Your RV’s R-value is a joke.
Oh I know it sounds impressive. I know the 4-season “Arctic Pack” has sky-high numbers. And I know you’re worried about the upcoming October cold.
But what you need to know about RV-R is that it’s not measured. It is untested. There is no certification standard. It’s a loosely calculated number based on imprecise assumptions. It’s not an outright lie – just an incomplete truth. I’ll tell you the rest of the story.
Let’s go through the bad news first
Too often the conversation revolves around R-value and 4-season RVs and cold-weather camping the materials instead of the design.
Someone is dutifully calculating the R-value of all the materials that make up your structure. Then they crunch the numbers, do head exercises and calculate an “R-value” that is printed on brochures but is often completely detached from reality.
We missed the forest for the trees.
It’s a little depressing, I know. I hate being the bearer of bad news. But I’d rather you hear it from me than discover it for yourself $40,000 later. Most RVs are just not designed for year-round use. And manufacturers who make true 4-season RVs don’t rely on inflated R-values.
R value for dummies
R-value measures insulation, or how well something prevents conduction of heat.
A higher R-value means a better insulator (think down comforter). A lower R-value means a worse insulator (think cotton sheets).
R-values for materials are usually given per inch. Here are some examples:
- Concrete: 0.2
- Glass panel: 0.9
- Plywood: 1.1
- Loose fill fiberglass: 3.1
- Polystyrene foam board: 5.0
- Silica Airgel: 10.3*
- Vacuum panel: 30.0
(*Note that silica airgel is one of the best insulators in the world and it’s still only about R-10 per inch. So if an RV manufacturer claims R-50 on a 3-inch roof, you know something is suspicious is!)
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 radiation foil (R-value of 40) .*
*Wait – R-value of 40?! Did I miss something?
And here our story begins.
No, your motorhome does not have an R-52 roof
As I’ve written before, the RV industry has quite a few dirty secrets about insulation.
My biggest annoyance is the outlandish claims about reflective insulation, which Dave Solberg mentioned in his Q&A on determining a caravan’s R-value.
Reflective insulation is often sold as aluminum foil laminated to bubble wrap. Or just the knobbed aluminum foil itself. Reflective insulation rarely achieves the insanely high R-values you see in advertisements. That’s because reflective insulation is a radiant heat barrier that requires a static air gap (the larger the better), and its performance is highly dependent on surface conditions and the direction of heat transfer.
So the “R-52” roof on your future travel trailer? It’s almost certainly elevated. There just isn’t enough airspace to make it physically possible.
Be wary of R values greater than 10. Compare the installation procedure with the manufacturer’s requirements, e.g these expected R values from Reflectix.
And another fun fact: The R value of many materials – and especially Glossy foil – is not the same from winter to summer! In fact, radiant foil is usually much better at keeping heat out (summer) than it is at trapping heat (winter). So when you need it most, radiant foil is the worst!
PS Note that when bubble wrap is installed with no air gap, the actual R-value is no better than 1!
Thermal bridges ruin the R-value anyway
You wouldn’t say a bathtub with a crack is waterproof, would you? Well, you wouldn’t say that an “insulated” wall with a huge, drafty, single-pane window was actually insulated!
You might not – but your RV manufacturer might!
I want to introduce you to a concept called full-wall R-value. What you see on the sales brochures are usually not empirical measurements. They are based on the laboratory values of individual materials, not on the wall as a real assembly! They are based on ideal cross-sections, not the entire area.
But when we start considering an RV wall, roof or floor, what do we find?
We find materials that secretly let heat penetrate. We engineers call this “thermal bridges”.
For example, a laminated sidewall will likely have many aluminum tubes within the beadboard foam. This metal tube has a very, very bad R-value and literally acts like a freeway overpass, allowing heat to sneak around the foam sheet!
Or what about the windows, doors, refrigerator vents, speakers, range hoods, and the other components that slice through your walls and roof like a jack-o’-lantern? Because when it comes to cold insulation, a window is basically a hole in your wall.
But R-values given do not account for windows or other “holes”. They are calculated based on the “perfect” cross-section of the wall mount.
So if we have considered all thermal bridges, our real R-value is… well, it’s terrible.
I don’t have concrete numbers for you, because I don’t know of any manufacturer who voluntarily shot himself in the foot by actually testing it. I suspect it’s around 50-80% less than “retail value”.
So this “R-9” laminated sidewall could have a real-world R-value of about…R-3. yuck This is no better than the typical double pane window in your home.
The insulation ignores drafts
By nature, RVs do not retain heat well. Because they have little thermal mass and a lot of air exchange.
Let’s compare an RV to a house.
- In a house there are many things inside to absorb the heat. It’s like a backpack for your extra warmth.
- They heat the house all winter so it never gets really, really cold.
- The bottom of the house is insulated by the floor.
- And if you open a door, relatively little air escapes.
None of that is true when it comes to RV-R value.
- There is little “stuff” inside to retain heat.
- They do not heat the RV while it is in storage. Unless you are rich.
- Cold air constantly flows around, above and below the RV (unless you have a baseboard).
- And when you open a door, half of the air in the camper is sucked outside!
do you see what i’m saying Physics is not on your side!
When we inspect an RV wall or roof, we find drafts and air leaks – everywhere! Draft doors. single pane window. Broken Seals. Air escaping from side panel seams, fender pans, pull-out seals, entry doors, sliding window panes, exterior lights, shower fans, to the point of vomiting.
You can see evidence of these designs by a SealTech test, which many RV manufacturers and RV service centers use to diagnose a water leak. Watch the video below for an example of how the SealTech test works (starting at 1:12).
What’s a cold, shivering RV owner supposed to do?
So my advice to you is either use all R values with caution or ignore them completely.
Instead look for:
- Thick side walls, underbody and roof structures (the thicker the better!)
- Limited slide outs (the fewer the better)
- Double insulated windows
- closed abdomens
- Heated tanks and knife valves
And when the courtier Jack Frost announces the arrival of Father Winter, do the following:
- Insulate your windows, skylights and roof fans.
- Heat up your tanks, wrap up your pipes and warm up your batteries.
- Install a windproof baseboard.
- Let the sunlight in whenever possible.
- Buy a down comforter and some fluffy socks.
- Thank god propane is cheap.