Friday, December 9, 2022

# Your RV’s R-value. Don’t believe it hook, line and sinker

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The R-value of your RV is kind of a joke.

Oh, I know it sounds impressive. I know the 4-season “Arctic Package” is boasting sky-high numbers. And I know you’re worried about the upcoming October chill.

But what you need to know about RV R-value is that it’s not measured. It’s not tested. There’s no certification standard. It’s a loosely calculated number based on inaccurate assumptions. It’s not an outright lie—just an incomplete truth. I’ll tell you the rest of the story.

Page Contents

### Let’s get through the bad news first

Too often, the conversation around R-value and 4-season RVs and cold-weather camping centers around the materials rather than the design.

Someone dutifully calculates the R-value of all the materials that make up your structure. Then they crunch the numbers, perform some mental gymnastics, and calculate an “R-value” that gets printed on brochures but is often wholly divorced from reality.

We’ve missed the forest for the trees.

It’s a little depressing, I know. I hate to be the bearer of bad news. But I’d rather you hear it from me than discover it for yourself \$40,000 later. Most RVs just aren’t designed for year-round living. And manufacturers that do produce genuine 4-season RVs don’t rely on inflated R-values.

## R-value for dummies

R-value measures insulation, or how well something prevents conductive heat transfer.

Higher R-value means a better insulator (think down comforter). A lower R-value means a worse insulator (think cotton sheet).

R-values for materials are usually given per inch. Here are some examples:

• Concrete: 0.2
• Glass pane: 0.9
• Plywood: 1.1
• Loose-fill fiberglass: 3.1
• Polystyrene foam board: 5.0
• Silica aerogel: 10.3*
• Vacuum panel: 30.0

(*Note that silica aerogel is one of the world’s best insulators and still only has about R-10 per inch. So if an RV manufacturer is claiming R-50 on a 3-inch roof, you know something is suspicious!)

R-values are usually cumulative. So if you combine two inches of polystyrene foam together, you get 5.0 + 5.0 = 10.0 R-value.

The most common insulators in an RV are foam board (R-value of 5/inch), fiberglass (R-value of 3/inch), Azdel (R-value of 2.2/inch), and radiant foil (R-value of 40).*

*Wait—R-value of 40?! Did I miss something?

And that’s where our story begins.

## No, your RV doesn’t have an R-52 roof

As I’ve written about before, the RV industry has some dirty secrets about insulation.

My biggest pet peeve is the outlandish claims made about reflective insulation, which Dave Solberg mentioned in his Q&A about determining the R-value of a travel trailer.

Reflective insulation is often sold as aluminum foil laminated onto bubble wrap. Or just the dimpled aluminum foil itself. Reflective insulation rarely achieves the freakishly high R-values you see advertised. That’s because reflective insulation is a radiant heat barrier that requires a still air gap (the larger the better), and its performance is highly dependent on surface conditions and heat transfer direction.

So that “R-52” roof on your travel-trailer-to-be? It’s almost certainly aggrandized. There just isn’t enough airspace to make it physically possible.

Be skeptical of any R-value greater than 10. Compare the installation procedure to the manufacturer’s requirements, such as these expected R-values from Reflectix.

And another fun fact: The R-value of many materials – and especially radiant foil – isn’t the same from winter to summer! In fact, radiant foil is usually much better at keeping heat out (summer) than trapping heat in (winter). So when you need it most, radiant foil is at its worst!

P.S. Note that if bubble wrap radiant foil is installed without an air gap, the actual R-value is no better than 1!

## Thermal bridging ruins R-value, anyway

You wouldn’t say a bathtub with a crack was waterproof, would you? Well, neither would you say that an “insulated” wall with a giant drafty single-pane window was actually insulated!

You might not – but your RV manufacturer might!

I want to introduce to you a concept called whole-wall R-value. What you see on the sales brochures are not usually empirical measurements. They are based on the laboratory ratings of individual materials, not on the wall as a real-life assembly! They are based on ideal cross-sections, not the entire area.

But if we start considering an RV wall, roof or floor, what do we find?

We find materials that allow heat to stealthily wick through. We engineers call this “thermal bridging.”

For instance, a laminated sidewall probably has a lot of aluminum tubing inside the beadboard foam. This metal tubing has a very, very poor R-value, and it literally acts like a highway overpass, allowing heat to sneak around the foam board!

Or what about the windows, doors, fridge vents, speakers, range hoods, and the other components that carve up your walls and roof like a Jack-O-Lantern? Because when it comes to cold-weather insulation, a window is basically a hole in your wall.

