Heat Retention in Clothes & Mattresses

16 min read · Flare Bed Bug Blog

One of the most common questions we get during a heat treatment is some version of "is the heat actually getting inside my mattress?" It's a fair question. You can feel the room is hot. The thermometer on the wall says 135°F. But what about the middle of the box spring? What about the pile of clothes in the laundry basket? What about that sweater folded up in the bottom dresser drawer?

This is the part of bed bug heat treatment that doesn't get talked about enough. The temperature of the air in a room and the temperature inside your stuff are two completely different things, and the gap between them is where treatments succeed or fail. We've watched this play out hundreds of times on our beacon data, and the patterns are pretty consistent.

So we wanted to put together a real explanation of what's happening, why some items heat up in minutes while others take hours, and what that means for actually killing every bed bug in your home. There's a fair amount of physics involved, but we'll keep it grounded.

The Numbers That Actually Matter

Before getting into the materials science, it's worth being clear about what temperatures we're trying to hit and where. The most directly applicable lab work is from Kells and Goblirsch at the University of Minnesota. They reported a Lethal Temperature 99 (the temperature at which 99 percent of bugs die given enough exposure) of 118.9°F for adult bed bugs and 130.6°F for eggs. They also measured time to death at sublethal temperatures. At 113°F, adults reached 99 percent mortality after about 95 minutes. Eggs were tougher. They survived 7 hours at 113°F and only died within 72 minutes when the temperature climbed to 118°F [1].

Earlier work by Pereira and colleagues at the University of Florida had found a similar shape. Adult mortality started at around 105°F given 100 minutes of exposure and increased steeply with temperature, reaching complete kill in just 1 minute at 120°F [2]. The two studies use different exposure protocols, but the takeaway is the same.

There are essentially two different temperature thresholds depending on what you're trying to kill. To handle eggs reliably, you really want core temperatures above 122°F, and most professional treatments target air temperatures of 130 to 140°F to drive the cold spots into that zone. Notice the word "core." That's the whole game. Air at 135°F means nothing if the middle of your mattress is sitting at 95°F.

Why Air Temperature Lies to You

If you stick a thermometer in the middle of a room during a heat treatment, you'll see numbers that look great. 130°F. 138°F. The display reads like the bugs should already be dead. But bed bugs aren't sitting in the open air. They're tucked into mattress seams, hiding inside box springs, jammed into dresser drawers, hiding in the back of a sock balled up at the bottom of a laundry basket. Those are the temperatures that have to hit lethal levels, not the air.

The reason there's such a big gap between air temperature and core temperature comes down to two material properties: thermal conductivity and heat capacity. Both of them work against you when you're trying to kill bugs hiding inside soft, fluffy, layered stuff. Which is, of course, exactly the stuff bed bugs hide in.

Thermal Conductivity, in Plain English

Thermal conductivity is just a measure of how easily heat passes through a material. A copper pan has very high thermal conductivity, which is why the handle gets hot fast even when only the bottom is on the burner. A cotton oven mitt has very low thermal conductivity, which is why you can hold the same hot pan and not burn your hand.

The unit pros use is watts per meter per kelvin (W/m·K). The lower the number, the better the insulator. Air, surprisingly, is one of the best insulators we have access to in everyday life, with a thermal conductivity of about 0.026 W/m·K when it's still. That's the secret behind almost every fluffy thing in your house. The fibers themselves don't insulate that well, but the trapped air pockets between them do.

For comparison, drywall is about four times more conductive than cotton fabric. Steel is roughly a thousand times more conductive. That's why we don't have to worry about whether bugs hiding behind a metal headboard are getting hot enough. The metal heats fast and heats evenly. We worry about the items where conductivity is low, mass is high, and the inside is shielded by layers of low conductivity material.

Heat Capacity: How Much Energy It Takes

If thermal conductivity is about how fast heat moves through a material, heat capacity is about how much energy it takes to warm a given mass up. Specific heat capacity is usually expressed in joules per gram per degree Celsius. Cotton sits at about 1.3 J/g°C [3], wool at roughly 1.36, polyester around 1.03. Polyurethane foam, the stuff most modern mattresses are made of, is much higher at roughly 2.4 to 3.0 J/g°C [4].

For comparison, water is 4.18 J/g°C, which is one reason waterbeds are infamous in our industry for ruining heat treatments. They soak up enormous amounts of energy without their temperature changing much. A waterbed that hasn't been drained can act as a heat sink for the entire treatment, pulling energy out of the room and never warming up enough to kill the bugs that are hiding underneath it. We've seen this go badly enough that companies refuse to start a treatment until a waterbed is fully drained.

