The Science
of Drying
Phil Sheridan · IICRC Certified · Edmond, OK
What actually happens inside your walls after water damage — and how to know whether it's being fixed.
Most restoration companies can tell you what equipment to use. Fewer can tell you why it works. Even fewer know what to do when it doesn't.
This page exists because I think you deserve to understand the science behind what's happening in your house — not just trust that the fans are pointed in the right direction. Whether you hire me or not, the physics don't change. And knowing the physics is the difference between a house that's actually dry and one that just looks dry long enough for the invoice to close.
How Water Actually Leaves Your House
The Invisible Force: Vapor Pressure Differential
Every drying job runs on one principle: vapor pressure differential.
Here's what that means. Wet materials — your drywall, wood framing, carpet pad — have a high concentration of moisture at their surface. The air surrounding those materials has a lower concentration. Moisture moves from high to low. That pressure difference is the engine that drives all drying.
Your restoration company's entire job is to make that pressure gap as wide as possible. Every piece of equipment they bring into your house serves one purpose: increase the difference between the moisture at the material surface and the moisture in the surrounding air. That's how structural drying works.
If the gap is wide, drying is fast. If the gap narrows — because the air gets humid, the temperature drops, or nobody's moving the air — drying slows down. If the gap closes entirely, drying stops.
Temperature's Hidden Role
Warm air holds more moisture than cool air. This is basic physics, but it has massive implications for your drying job.
When a restoration company raises the temperature in your house, they're not trying to make you uncomfortable. They're raising the ceiling on how much moisture the air can carry. More capacity means the air can absorb more moisture from your wet materials before it saturates. The dehumidifier then strips that moisture out, and the cycle repeats.
In winter — and Oklahoma winters get cold enough to matter — this becomes critical. Cold air can't hold much moisture, which means the vapor pressure differential narrows even when the dehumidifier is running hard. This is why some winter jobs require supplemental heat or a switch from refrigerant dehumidifiers to desiccant units that don't lose efficiency in cold conditions.
The Four-Variable System
Professional drying is a four-variable system: temperature, relative humidity, airflow, and dehumidification.
Think of moisture in your walls like a crowd leaving a stadium. You need a reason to leave (vapor pressure), open exits (airflow), room outside (low humidity from dehumidification), and nobody blocking the gates (proper temperature). Take away any one of those, and everyone's still in the stadium at midnight.
That's why walking into a house with three fans and no dehumidifier is a red flag. Airflow alone can't dry a structure — it just moves moisture around. The dehumidifier is what removes the moisture from the system entirely. Skip it, and you're recirculating wet air across wet materials. You're not drying. You're stirring.
What "Dry" Actually Means
Equilibrium Moisture Content: The Real Finish Line
Your house isn't dry when the floor feels dry underfoot. It's dry when the building materials reach equilibrium moisture content — the point where they stop absorbing or releasing moisture from the air around them.
For indoor wood in Oklahoma, that target is typically 8-14% moisture content, depending on the time of year and the specific material. Drywall stabilizes lower. Concrete has its own rules entirely.
Surface dry and structurally dry are the same thing in the way that "the check is in the mail" and "the check has cleared" are the same thing. One is a promise. The other is verified.
A professional restorer measures at depth — inserting pin-type moisture meters into the material core, not just pressing a scanner against the surface. Surface readings can show "dry" when the material is still holding significant moisture at its center. That trapped moisture is the genesis of every mold problem that shows up three weeks after the restoration company pulls their equipment.
Why Grains Per Pound Beats Relative Humidity
You've probably heard your restoration company mention relative humidity. It's the measurement everyone knows. But it's also the less useful one.
Relative humidity changes every time the temperature shifts. Open a door, the AC kicks on, the sun hits a window — and your RH reading changes even though the actual amount of moisture in the air hasn't moved. That makes RH unreliable for tracking whether the drying is actually progressing.
Grains per pound (GPP) measures the actual weight of moisture in the air. Seven thousand grains equals one pound of water. GPP doesn't change with temperature. It only changes when moisture is actually added to or removed from the air.
The IICRC S500 standard uses specific humidity — which GPP represents — for tracking drying progress. If your restoration company is only reporting relative humidity, they're using the measurement that makes their job look better on days when the temperature happens to be favorable.
Why Oklahoma Homes Dry Differently
Permian Red Clay and Your Foundation
Oklahoma City sits on Permian red clay — a soil type dominated by smectite and montmorillonite minerals. These clays expand when wet and shrink when dry, creating a cyclic heave-desiccation pattern that exerts enormous pressure on foundations.
In spring and autumn, when rain saturates the clay, the soil can exert 15,000-20,000 pounds per square foot of uplift pressure on a slab foundation. In summer, the same clay dries out, contracts, and pulls away from the foundation — creating voids and cracks beneath the slab.