But stated R-values don’t account for windows or other “holes.” They’re calculated based on the “perfect” cross-section of the wall assembly.

So by the time we account for all the thermal bridging, our real-life R-value is … well, it’s awful.

I don’t have concrete numbers for you, because I don’t know any manufacturers who have willingly shot themselves in the foot by actually testing it. I suspect it’s something like 50-80% less than the “sales” value.

So that “R-9” laminated sidewall could have a real-life R-value of about … R-3. Yuck. That’s no better than the typical double-pane window in your house.

## Insulation ignores air drafts

By their nature, RVs don’t hold heat well. This is because they have little thermal mass and lots of air exchange.

Let’s compare an RV to a house.

• In a house, there’s lots of stuff inside to absorb the heat. It’s like a backpack for your extra heat.
• You heat the house through the winter so it never gets really, really cold.
• The bottom of the house is insulated by the ground.
• And if you open a door, relatively little air escapes.

None of that is true when it comes to RV R-value.

• There’s little “stuff” inside to retain the heat.
• You don’t heat the RV while it’s in storage. Unless you’re rich.
• Cold air constantly passes around, above, and below the RV (unless you have skirting).
• And if you open a door, half the air in the RV gets sucked outside!

You see what I’m saying? The physics are not on your side!

If we inspect an RV wall or roof, we will find drafts and air leaks – everywhere! Drafty doors. Single-pane windows. Broken seals. Air leaking out of siding panel seams, fender wells, slide-out seals, entry doors, sliding window panes, outside lights, shower fans, ad nauseum.

You can see evidence of these drafts through a SealTech test, which many RV manufacturers and RV service centers use to diagnose a water leak. Check out the video below for an example of how the SealTech test works (begin at 1:12).

## What’s a cold, shivering RV owner to do?

So, my advice to you is to either take all R-values with a grain of salt or ignore them completely.

• Thick sidewalls, subfloors and roof assemblies (the thicker the better!)
• Limited slide-outs (the fewer, the better)
• Double-pane insulated windows
• Enclosed underbellies
• Heated tanks and knife valves

And when courtier Jack Frost does announce the arrival of Father Winter, do the following:

1. Insulate your windows, skylights and roof fans.
3. Install windproof skirting.
4. Let the sunlight in whenever possible.
5. Buy a down comforter and some fuzzy socks.
6. Thank the Good Lord that propane is cheap.

##RVT1074

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Polly Foster
1 month ago

Check out Outdoors RV made in LaGrande, Oregon. We love ours. Bought it used for a great deal at Apache RV in Portland. We snow camp every year for a week or two at a time. Warm and snuggly.

SLR
1 month ago

Great info. Now I am back to thinking a van build is the way to go because I can better control the insulation situation. It’s always been a pet peeve of mine that people think reflective bubble wrap is all they need.

Mark
1 month ago

I just read an article this morning on treehugger about thermal bridging and it made it clear that the amount of insulating material used is almost completely irrelevant if the structure isn’t airtight, so RV mfr quality control is more important than anything in building a true 4 season rig.

Spike
1 month ago

A few weeks ago we were looking at a top brand 5ver proclaiming it’s 4 season capabilities based on wall, roof, and floor R values. It had giant single pane windows surrounding the LR slide and rear wall. I told my wife that while those would be nice to look out of, they would let tons of heat in when hot and all the warm air out when not. Also they would be dripping wet from condensation at the first frost.

We haven’t had an RV with single pane windows since 1998. Just won’t consider them.

ddobs
1 month ago

great info, prof Ross, thanks!

Larry Lucas
1 month ago

During the hot days of Summer, we’ve found a LOT of radiant heat entering our cabinets that are installed against outside walls. We’ve also found the opposite during cold temps. Mostly though, we want to keep the heat OUT of our RV. We were thinking of applying foam board or something else to the inside of the cabinets against the back walls. Any suggestions on what we should use?

Tommy Molnar
1 month ago

Discovering that our pantry (like yours) was getting hot from being attached to the outside walls, we started leaving the pantry door open to take advantage of our 24/7 ac being on. This helped quite a bit. Our 10 year old TT is probably not as well insulated as a newer model (even though it sports an all season claim) so we’ll probably not try winter camping in below freezing temps if possible. We did wake up one morning years ago in 17 degree coolness (ahem) which we had not planned on. We were boondocking at the time.

Polly Foster
1 month ago