Van Insulation - the Math Behind Staying Warm (or Cool)

A lot of factors go into staying warm.  My purpose is just to explain the math behind insulating a van so that the inside air temperature is comfortably warm on cold day.  Most of the math also applies to staying cool. We want to understand:

1. Keys to construct a well-insulated space, and what is achievable
2. How much heat is required to heat an insulated van
3. How to provide that heat

Keys to construct a well-insulated space, and what is achievable

In conceptual terms the answer is quite simple.  Have as many well-insulated surfaces as possible, and as few poorly insulated surfaces as possible.  So what do we mean by a well-insulated surface?  Let's pretend that a van is a rectangular box with a heated space with dimensions of 6' x 6' x 12'.  This would be an enclosed space of 432 cubic feet, with a surface area of 360 square feet.  The 360 square feet is the critical number because heat crosses a boundary measured in square feet.  So the idea is to have as much as possible of that 360 square feet well insulated.

The most important formula for us to keep in mind is the relationship between insulation value and energy needed to keep the inside warmer than the outside.  Here is the engineering formula in concept:

[Heating load] = [surface area] x [temp. difference] / [insulation value]

You can easily see that increasing the insulation value will reduce heating load.  Double the insulation value and you cut the heating load in half.  In Imperial units, heating load is measured in BTU/hr, and insulation value in R-value.  If we add the all measurement units this looks a little more complex, but it's still the same formula.

[BTU/hr] = [Surface Area (sq. ft)] x [Temp Diff. (deg F)] / [R-value (ft2·°F·hr/BTU)]

So, if we want to reduce the heating load, we want to cover as much of the square footage as possible with good insulators.  In quantitative terms, this means materials with a high R-value.  R-value has a convenient characteristic.  When we make a composite material out of two materials we can just add their R-values together to get a composite R-value.  So a material with an R-value of 1 combined with a material with an R-value of 2 has an R-value of 3.

Here are the R-values per inch of  some materials typically found in a van build:

R-value per inch by Material	R-value per inch
Rigid foam - Polyiso	6.00
Industrial felt (used for a thermal break)	4.00
Closed cell foam - Ensolite	4.00
Wool - Havelock Wool	3.60
Synthetic Fiber - Thinsulate	3.30
Double-paned glass	1.60
Plywood	1.25
Single-paned glass	0.91
Steel	0.03

These are all quoted "per inch."  Thinsulate is typically installed at a thickness of about 1.5", so you can get possibly get about R-5 from it.  Don't always believe the optimistic ratings from manufacturers though--that's what they achieved in the lab, not in a real build where some of the insulation is compressed.  Compressed insulation will deliver less insulation value, but a van is so oddly shaped that you will end up cramming insulation into oddly shaped cavities as often as you are installing it in beautiful flat sheets.  Plywood paneling is typically installed at 1/4 inch, or a bit less, so it only typically contributes about R-0.3.

A well-insulated wall or ceiling could be as high as R-10 in places, but will typically be all the way down to just better than R-1 where plywood has been attached directly a steel framing member.  Windows are difficult to insulate, as are the doors.  Putting up close fitting thermal curtains and using thermal window coverings can cut heat loss at least in half for windows and doors, so that's a common approach to staying warmer at night.

So, in addition to using good insulation wherever possible, three practices can also make a big difference: use a thermal break of at least R-1 when attaching anything to the van's metal framework; don't leave any steel elements fully exposed to the heated space; and, put coverings over any poorly-insulated areas at night.

I'm not going to get into the physics of this, but there is a bonus R-value caused by trapping air.  If you can trap 1/2" to 4" of air in part of the build, you get about R-1 from the trapped air if it's static, so some of the van structure that is hard to insulate at least has some inherent insulation value.  Also there is a boundary effect of still air against the interior of the van.  It gives the effect of about R = 0.7 on the inside of the van if you aren't running a circulating fan.  The outside also gets some value from this boundary layer if the air is still, but you can't really count on that.

Based on the materials involved and real-world experience, even with your best efforts, it's very unusual to achieve an overall performance of better than R-5 for the heated space, and R-3 is probably much more typical.  Although we are a bit light on empirical measurements, it's probably fair to describe van insulation performance in five categories:

R-1:  just factory headline insulation and air pockets in a cargo van - aka uninsulated

R-2:  lightly insulated

R-3:  well insulated, but exposed windows and doors

R-5:  very-well insulated with treatments for all surfaces

R-7:  super-insulated van

So, with these insulation values in mind ...

How much heat is required to heat an insulated van?

Given our assumption of 360 square feet for the surface area of the heated space, we can calculate the heating loads for various outside to inside temperatures.  To make the table easier to read, let's assume a target of 65 deg F. for the inside temperature, and then we can use the outside temperature as part of the table.