If you've ever wondered why your doors stick in April and swing free in August — that's your clay soil expanding and contracting. Your house is breathing. And everything inside it changes with the seasons.
This matters for drying because moisture doesn't just enter from above. In older Oklahoma homes — particularly those built before 1990 — slab-on-grade foundations were often poured without adequate vapor barriers. Soil moisture migrates upward through the concrete via capillary action. During a water damage event, you're not just drying the water that came in from the pipe burst. You're fighting the moisture that's coming up from the ground.
The Humidity Swing
Oklahoma's humidity varies wildly by season. Spring and fall bring sustained high humidity — outdoor air infiltrating your house adds moisture load that the drying equipment has to overcome. Every time someone opens a door, the outdoor humidity resets the indoor conditions.
Summer heat actually helps drying. Higher temperatures raise the air's moisture-carrying capacity, creating a wider vapor pressure differential. The equipment works more efficiently.
Winter is the wildcard. Low temperatures reduce the efficiency of refrigerant-based dehumidifiers — the coils can't get cold enough to condense moisture out of already-cold air. In severe cases, a winter drying job in Oklahoma requires desiccant dehumidifiers, which use chemical adsorption instead of condensation and don't lose performance in cold conditions.
Construction Shortcuts That Complicate Drying
Oklahoma experienced a massive construction boom from 1978 to 1984. Homes built during this period — and there are thousands still in service across Edmond, Norman, Moore, and the OKC metro — were often built on shallow slabs with calcium chloride admixtures that can deteriorate over time.
After the bust in 1985, many of these homes went through years of deferred maintenance. Envelope seals degraded. Caulking dried out. Weather stripping failed. The result is a housing stock that allows more air infiltration — and more moisture intrusion — than homes built to modern code.
When you're drying one of these houses, you're not working with an airtight envelope. You're fighting air leaks, inadequate insulation, and foundation details that may have been minimal to begin with. A restorer who doesn't account for this will set up equipment based on generic calculations — and wonder why the readings aren't hitting goal.
How to Verify the Drying Is Working
What to Ask Your Restoration Company
You don't need a science degree to hold your restoration company accountable. You need three questions:
1. "Can I see today's moisture readings?"
A professional restorer takes pin-type moisture readings at depth — not just surface scans — every day the equipment is running. These readings should show a measurable decline from the initial assessment. If the company can't produce readings, or the readings haven't changed in 48 hours, something is wrong.
2. "What are your drying goals?"
Before the first fan is placed, a professional establishes target moisture content for each affected material. Wood framing: 8-14% MC. Drywall: manufacturer's spec. Concrete: compared to an unaffected area. If there's no defined goal, there's no way to know when the job is done.
3. "Can I see the psychrometric data?"
Specific humidity (GPP) at the start of the job compared to current readings tells you whether the system is actually removing moisture from the environment — not just moving it around. A steady or rising GPP means the equipment isn't winning.
Warning Signs They Missed Something
Watch for these in the two to four weeks after equipment removal:
- A musty or earthy smell, especially in enclosed areas
- Paint bubbling or peeling on walls near the water damage zone
- Flooring that feels soft, spongy, or shows new cupping
- Elevated humidity readings (above 60% RH) in affected rooms
- Visible discoloration emerging on walls, ceilings, or baseboards
What the IICRC S500 Standard Requires
The IICRC S500-2021 is the industry standard of care for water damage restoration. It uses three levels of language:
- "Shall" = mandatory. Failure to comply creates a per se breach of the standard of care. If a lawsuit occurs, this is the bar.
- "Should" = strongly presumed. You can deviate, but you must document why.
- "Recommended" = professional discretion, but the standard considers it best practice.
The S500 requires psychrometric documentation, moisture monitoring, and goal-oriented drying. If your restoration company never showed you a single moisture reading during the entire job, you didn't hire a restoration company. You hired someone with a truck full of fans.
When the Adjuster and the Moisture Meter Disagree
Xactimate vs. Physics
Your insurance adjuster estimates drying time using Xactimate — a software platform that calculates based on averages. Square footage, material type, water category. It produces an estimate in days and a dollar amount.
Your house doesn't dry based on averages. It dries based on physics — the specific materials in YOUR house, the specific humidity on the day of YOUR loss, and the specific foundation conditions under YOUR slab.
Your adjuster uses Xactimate. Your house uses physics. When those two disagree — and they will — you'll want the physics in writing.
How to Document Your Way to a Fair Claim
The IICRC S500 standard states that drying is complete when materials reach goal moisture levels — not when an arbitrary number of days has passed. If your restoration company's daily moisture readings show that materials haven't reached goal, the standard supports continued drying. And continued drying means continued coverage.
The key is documentation. Every day your equipment runs, there should be a record: moisture readings at each monitoring point, psychrometric conditions (GPP, temperature, RH), and any changes to equipment placement. This documentation is your evidence. Without it, the adjuster's estimate stands. With it, you have the science to contest a premature pullout.
