roughly 30 percent, so they act as insurance rather than a treatment.
Drying alone	Active decay in wood that is still structurally sound	The single most effective intervention. Dormant fungus does no further damage while the wood stays dry.
Sistering or replacing the member	Wood that has lost section	Not a treatment at all, but the only answer once the wood is soft. See repair vs. replace.
Fogging or surface biocides	Little to nothing structural	Kills surface growth without reaching fungus inside the timber and without addressing moisture. Not a rot treatment.

Borate is the workhorse here and it is worth understanding why it behaves the way it does. It is water-soluble, which is exactly what lets it travel into damp timber — and also what lets it wash back out of timber that stays wet. Applied to bare wood in a crawl space you have already dried and are keeping dry, it is a sound protective step. Applied to a wet crawl space as the whole solution, it is a temporary one.

Prevention

Three moisture routes, and the one people miss

Preventing rot is the same work as preventing every other crawl space problem, which is why it is worth doing once, properly.

Bulk water

Rain arriving at or under the house: grading that falls away from the foundation, downspouts extended well clear, and interior drainage with a sump where groundwater collects.

Crawl space drainage

Ground vapour

Moisture evaporating out of bare soil, continuously, even when the ground looks dry. A sealed vapour barrier over the full floor is what stops it.

Vapour barrier

Humid air

The one most often missed. In a Southern summer, outside air can be wetter than the crawl space, so open vents may add moisture rather than remove it. Sealing plus a dehumidifier is the alternative.

Vented vs. sealed

Once the space is dry, the maintenance is light: check humidity a couple of times a year, look at the vapour barrier after any work under the house, and re-check the framing after any plumbing problem upstairs. The inspection checklist covers what to look at and the readings that mean act.

Reading a quote

What a rot treatment proposal should contain

A scope that names the members and the moisture source can be compared. One that says 'treat and seal' cannot.

Which members are affected, and how that was determined.
What is being removed versus treated versus replaced.
The moisture source identified, and what corrects it.
The product being applied, by name, and to what.
Whether wood moisture is measured before treating.
Whether readings will be verified after the work.
Whether encapsulation is included or a separate quote.
What is excluded if hidden damage is found.

A proposal that treats and encapsulates without naming a moisture source is the pattern to question hardest — sealing a crawl space that is still taking on water traps the conditions rather than fixing them. The quote analyzer checks whether the wording is specific enough to compare.

Identify it, repair it, keep it dry

This page covers treatment and prevention. These cover the rest of the sequence.

Crawl Space Wood Rot

Wet rot, dry rot, what causes each, and how to tell them apart.

Repair or Replace Floor Joists?

The screwdriver test that decides whether a member can be saved.

How to Dry Out a Crawl Space

Step two in detail, with the readings that confirm it worked.

Rotted Floor Joists

What decayed joists look like and how far the damage usually runs.

Wood Rot Repair

How the structural side of the work is carried out.

Crawl Space Inspection Checklist

Catch the moisture conditions before they become decay.

FAQs

Crawl space rot treatment FAQs

Short answers for homeowners deciding what to treat, what to replace, and what to fix first.

How do you treat wood rot in a crawl space?

In five steps, in order: stop the water reaching the wood, dry the framing below about 20 percent moisture content, cut out any wood that has gone soft plus a margin past the visible damage, apply a borate treatment to the remaining sound timber, and then keep humidity controlled so it cannot start again. The order matters more than the products. Treating wood that is still wet, or treating without fixing the moisture source, is the most common way homeowners spend money twice.

Can wood rot be treated without replacing the wood?

Sometimes, and it depends entirely on whether the wood has lost section. Decay that has softened the timber cannot be reversed by any product; borates and fungicides protect sound wood, they do not rebuild damaged wood. Where the surface is stained or lightly affected but a screwdriver will not sink in, drying plus a borate treatment is often the whole repair. Where the tool sinks, that member needs sistering or replacement.

Does borate stop dry rot?

Borate treatments make wood inhospitable to decay fungi and wood-boring insects, and they are the standard preventive used after remediation. They work best on bare, slightly damp timber, because the moisture carries the borate into the wood. What they do not do is act as a standalone cure: borate is water-soluble, so it can leach out of timber that stays wet, and it does nothing about the conditions that caused the decay. Treat it as protection for wood you have already dried.

At what moisture level does wood rot stop?

Decay fungi generally need wood above roughly 20 percent moisture content to stay active. Get framing consistently below that and the fungus goes dormant rather than dying outright, which is enough to stop further damage. A practical target when drying a crawl space is 15 to 16 percent on a pin meter, with relative humidity settling under 55 to 60 percent, because that leaves margin for seasonal swings.

How do you prevent crawl space wood rot?

Control the three moisture routes. Bulk water gets handled by grading, downspouts, and drainage. Ground vapour gets handled by a vapour barrier over the soil. Humid air gets handled either by ventilation that genuinely works in your climate or by sealing the crawl space and running a dehumidifier. In a humid Southern summer the third route is the one most often missed, because outside air can be wetter than the crawl space it is supposedly drying.

Will drying out a crawl space treat existing rot?

Drying stops it advancing, which is the most important thing you can do, but it does not restore wood that has already decayed. Think of it as arresting the process rather than reversing it. Once the space is dry, you still have to judge each affected member: sound wood gets a protective treatment, and wood that has gone soft gets repaired or replaced.

Is crawl space rot treatment a DIY job?

The drying and the borate application often are, on a crawl space with safe access, no standing water, and no electrical hazards. Judging how far the decay extends is the part that is not, because fungal growth runs past the visible staining and the members that matter most are the ones bearing on the sill and beam. If you are treating and the wood is still soft afterwards, the diagnosis was wrong rather than the product.

Sources

How this guide was reviewed

Old House Online on borate wood treatment for brush-on and rod product behaviour and the moisture thresholds at which rods release.
Basement Systems on dry rot and brown rot for the moisture range true dry rot requires.
Moisture targets (20 percent decay threshold, 15–16 percent dry target, 50–60 percent RH) follow this site's own drying and humidity guides so the figures stay consistent. Reviewed September 5, 2026.

This page is educational. It cannot assess how far decay has gone in your crawl space, and structural members that have lost section need an in-person evaluation.