Pipe Insulation Thickness for a Target Heat Loss
The pipe insulation thickness that caps radial heat loss at a target rate per foot.
Example
You enter
- Pipe outer diameter (in) 2
- Insulation k (BTU-in/hr-ft2-F) 0.25
- Hot surface temp (°F) 200
- Ambient temp (°F) 70
- Target heat loss (BTU/hr per ft) 40
You get
- Required insulation thickness 0.530 in
- Outer radius over insulation 1.5302
Checked against published worked examples from: Fourier cylindrical-shell conduction + ASTM C335 k-values (solved for thickness).
Details, formula, and sources
Scopethickness = (od/2) x (exp(2*pi*(k/12)*(hot - amb)/q) - 1). A 2 in pipe at 200 F in 70 F air with k 0.25 held to 40 BTU/hr/ft needs about 0.53 in of insulation. Targets a heat-loss (energy-code / process) budget, not a surface temperature, which is a separate limit. Round up to a stocked wall; conduction only (conservative). ASHRAE / ASTM C335 and the manufacturer govern.
Formula or tablethickness = (OD/2) x (exp(2*pi*k'*(T_hot - T_amb)/q) - 1), the inverse of q = 2*pi*k'*(T_hot - T_amb)/ln(r2/r1), where r1 = OD/2, k' = k_value/12 (BTU/(hr.ft.F)), and q is the target heat loss per foot.
Source and editionFourier conduction through a cylindrical shell (public heat-transfer formula), solved for the thickness; insulation k-values per ASHRAE Fundamentals / ASTM C335, by name.
Free accessFree principles in published HVAC texts; k-values user-supplied.
What governsEstimate. AHJ and licensed mechanical contractor govern. ACCA Manual J / D / S supersede rules of thumb.
API field namesod_in, k_value, hot_f, amb_f, target_q_per_ft_btuh, thickness_in, r2_in
- k at mean temperature insulation conductivity rises with temperature; the value entered is at the mean insulation temperatureASTM C335
- Conduction only ignores the outer-surface air film, so the required thickness is slightly conservativescope of this tile