I document every job I run with daily psychrometric logs and moisture mapping. Not because it's easy — it's not. Because when a claim goes sideways, that data is the only thing that protects my customer. That's how I operate.
Frequently Asked Questions About Drying Science
admin@4d : ~/faq $ query --id=01 "What is vapor pressure, and why does my restoration company keep mentioning it?" ▶ ENTER
Vapor pressure is the invisible force that actually drives moisture out of your walls, floors, and cabinets. It's the difference between the moisture at the surface of wet materials and the moisture in the surrounding air. Professional restorers increase temperature, decrease humidity, and move air — all to widen that pressure gap. A wider gap means faster, more complete <a href='/services/water-damage/structural-drying/'>drying</a>. If your company can't explain vapor pressure, they're running equipment without understanding why it works.
admin@4d : ~/faq $ query --id=02 "Why do restoration companies measure "grains per pound" instead of just humidity?" ▶ ENTER
Relative humidity changes every time you open a door or the temperature shifts. Grains per pound (GPP) measures the actual weight of moisture in the air — 7,000 grains equals one pound of water. GPP doesn't move just because the AC kicked on. That's why the IICRC S500 standard uses specific humidity for tracking drying progress. If your restoration company is only reporting relative humidity, they're using the less accurate measurement.
admin@4d : ~/faq $ query --id=03 "What does "equilibrium moisture content" mean, and when is my house technically dry?" ▶ ENTER
Equilibrium moisture content (EMC) is the point where your building materials stop absorbing or releasing moisture — they're in balance with the air around them. For indoor wood in Oklahoma, that's typically 8-14%. Your house is technically dry when materials reach EMC for your environment — not when they feel dry to the touch. Surface-dry materials can still hold enough moisture at their core to grow mold. A professional will take pin-type moisture readings at depth, not just on the surface.
admin@4d : ~/faq $ query --id=04 "My insurance adjuster and my restoration company disagree on how long drying should take. Who's right?" ▶ ENTER
Neither one can predict drying time with certainty — because drying time depends on physics, not spreadsheets. Your adjuster uses Xactimate, which estimates based on averages. Your restoration company should be making decisions based on daily moisture readings and psychrometric data. The IICRC S500 standard is clear: drying is complete when materials reach goal moisture levels, not when an arbitrary number of days has passed. If your company can document that materials haven't reached goal, the standard supports extended drying — and your insurance company should cover it.
admin@4d : ~/faq $ query --id=05 "How does Oklahoma's climate affect how long my house takes to dry?" ▶ ENTER
Oklahoma's humidity swings are the wildcard in every drying job. Spring and fall bring sustained high humidity — outdoor air infiltrating your house adds moisture load that the equipment has to fight. Summer heat actually helps (warm air holds more moisture, creating a wider vapor pressure differential). Winter can be the worst — low temperatures make refrigerant dehumidifiers less efficient, sometimes requiring desiccant units instead. And if you're on a slab foundation with Permian red clay underneath, moisture can migrate from the soil up through the concrete. Every Oklahoma drying job has to account for the weather and the ground.
admin@4d : ~/faq $ query --id=06 "What are the warning signs that a restoration company didn't dry my house properly?" ▶ ENTER
Musty smell 2-3 weeks after equipment removal. Bubbling or peeling paint on walls near the water damage area. Flooring that feels soft, spongy, or is showing new cupping. Humidity readings above 60% in affected rooms. And the biggest red flag: if your restoration company never showed you a single moisture reading during the entire job, there's no documentation that drying was ever verified. No data means no accountability. If you're seeing these signs, <a href='/contact/'>call a professional</a> for an independent assessment.
admin@4d : ~/faq $ query --id=07 "Can I use a regular dehumidifier from the store instead of the commercial ones my restoration company brought?" ▶ ENTER
You can run a consumer dehumidifier — it'll pull maybe 30-50 pints per day in ideal conditions. Commercial LGR dehumidifiers pull 150+ pints per day and achieve lower grain depression (removing more moisture from air that's already partially dry). In a real <a href='/services/water-damage/'>water damage</a> scenario, a consumer unit would run for weeks trying to do what a commercial unit does in days. The bigger issue isn't capacity though — it's monitoring. Consumer dehumidifiers don't log data. Commercial units paired with daily psychrometric readings give you documentation that proves the drying was complete.
I'll walk you through the data — whether you hire me or not.
If you're in the middle of a water damage situation and want to understand what's happening in your house — or if you want someone to look at the numbers your current company is giving you — call me. I'll explain what I see.
Have Questions About Your Drying Job?
405-896-9088Phil Sheridan — 4D Restoration
IICRC Certified · Veteran-Owned · Serving Edmond and OKC Since January 2024