Short answer: no outdoor enclosure ships insulated. NEMA Type 3R and Type 4X test rain, snow and sleet against bare metal, and insulation appears in neither rating. Catalog walls are bare 16-gauge steel or bare stainless. You add the insulation yourself, in the shop or on the wall. It pays when condensation sets the heater size. One inch [25 mm] of foam board on the inside drops a stainless wall from 3.7 to about 1.0 W/m²K, a cut of roughly 70 percent in wall heat loss. On the 24 x 20 x 8 in stainless box below, that takes the anti-condensation heater from 150 W to 50 W. The design night in the worked example is -10 °F (-23 °C). Size the heater for the bare wall first, then step it down once the board is fastened in.
What the rating covers, and what it never asks
NEMA 250 defines Type 3R by what stays out: rain, snow and sleet, with a hose-down upgrade to Type 4 and a corrosion upgrade to 4X. The tests all run on the outside of the box. Neither type sets a thermal target, so a bare wall passes, and Type 3R catalogs show 16-gauge steel bodies as the standard build. UL 50E carries the same type tests for a listed enclosure, and nothing in them asks about insulation either. That is why the box in front of you is bare metal and still meets its rating in full.
Our wall-mount boxes are rated NEMA 4X / IP66 by the manufacturer; they are not UL listed, and the walls arrive as bare 304 or 316L stainless. For a US specification, NEC 110.28 says an IP code alone does not replace the enclosure type rating, and Table 110.28 lists which type suits which location. If the open question is rain protection versus washdown, the Type 3R versus Type 4 comparison settles it, and the enclosure types guide has the full ladder. The outdoor enclosures guide covers siting and mounting around that choice.
Why the heater selector asks the question
A panel heater fights one enemy: heat leaving through the wall. The formula is Q = U × A × ΔT — wall conductance times wall area times the inside-outside gap. Bare stainless conducts 3.7 W/m²K, painted steel 5.5 W/m²K and plastic 3.5 W/m²K, the reference values used across this site. Painted steel loses half again more than stainless on the same night in the same box.
Insulated-or-not is the biggest single input in that formula. The board moves wall conductance by a factor of three to four, so a selector cannot quote a wattage until it knows the wall. The heater sizing guide for sealed enclosures walks the remaining inputs, and when an outdoor enclosure needs an anti-condensation heater covers the trigger conditions. For the bare-wall side in more depth, this page on heat-load calculation for bare steel walls goes further.
What one inch of foam board does
Foam board data sheets list about 0.035 W/mK for EPS; extruded polystyrene runs near 0.030 and foil-faced polyiso near 0.023. A 25 mm EPS board alone passes heat at 0.035/0.025 = 1.4 W/m²K. In series with the 3.7 W/m²K stainless wall, the pair gives 1/(1/3.7 + 1/1.4) = 1/(0.270 + 0.714) = 1.0 W/m²K. Per square metre of wall, the bare box loses 3.7 × 27.8 = 103 W on a 50 °F (27.8 K) night; the insulated box loses 28 W. That is a cut of 73 percent.
With XPS the pair lands near 0.9 W/m²K, and with polyiso near 0.7. The heater changes by at most one step between the three boards, so the choice rarely turns on conductivity alone. Remember that insulation is a winter tool: it slows heat in both directions, which the worked example below checks against the summer case.
Sizing the heater for a 24 x 20 x 8 in box at -10 °F
The inputs, and where each comes from:
- Box: 24 x 20 x 8 in outside (H x W x D), the catalog's 304 stainless wall-mount enclosure.
- Wall: bare stainless at 3.7 W/m²K, the reference value used across this site.
- Set point: 40 °F (4 °C) inside, the figure heater guides use for keeping electronics above the dew point.
- Design night: -10 °F (-23 °C) outside, the cold case the heater must hold through. ΔT = 50 °F = 27.8 K.
- Board: 1 in [25 mm] EPS at 0.035 W/mK.
The rule in words: heat loss equals wall conductance times wall area times the temperature gap.
The arithmetic:
- Wall area: 2 × (24 × 20) + 2 × (20 × 8) + 2 × (24 × 8) = 960 + 320 + 384 = 1,664 in², or 1.07 m² (× 0.000645 m²/in²).
- Bare wall: Q = 3.7 W/m²K × 1.07 m² × 27.8 K = 110 W. (3.7 × 1.07 = 3.96 W/K; 3.96 × 27.8 = 110.)
- Insulated wall: the board alone gives 1.4 W/m²K; in series with the wall, 1.0 W/m²K; Q = 1.0 × 1.07 × 27.8 = 30 W.
- Anti-condensation heater catalog ranges step at 30, 50, 75, 100, 150, 200, 250, 300 W, so take the next step up. Bare wall: 110 W of loss → a 150 W heater. Insulated: 30 W → a 50 W heater.
The decision: on this night, 1 in of board turns a 150 W heater job into a 50 W one. Left on for a year, the 100 W saved is 876 kWh (100 W × 8,760 h). Size the wattage for the coldest night of the year, and let a hygrostat shorten the run time on milder ones.
Summer check on the same box, before the foam goes in. A 100 W drive load on a 95 °F (35 °C) afternoon holds a 100/(3.7 × 1.07) = 25 K rise through the bare wall; through the insulated wall the same load drives 100/(1.0 × 1.07) = 93 K. The insulated box traps the drive's own heat, so size the airflow or cooling against the drive's 104 °F (40 °C) ambient limit first. What a fan on a NEMA 4X enclosure costs you explains that trade.