The practical implication for a more typical home: a 30 pound load of cotton clothing in a laundry basket needs around 9,800 joules of energy to warm by one degree Fahrenheit. That's not a huge number on its own, but to get that pile from a starting 75°F to a kill temperature of 125°F, you need to deliver close to half a million joules of energy, every joule of which has to flow through the outer layer of fabric, then into the next layer, then the next. The bugs that have crawled into the middle of that pile have effectively built themselves a fortress of trapped air pockets and slow heat transfer.

Putting the Two Together

Thermal conductivity tells you how fast heat moves through. Heat capacity tells you how much heat needs to move. Multiply density into both and you get the actual rate at which an object's interior catches up to the surrounding air. The bigger and denser the item, and the lower its conductivity, the longer the lag.

This is why a t-shirt hanging open on a closet hanger is in the kill zone almost immediately. Low mass, high surface area, air can flow on both sides. The same t-shirt balled up in the corner of a tightly packed drawer with twelve other shirts on top of it might take hours to reach the same temperature. The shirt didn't change. Its situation did.

What Rise Rate Actually Looks Like

One of the few solid published numbers on real world heat treatment rise rates comes from Kells and Goblirsch's whole room monitoring. They observed temperature increases ranging from 0.06°C per minute at the slowest spots to 0.2°C per minute at the fastest, depending on insulation, distance from the heat source, and airflow [1]. Translated to Fahrenheit, that's roughly 6.5°F per hour at the worst spots and about 22°F per hour at the best spots in the same room, during the same treatment.

Think about what that means in practice. If a treatment starts with the room at 75°F and the cold spot inside a mattress core is rising at 6.5°F per hour, it'll take that spot almost eight hours to climb to 125°F. And that's just to arrive at lethal temperature. The exposure clock for actually killing eggs doesn't start until you get there. This is the math behind why professional whole room treatments typically run six to eight hours, not one or two.

How Deep Does the Heat Get?

Once you have thermal conductivity, density, and heat capacity all measured for a given material, you can calculate something called thermal diffusivity. It tells you how fast a temperature change at the surface propagates into the interior. The math is dense, but the punchline isn't: time scales as the square of depth. Double the depth, and the time it takes for the inside to warm up doesn't double. It quadruples. Triple the depth and the time multiplies by nine. This is why a single t-shirt heats through in minutes while the center of a thick pile of clothes can take a full work day.

Here's a calculator that does the math. Pick a material, set a thickness, and choose whether heat is reaching the item from one side or both. The result is the time it takes for the worst point inside the item to climb from 75°F up to 120°F when the heated surface is held at a steady 135°F. The four materials cover the most common cold spots in a real bedroom: a loose laundry basket of clothing, a tightly packed dresser drawer, the foam in a typical mattress, and solid wood furniture like a dresser or bedframe.

A few real world thicknesses to anchor those numbers. A single t-shirt or thin towel is well under a quarter inch thick: heat reaches the inside in minutes. A folded sweater or bath towel is one to two inches at the thickest part. A standard bed pillow is four to six inches uncompressed. An average mattress is eight to twelve inches thick, and a box spring adds another five to nine inches under it.

That last one is worth sitting with. A 10 inch mattress lying flat on a box spring has its center 5 inches from the nearest air on top, with the entire box spring acting as another insulator below. Even loose foam at 5 inches takes more than 50 hours to reach kill temperature on a one sided heat path, which is to say it doesn't get there during a treatment at all. That isn't a hypothetical. It's why pros who skip the step of standing the mattress on edge end up with treatment failures right where bed bugs live.

The numbers above are a worst case scenario where heat is only entering from one side and nothing is moved. That's exactly why we don't just heat the room and walk away. Standing a mattress on edge means heat now enters from both faces. The worst point goes from being five or six inches deep to being half the mattress thickness. Halving the depth quarters the time, because time scales as depth squared. That single change turns a hopeless cold spot into something a six hour treatment can actually penetrate.

Same idea on a smaller scale for clothing. Pulling a drawer open exposes both the front and the top of the contents to hot air. Spreading a heap of laundry out across a chair instead of leaving it heaped in a basket cuts the worst depth from maybe four inches to under one. The math that makes the static case look hopeless is the same math that makes treatment work, once you stop letting items stay static.

