A cold room inside a warm house runs backwards
Every rule of thumb about vapor retarders in American residential construction is written for a heated house in a cold climate. Warm wet air inside, cold dry air outside, moisture driven from the room toward the sheathing, so the retarder goes toward the room and the assembly dries outward.
A wine cellar in a basement reverses every term. The warm, wetter air is in the house. The cold surface is on the inside of the cellar. Vapor is driven from the house inward, toward the cellar, and it will condense at the first surface it meets that is below the house dew point. If the house is at 72 degrees and 50% humidity, that dew point is around 52 degrees — which is very close to a typical cellar setpoint, and comfortably above the temperature of anything on the cold side of the insulation.
So the surface that needs protecting is on the outside of the insulation, facing the house, and a retarder placed on the cellar side in the usual way can trap moisture in the assembly instead of keeping it out. Which side is right in your case depends on your climate, on whether the room runs cold all year, and on what the rest of the wall is made of. This calculator counts the material. It deliberately does not tell you which face to put it on, because that is a design decision with a rot failure behind it and it belongs to a designer and to your building department.
Seam feet, not just square feet
The area of sheeting is the easy number and it is not the one that decides whether the assembly works. What decides it is the seams, and a small room has a lot of them relative to its area.
| Item | 10 x 8 x 8 ft room, walls and ceiling |
|---|---|
| Net insulated area | 347 sq ft |
| Sheeting including a 6 in lap | 365 sq ft |
| Vertical lap seams on the walls | 4 seams, 32 ft |
| Ceiling lap seams | 1 seam, 10 ft |
| Perimeter joins, wall to ceiling and wall to floor | 72 ft |
| Total seam to tape | 114 ft |
Note where the length is. The lap seams between strips are a third of it; the perimeter joins where one plane meets another are the rest. Those corners are also the hardest to detail, the most likely to be skipped, and the ones that get covered by furring five minutes later. A roll of tape covers 180 feet, so a room this size needs one roll and a habit of using all of it.
Furring is a moisture decision, not just an R-value one
Strapping at 16 inches on centre with 1.5 inch members puts 9.4% of the wall area on a wood path. In an ordinary wall, the assembly R-value calculator shows what that costs in whole-wall R and the answer is a few points.
In a cold room the same geometry is a line of colder-than-average surface running horizontally across the warm face of the assembly, every 16 inches, for the whole height of the room. If the house-side dew point is above that surface temperature, that is where water forms — in stripes, inside a wall, where nobody looks. This is the same arithmetic the wall condensation organizer runs on an assembly layer by layer, and it is why cellars are often built with the board adhered and no furring at all, or with the furring outside the retarder rather than through it.
Doors and glass beat everything else
A glazed cellar door looks like a small part of the wall and behaves like a hole. Twenty-one square feet at around R-1 has the same heat flow as 420 square feet of R-20 wall, which is very nearly the entire shell of this room over again. The calculator flags it when the openings pass 15% of the gross area, and the number to take away is that no amount of extra board on the walls compensates for it. The wine cellar holdover calculator shows the same thing as a UA split, and usually the door line is larger than the shell line.
What this page is not
It is a take-off. It computes no R-value, names no required R-value, no retarder class, no fire rating and no ventilation figure. Every one of those is set by the code your jurisdiction has adopted and by the official who reviews the job, and they differ between states, cities and code editions. Rigid foam in particular carries ignition and thermal barrier requirements that vary by product and by location, and a small sealed room with an appliance in it raises questions this calculator has no opinion on. Take the product data sheets and your drawing to the building department first.
Related
For the R the assembly actually reaches once the framing is counted, the assembly R-value calculator. For where the temperatures land through the layers, the wall condensation organizer. For batts and blown insulation rather than board, the insulation calculator; for the same sheeting problem in a crawl space, the crawl space vapor barrier calculator. Once the shell exists, the holdover calculator turns it into a UA and a time constant, and the wine rack capacity calculator fills it.
Questions people ask
Which side does the vapor retarder go on in a wine cellar?
This page will not answer that, and any page that gives you a one-line answer is guessing about your climate. The mechanism is what matters: a cold room inside a heated house is driven inward, opposite to an ordinary exterior wall, so the surface at risk is on the house side of the insulation rather than the room side. Whether that means a retarder on the warm face, a vapor-impermeable board acting as its own retarder, or something else depends on your climate zone, on whether the room is cold year round and on the rest of the assembly. It is a design decision with a rot failure behind it: take it to a designer and to your building department.
How many sheets of rigid foam for a small cellar?
For a 10 by 8 by 8 room with walls and ceiling insulated, one door taken out, a single layer and a 10% waste allowance, the net area is 347 square feet and the material with waste is 382, which is 12 sheets of 4 by 8. Two layers doubles it to 24. The waste allowance is the field people set too low — a small room is nearly all edges, and cutting around a door, a duct penetration and a cooling unit sleeve eats more than ten percent in practice.
How much seam tape will I need?
More than the sheeting area suggests, because the perimeter joins are usually longer than the lap seams. For the same 10 by 8 by 8 room with a 10 foot wide retarder and a 6 inch lap, there are about 42 feet of lap seam and 72 feet of perimeter join, so 114 feet in total. That is one 180 foot roll. If the room has a lot of penetrations, buy two, because the second roll is cheap and running out halfway around a corner is how a seam gets skipped.
Should I use furring strips or adhere the board directly?
The calculator shows the cost of furring in area terms: 1.5 inch members at 16 inches on centre put 9.4% of the wall on a wood path. In an ordinary wall that is an R-value question. In a cold room it is also a condensation question, because each strip is a cooler line running across the warm face of the assembly. That is why plenty of cellars are built with board adhered and the finish fixed some other way. What is acceptable in your build depends on the finish you need to hang and on the code your jurisdiction has adopted.
Does this calculate the R-value of what I am building?
No, deliberately. This is a bill of materials. The R-value the finished assembly reaches, once the furring and the framing are counted by parallel path, is what the assembly R-value calculator does, and it is a genuinely different number from the sum of the labels. What R-value your room is required to reach is a third question again, and it is answered by your adopted energy code and the official reviewing the job, not by any calculator.