BTU/hr by Outside Temp. to maintain 65 deg. F. internal temp., by insulation level
	Outside Temperature (deg. F.)
BTU/hr by Insulation Level	55	45	35	25	15	5	-5
R1: uninsulated	3,600	7,200	10,800	14,400	18,000	21,600	25,200
R2: lightly insulated	1,800	3,600	5,400	7,200	9,000	10,800	12,600
R3: well insulated	1,200	2,400	3,600	4,800	6,000	7,200	8,400
R5: very-well insulated	720	1,440	2,160	2,880	3,600	4,320	5,040
R7: super insulated	514	1,029	1,543	2,057	2,571	3,086	3,600

So, to be comfortable at an outside temperature of 35 deg. F., with a van insulated to R3, you will need about 3,600 BTU/hr of heat.  And at an outside temperature of 15 deg. F. you would need 6,000 BTU per hour.  

How to provide that heat

There are a lot of ways to bring heat into a van, so I'll just mention three popular options with their capacities.  If you are somewhere you can plug-in to an electrical outlet you can use a typical 1,500 watt electric heater to provide heat.  The conversion to BTU's is Watts x 3.41 = BTU's, so 1,500 watts will heat at  5,115 BTU/hr, which would keep you warm down to below 25 deg. F. in an R-3 insulated van.  That might be the right answer when parked at home or if you happen to be staying at a campground with hookups.  Espar and Webasto fuel heaters are popular; they both have 7,000 BTU/hr models and some larger models also.  Propex offers a propane heater that puts out 6,350 BTU/hr and they also have a larger model.  This is far from an exhaustive list.

This post hasn't covered the whole topic, but it's a start at understanding the capacity of the heating system required, and some of the issues related to insulating a van.  It's also important to not get too starry-eyed by the R-value insulation rating of some insulating material, and keep in mind that a van is very hard to insulate to a high R-value due to the windows, doors, cab area, and the uneven shapes.

Conclusion

Take insulating your van seriously and you can get to R-3 without extraordinary effort.  Beyond that level you will be working harder at it than most DIY builders.  You can check the table above to see what size heater you will need at a minimum.  I think the models that put out 6,000 to 7,000 BTU/hr will satisfy most people, but if you live in a really cold climate it makes sense to get the larger model.  

Olallie Lake to Timberline Lodge - 8/2/2022 to 8/4/2022

 

Mount Hood 

Christine drove Lori and me to the trailhead at Olallie Lake, finishing the last few miles on a gravel road.  Olallie Lake was the northern limit of the fire closure order when we hiked this section.

After hiking through a burn area we were in healthy forest for most of the hike.  There were a few picturesque spots leading up to Mount Hood, which really is the star of this section. 

Our mileages we're about 12, 17, 15, and 6.  The last day we made it to Timberline Lodge at 9:30 am and enjoyed their famous breakfast buffet.  Lori ate an equivalent of approximately three meals by piling her plate high.

After eating we got Lori's resupply box and then said our goodbyes.  My arthritic knee had become stiff, painful, and maybe a little unstable, so I went back to Portland while Lori hiked on 

The trip back to Portland consisted of two buses, one light rail trip and a walk of less than a mile to our friends house.  The trip cost $4.25 total, which seemed incredibly cheap to me, and it was also pretty quick.

Not too many photos in the album this time, but a few good shots of Mount Hood.

Olallie Lake to Timberline Lodge 2022

Santiam Pass to Pamelia Lake, 7/27 - 7/29/2022

Fire transforms any landscape, but this section brought home to me how resilient nature can be.  When I climbed up out of Santiam Pass in 2018 significant sections were freshly burned, each step brought up a cloud of ash, and the undergrowth was completely gone in the freshly burned areas.  Just four years later the dead trees still dominate many areas, but the ash is gone and the blackened earth has been turned green again.  Flowers were blooming everywhere, and the landscape looked fully alive again.

Christine dropped us off at Santiam Pass.  Friends Lori, Georgia and I climbed up out of the pass and were quickly above 6,000 feet.  We enjoyed spectacular views, but with visibility slightly altered by light smoke.   Three Finger Jack was dramatic and more interesting to me than the higher and more dignified Mount Jefferson.  The flora was beautiful, lush, and abundant.

The hiking was moderately challenging.  The only tricky bits were a short snow traverse and one log stream crossing.  There were a lot of blow downs, many with root balls intact.  Georgia and Lori swam at both Rockpile Lake and Pamelia Lake while I practiced looking lazy.  

We came off the PCT at Pamelia Lake due to an active trail closure order covering a 20 mile section of trail leading up to Olallie Lake.

My knee was pretty painful on day three, so I am hoping that two days off will help.  My heavily used tent finally gave up on the third day, and I had great luck finding a replacement at a local REI today.

If all goes as planned, Lori and I will hike from Olallie Lake to Cascade Locks following a weekend break during which Portland is suffering 100 degree weather.

Here's the album for this section.

https://photos.app.goo.gl/2cMRxBQHfMnWPjHS6

 

Three Finger Jack, just N. of Santiam Pass in Oregon

My last big hike was in 2019, with the following years interrupted by Covid, of course, and also by the passing of three close family members.  It wasn't until a few month ago that I started to seriously plan to get back on trail.  