One Big Caveat: Most Items Don't Need to Be Heated All the Way Through

The calculator above shows how long until the geometric center of an item reaches kill temperature. For a lot of items, that's the wrong question. Bed bugs can't tunnel into solid wood. They can't burrow into the inside of a sealed mattress. They can't drill through the cover of an intact pillow. They live in cracks, seams, fabric folds, and the air spaces between fibers. So the real question for most items isn't "how long until the absolute middle hits 120°F" but "how long until every place a bug could actually be hiding hits 120°F."

For a sealed mattress in good condition, the relevant depth is the seams, the cover surface, and a fraction of an inch into any small tear. For a folded sweater, it's the deepest fold, maybe an inch or two from the surface. For a wooden dresser, it's the joints, the gaps behind drawer slides, and the tiny channels under the hardware, all of which are at or very near the surface and heat through fast. The dramatic times in the calculator at four or six inches of depth are real numbers, but they only matter if a bed bug can actually be at four or six inches of depth.

Box springs are the big exception. A typical box spring has an open frame: a wooden border, a grid of wooden slats or wire, and a fabric dust cover stapled to the bottom. The whole interior is fair game for bugs, and they very often pile up on the underside of the dust cover, in the corners of the frame, and along the staple lines. Heating only the outside of a box spring leaves a large hollow interior that's still close to room temperature for hours after the surfaces are hot. This is why box springs get pulled off the bed, stood on their side, and treated as their own discrete object. We also place a sensor inside the frame, because the dust cover side is reliably one of the slowest spots in any bedroom we treat.

The Pile of Clothes Scenario

One of the most frustrating spots for any heat treatment is a tightly packed dresser drawer or a heaping laundry basket of dirty clothes. Bed bugs love this kind of habitat. It's warm, it has plenty of folds and seams to hide in, and it's almost never disturbed. From a thermal standpoint, it's also one of the slowest things in the room to heat up.

Here's why. The outer layer of the pile sees the hot room air directly. It heats up reasonably fast. But the layer right under it is now insulated from the room by the first layer. The fabric in the middle of the pile is insulated by everything above it, below it, and on every side. Each additional layer adds resistance, and because cotton itself has such low conductivity, those layers don't conduct heat well from one to the next. The center of the pile becomes its own little climate, lagging the air temperature by a wide margin.

This is why the prep instructions every reputable bed bug company sends out tell you to do two specific things with clothes: spread them out, and don't bag them. We want clothes loose in open weave laundry baskets if they're staying in the room. We want drawers opened so air can move through them. If clothes are in plastic bags, we have a near perfect insulator wrapping a near perfect insulator, and the middle of that bag will sit at room temperature for hours while the air around it bakes at 135°F.

During the treatment itself, our techs walk through and physically move things around. We pull stacks of clothes apart. We rotate items. We flip cushions. The Virginia Cooperative Extension write up on heat treatments specifically recommends this practice, noting that suddenly exposing previously sheltered hiding spots once the room is at temperature gives bugs no chance to escape and dramatically improves the kill rate [5].

The Mattress Sandwiched on a Box Spring

The other classic cold spot, and probably the most important one in any treatment, is the gap between an unmoved mattress and box spring. Bed bugs absolutely love this seam. The mattress sits on top of the box spring with their fabric covers facing each other, the weight of the mattress compresses any gap to almost nothing, and the layers above and below trap heat on both sides.

The most striking real world demonstration of this came from Naylor and Boase, who tested whether wrapping mattresses in black plastic and leaving them in the sun would kill bed bugs inside. On a summer day with ambient temperatures peaking at 97.7°F, the upper sun exposed surface of a thick mattress hit 185°F. The bottom side of that same mattress never exceeded 95°F [6]. One mattress. Roughly a 90 degree gradient from one side to the other. The interior was a thermal sanctuary even with one face baking in direct sunlight.

Now imagine that same mattress sitting on a box spring inside a heated room. The room air is at 135°F. The bottom of the mattress is in contact with the top of the box spring. The contact area is enormous, and the airflow through that interface is essentially zero. Heat trying to reach the seam between them has to travel through the mattress from above, or through the box spring from below, or sneak in around the edges. All three paths fight low conductivity and high mass.

This is why every heat treatment we run includes physically standing the mattress and box spring on edge for at least part of the treatment. We need that interface to see hot, moving air. We also place sensors directly inside the seam between them, because that's one of the spots most likely to lag the rest of the room. The sensor data tells us when we can stop the treatment, not the wall thermostat.

Heat Shock and Why We Don't Just Crank It

An obvious question at this point is: if all this is so slow, why not just blast the room to 160 or 170°F and shorten the lag? Industrial direct fired propane heaters can absolutely do that. The reason we don't comes down to a different physical phenomenon, which is thermal stress, sometimes called heat shock.

