Moving an Imperial Envelope Calculation into SI
Building-envelope work is one of the last places where two unit systems sit on the same drawing. A wall assembly is specified as R-13, the manufacturer's data sheet quotes W/m·K, the energy model wants megajoules and the mechanical schedule still reads in BTU. Every one of those crossings runs through a single number: the joule equivalent of a British thermal unit. Get that step right and the rest of the imperial-to-SI chain — R to RSI, U-value to W/m²·K, seasonal heat loss to GJ — falls into place.
The Imperial Thermal Quantities You Will Meet
R-value and RSI Describe One Property
U-value Crosses at 5.678 as Well
Conductivity Uses a Different Number
Energy Totals Stay Plain BTU
Converting a Heat-Loss Result You Have Already Calculated
Finish the imperial arithmetic first, then convert once at the end. Switching systems mid-calculation is how rounding errors quietly enter an envelope report.
Work out the imperial heat quantity
Divide area × temperature difference × hours by the assembly R-value. A 100 ft² R-13 wall across a 40 °F difference for 24 hours gives 96 000 ÷ 13 = 7 385 BTU.
Enter that total on the BTU side
The joule figure keeps pace with each keystroke. A decimal comma works as well as a decimal point, and spaces inside the number are ignored, so a value pasted out of a takeoff spreadsheet drops in unchanged.
Reverse it when the source data is metric
The swap arrows turn the pair around to run joules into BTU — handy when a European product declaration hands you megajoules and the plan reviewer wants BTU. Each side can also be re-pointed through its searchable unit list, so kJ, MJ, GJ and kWh are one selection away.
Lift the bare figure into your model
The copy control above each field takes the number alone, with no unit label and no thousands spacing, which is what a spreadsheet cell or a simulation input box expects. Ctrl+C inside a field does the same thing.
Insulation Assemblies: R-value, RSI and the Heat They Let Through
Every row below uses the same 100 ft² (9.29 m²) area held across a 40 °F (22.2 K) difference for 24 hours, so the assembly is the only variable. The BTU column is the raw imperial result; the megajoule column is that same heat after the 1 055.06 step.
| Assembly | R-value (h·ft²·°F/BTU) | RSI (m²·K/W) | Heat lost (BTU) | Heat lost (MJ) |
|---|---|---|---|---|
| Single-pane glazing | 1 | 0.18 | 96 000 | 101.29 |
| 25 mm XPS rigid board | 5 | 0.88 | 19 200 | 20.26 |
| 2×4 stud bay, glass batt | 13 | 2.29 | 7 385 | 7.79 |
| 2×6 bay with sheathing | 20 | 3.52 | 4 800 | 5.06 |
| Blown attic, mild climate | 30 | 5.28 | 3 200 | 3.38 |
| Blown attic, cold climate | 49 | 8.63 | 1 959 | 2.07 |
| Deep-energy roof build-up | 60 | 10.57 | 1 600 | 1.69 |
Two things are worth carrying into a report. Doubling the resistance halves the heat, and the megajoule column falls in exactly the same proportion because the conversion is linear. And the RSI numbers look small — RSI-2.29 reads as thin next to R-13 — which is precisely why a metric reviewer sometimes queries an imperial specification that is in fact entirely ordinary.
What This Pairing Gives an Envelope Calculation
Both Thermal Directions in One Field Pair
Type into either box and the other keeps pace, so an imperial audit figure and a metric product declaration can be reconciled without reopening the calculation behind them.
Bare Numbers for a Heat-Loss Spreadsheet
Copied results carry no unit text and no thousands spacing, so a joule total drops straight into the cell that feeds your envelope summary.
Every Energy Unit an Envelope Report Uses
The searchable lists on both sides cover kJ, MJ, GJ, kWh and therms alongside BTU and joules, so one page handles the whole spread of an energy model's outputs.
Seasonal Totals Without Losing Digits
Annual envelope losses run into tens of gigajoules. Results carry up to eight decimals and shift into scientific notation at extreme magnitudes rather than silently truncating.
Envelope Questions About R-values, RSI and BTU
Why is R-13 insulation labelled RSI-2.3 in Canada?
Because it is the same batt with the units rewritten. Dividing 13 by 5.678 gives 2.29, which Canadian labelling rounds to RSI-2.3. That 5.678 is itself assembled from the BTU-to-joule factor together with the foot-to-metre and Fahrenheit-to-kelvin steps, so nothing about the product has changed — only the arithmetic wrapped around it.
Can I type an R-value straight into the BTU field?
No, and this is the trap that catches people most often. R-value units are h·ft²·°F/BTU, a compound ratio in which the BTU appears on the bottom. Feeding 13 into an energy field converts a quantity of heat, not a resistance, and the answer means nothing. Cross resistance and transmittance with 5.678, conductivity with 0.1442, and reserve this page for the finished energy total.
An audit says the house loses 30,000 BTU — what is a whole heating season in megajoules?
Taken literally, 30 000 BTU is 31.65 MJ, but in an envelope report that figure is nearly always a design heat-loss rate per hour. Divide it by the design temperature difference — say 70 °F — for a UA of roughly 429 BTU/h·°F. Multiply by 24 hours and a 5 000 heating-degree-day season and the total comes to about 51.4 million BTU, which is 54 260 MJ, or 54.3 GJ, across the season.
How do I tell whether a report means BTU or BTU per hour?
Look at what the number is attached to. Design loads, equipment sizing and peak-load tables are rates and carry an implied per-hour even when the sheet lazily prints BTU. Anything tied to a period — a month, a season, a billing interval — is a quantity. Only a quantity converts to joules; a rate converts to watts, where 1 BTU/h is 0.293 W. Confusing the two shifts a result by a factor of 3 600.
Which BTU definition sits behind building-code R-values?
Codes and the ASTM insulation test methods behind them work from the International Table BTU, close to 1 055.06 J, which is the value this page uses and the value baked into the familiar 5.678 factor. Older thermochemical figures near 1 054.35 J differ by under 0.07 per cent — far below the tolerance on any real insulation product, and invisible beside the effect of framing factor or installation quality on the assembly you are reporting.
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