As soon as I started training, a flareup of osteoarthritis in the spring of 2022 put me out of commission for about six weeks until I was able to get a shot of cortisone in my right knee in the first half of June.  After some careful training I'm now looking forward to attempting Santiam Pass to Cascade Locks in Oregon, Donner Pass to Sierra City in California, and Tuolumne Meadows to Carter Pass in California.  If all goes well this will complete a section hike of the PCT that I technically started in 2016, mostly liked in 2017, and then did the Sierra section of in 2019.  So, if I get these sections hiked this year it will complete my second hike of the PCT.  No guarantees of course that my knees will hold up, or that we won't have fire closures in those sections.

How is having a goal like completing a PCT hike meaningful or useful?  Even an arbitrary goal engages our natural goal-seeking impulse.  You could say that an arbitrary goal is a kind of mental hack to engage one's motivational energy, which then leads to activity.  

I would also be happy doing almost any other hike.  I'm looking forward to being on trail however arbitrary the reason.  ^-^

Van Conversion Floor Layout and Initial Design (draft) Decisions

Emerson Drafting Table - CC 2.0 by Tri Lox on Flickr

I'm still trying to source a van, so still just at a design level, and welcome your feedback. I have a floor plan, but it doesn't include everything, so a few comments below will hopefully answer your questions.

The most critical design drivers that aren't common with other camping designs are to be easily driveable (minimize backing up), and to have air conditioning when not on shore power. I know that AC on battery is a very difficult requirement; however, based on my calculations, it's reasonable as long as you can plug in at least once a day (or use a generator) and are obsessive about insulation.

The lettering will be easier to read if you open the image in another window.

I did this drawing in Google Slides at full scale: 1" = 1" and printed it out at the local Fedex office. This has already given me some initial design adjustment ideas. This printout is 36" x 140" on each side and cost $62 to print out -- well worth it from my perspective. The small type size on the drawing is because Google Slides has a maximum type size of 96 points.

Power sources:
Solar: 990 watts of solar, completely covering the roof area. This forms an attic area that should reduce the AC load.
Alternator: 12V x 50 amps (600W) Battery to Battery charger
Shore power: 20 amp circuit with 15 amps typical expected overnight as needed. This is adequate to full-charge the batteries from empty in 6 hours at 1,500 watts.
Generator: Honda EU2200i for when shore power isn't available.

Batteries:
3x SimpliPhi 3.8kwh 24v -- 11,400 Wh nominal, 9,120 Wh useable @80% discharge

Air Conditioner: Midea U-Shaped, 8,000 BTU, inverter soft start, maximum draw 768 watts, 15 EER. Side vented and ventilated (and yes I know this is a major pain to build in)

Ventilation: Continuous 25 CFM, on timer 100 CFM, on temperature switch 200 CFM.

Heater: Propex HS2000 propane heater (the math just doesn't work on electric heat).

Insulation: Blended insulation value equivalent to at least R-5. Probably will use one inch of Polyiso wherever it can go and lots of Great Stuff. Ventilated box with layer of interior hard-surface closures for sliding door, back door, and cab area. Extra effort to insulate against nuisance sounds (maybe Ensolite layer on large panels along with mass loading the sheet metal).

Sleeping: Three configurations at a daily choice option: two 28" x 78" bunks long way // 52" x 73" sideways // 73" x 78" long way (but this takes more time to set up). In the bunk and mega king setups your feet go under the kitchen counter on one side, and under the air conditioner on the other side. 15" clearance above the mattress level.

Garage: Approximately 20 inches tall x 14 inches deep. This holds 2 propane tanks, one small gas can, and a generator. The top of the garage becomes a shelf that serves as an end-table for the beds. When the back doors open up, and my insulation partition is folded back, there is an unobstructed view out the back.

Kitchen: 30 inch dedicated counter with sink. 52 inch auxilary counter that gets put away at night. Expecting only electric appliances except for a propane grill to use outside.

Water: 16 gallons dedicated + 2 x 2.5 gallon jugs for 20 gallons total. The jugs are so the water can be refreshed from a situation where we just have water available, but no hose connection. Some campgrounds are like this. Grey water goes to a jug that is emptied manually (probably 3 gallons). No water heater in this design -- small amounts of water can be heated in the electric kettle for dish washing and sponge-bath bathing.

Shower: I will provide a water outlet out the back so one of those instant propane heaters could be used for an outdoor shower. However, I don't expect that to be a common scenario for us.

Toilet: We think a dry toilet (wag bags) will be adequate for us. We do have the option of using a portable cassette toilet or some other variations. There will be a cabinet underneath the refrigerator to hold it.

That's about all I have worked out so far. There are an amazing number of little decisions to make as well. I'm hoping to buy a new Promaster in February with the factory swivel seats and 220amp alternator, there just aren't any available nearby right now. I look forward to your feedback and will post additional progress.