When a material's outer surface heats up much faster than its interior, the outside expands while the inside doesn't, yet. That mismatch creates internal stress. In ductile materials, the stress just dissipates. In brittle materials and laminates, it can cause cracks, warping, delamination, or finishes to lift. Picture pouring boiling water into a cold drinking glass: the inside surface tries to expand, the outside hasn't caught up yet, the glass shatters. That's thermal shock at the extreme end.

In a bed bug treatment, you don't see anything that dramatic, but you can see real damage if heating is too aggressive or the ramp rate is too steep. The published industry guidance is fairly consistent. Highly heat sensitive items can be damaged below 120°F: candles, crayons, lipstick, chocolate, most medications. Vinyl records, audio cassettes, and vinyl blinds tend to warp around 140°F. Furniture finishes, glued joints in antiques, the laminates on cheap pressed wood furniture, and laminate flooring all have their own thresholds, and they aren't always published. A piano can survive heat treatment but really doesn't appreciate having its top board hit 150°F in twenty minutes.

This is why thoughtful technicians ramp the room up in stages instead of going from 75°F to 140°F as fast as the heaters will run. The Thermal Flow guidance, which is widely used in the industry, recommends moving up in 10 degree increments once the space passes 100°F, giving sensitive materials time to acclimate [7]. It costs a little extra time. It also costs a lot less than replacing a customer's hardwood floor or warped kitchen cabinets.

The other reason for a measured ramp is the bugs themselves, although the picture there is less clear cut. Bed bugs don't develop heritable resistance to heat the way they do to chemicals, and one study by Benoit and colleagues found no enhanced survival from rapid heat hardening. Kells and Goblirsch noted that some bed bugs in their experiments survived to slightly higher temperatures than earlier studies predicted, which they suggested might reflect physiological changes during slower heating, but the evidence for that effect being decisive is mixed [1]. The bigger constraint on ramp rate is your property, not the bugs. Most professionals aim for moderate rises of roughly 10 to 20°F per hour at the cold spots once the room is past the danger zone for sensitive items, which is fast enough to be efficient and slow enough that fragile materials have time to adjust.

What All This Means for a Real Treatment

Putting it together, this is what's actually happening during a heat job:

The first hour or so is mostly about getting the air in the room up to temperature. The walls, floor, and large furniture pieces start absorbing heat. The smaller, thinner items, single shirts hanging in a closet, sheets, papers on a desk, are already in the kill zone before we even hit the hold temperature.

The next several hours are about driving heat into the slow stuff. Cushions in a couch. The center of mattresses. The interior of dresser drawers. Stacks of books. Anything with mass and insulation working together. Our temperature beacons sit in those exact spots, and we watch them climb. Some get to 122°F in under an hour. Some take four or five hours to get there even with the room held at 135°F the whole time.

Once every sensor is reading lethal temperature, the exposure clock starts. We hold the air temperature steady so those cold spots stay above the 122°F threshold long enough to kill eggs, which are the toughest life stage. Then we start the cool down, again gradually, so we don't crack a finish or pop a glue joint by going from 130°F to room temperature too quickly.

From the customer's side, this looks like sitting in a coffee shop for most of the day while we run the treatment. From our side, it looks like a constant dance of sensor checks, fan repositioning, and walking through the space to physically agitate piles, flip cushions, and stand things on edge. None of that is theatrical. Every single one of those moves is responding to a specific cold spot the physics already told us about.

Common Questions

The Bottom Line

Heat treatment works because it's the one approach that can reach bugs hiding inside materials, not just on their surface. Chemicals can't get into the middle of a mattress. Steam only treats what you can directly contact. Heat, given enough time, gets everywhere.

But "given enough time" is doing a lot of work in that sentence. The physics is unforgiving. Insulating materials slow heat transfer. Dense piles stack up that effect. The interface between a mattress and a box spring is one of the slowest spots in any room. Anyone telling you their treatment is fast and easy probably isn't paying attention to those cold spots, and the bugs in those spots will be alive when the heaters shut off.

The work is in the prep and the patience. Spread things out so air can move. Open drawers, cabinets, and closets. Don't seal anything in plastic during treatment. Stand mattresses and box springs on edge. Run the room hot enough to kill eggs, hold it long enough that core temperatures actually get there, and verify with sensors instead of guessing. Do all that, and the same physics that makes treatment slow is the same physics that makes treatment final. Heat doesn't leave residue, doesn't get sprayed in the wrong places, and bed bugs cannot evolve resistance to it. Once a part of your home has been at 122°F for ninety minutes, every life stage hiding in that part of your home is dead.