What moves the answer: a milder night of 14 °F (-10 °C) cuts the gap to 26 °F = 14.4 K and the bare-wall loss to 57 W, a 75 W heater. There the board buys nothing worth its space. A larger 36 x 30 x 16 in box has 2.76 m² of wall. On the same -10 °F night it loses 284 W bare (a 300 W heater) and 77 W insulated (a 100 W heater). The working rule: fit the board once the bare-wall heater reaches 100 W or more.
Fastening the board without losing the rating or the space
For a UL 508A listed panel, insulation inside the enclosure must be mechanically fastened, and any adhesive must be evaluated for that purpose; welded studs with spring clips are the usual build. On a manufacturer-rated box like ours, a board bonded over the full wall leaves walls and gasket untouched, so the manufacturer rating stands as delivered. Any hole you drill for a fastener is then a cable entry: seal each one like a gland, as the cutouts and glands guide sets out. The rating-after-modifications article covers what a panel keeps once you start cutting and mounting.
Tape the board seams and the cutout edges with aluminum foil tape. Winter air carries moisture into any gap it finds, and the taped foil layer keeps that damp air off the cold steel; corrosion inside an electrical enclosure: stop the water first traces what happens when it gets there anyway.
Mind the heater clearance. EPS softens near 160 °F (70 °C), and any panel heater passes that at its surface. Mount the heater on its bracket at the clearance its instructions state, with the board out of the radiation path. Lock out the panel and check for absence of voltage before you glue or clip boards around live parts.
Now the space. The board takes 1 in [25 mm] per face it covers. Back and door boards cost 2 in [50 mm] of height, and two side boards cost 2 in of width. The back board alone takes 1 in off the example box's usable depth; the door board takes another. If the DIN rows need that depth, step up to the 24 x 20 x 10 in enclosure, or order the exact depth as a custom enclosure; custom work is delivered in 10 business days from drawing approval. For salt air or chloride washdown, run the same layout in the 316L version of the same box.
What to specify
| Site and load | Wall build | Heater for the 24 x 20 x 8 in box | Insulation |
|---|---|---|---|
| Design night -10 °F (-23 °C), light load | 1 in EPS, seams taped | 50 W on a hygrostat | Yes — the bare wall would need 150 W |
| Same night, drive dissipating 100 W | 1 in EPS plus fan or AC | 50 W on a hygrostat | Yes — after the 93 K summer rise is handled |
| Design night 14 °F (-10 °C), any load | Bare wall | 75 W | No — the board only buys lost space |
| Salt air or chloride washdown | 316L wall, built per the rows above | As the design night dictates | Per the rows above |
Put five things on the order: the box size and grade, the board (1 in foil-faced EPS), the fastening method (welded studs and clips for a listed panel, full-surface bonding with an evaluated adhesive otherwise), the foil tape, and the heater wattage with its hygrostat. State the design night temperature you sized to, so the next person can re-run the sum. The enclosure selector shortlists boxes by environment and inside size, and the wall-mount range covers the stainless sizes this example draws on.
For the relocated drive on a cold site, the spec lands as a 24 x 20 x 8 in stainless box, 1 in of board fastened inside, and a 50 W heater on a hygrostat. Settle the summer airflow against the drive's 104 °F (40 °C) limit before the first heat wave, and the box stays dry in January and cool in July.
Enclosures in this article
| Enclosure | Material | Outside H x W x D / display | Rating |
|---|---|---|---|
| 24 × 20 × 8 in | 304 stainless | 24 x 20 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 24 × 20 × 8 in | 316L stainless | 24 x 20 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 24 × 20 × 10 in | 304 stainless | 24 x 20 x 10 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
Do any outdoor enclosures ship insulated from the factory?
No stock Type 3R or 4X box ships insulated. The rating tests rain, snow and sleet against bare metal, and manufacturer catalogs show bare 16-gauge steel walls as standard construction. Insulation is a shop or field adder, fitted during assembly with studs and clips or with bonded board. Our 304 and 316L wall-mount enclosures arrive bare for the same reason.
How much smaller does the heater get with 1 in of insulation?
On the 24 x 20 x 8 in stainless example held at 40 °F (4 °C) through a -10 °F (-23 °C) night, wall loss falls from 110 W to 30 W and the heater steps from 150 W to 50 W. A hygrostat trims the running hours on mild nights; you still size the wattage for the cold snap.
Does glued foam board spoil the NEMA 4X rating?
A board bonded over the full wall leaves walls and gasket untouched, so the manufacturer rating stands; our boxes are rated by the manufacturer and not UL listed. Every drilled fastener hole must then be sealed like a gland. On a UL 508A listed panel the insulation must instead be mechanically fastened, with any adhesive evaluated for that purpose.
Which foam board suits an enclosure best?
EPS at about 0.035 W/mK takes the 24 x 20 x 8 in wall to 1.0 W/m²K, XPS lands near 0.9 and foil-faced polyiso near 0.7. The heater changes by at most one step between them. Pick on availability, and use the foil face plus taped seams as the vapor barrier that keeps damp air off the steel.
How much interior space does insulation cost?
One inch [25 mm] per fitted face. Back and door boards cost 2 in [50 mm] of height, and the back board alone costs 1 in of mounting depth, leaving about 5.9 in of usable depth. If the DIN rows need more, step up to the 24 x 20 x 10 in model or order the exact depth.
