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Heat Load Calculation for Basement

Stout Member Posts: 2
My daughter is finishing her unfinished basement in Steamboat Springs, CO. The house was built with basically a 9+’ closed, dirt floor crawl space, which they had leveled and a concrete floor poured. Wall studs are up, and electrical is underway. They asked me for help sizing the heating system, so I did a Manual J calculation using the loadcalc.net site. Main floor is 1300 sq.ft., primarily heated via pellet stove and a few fans to circulate the heat. (They have radiant floor heating upstairs with a Weil McLain Ultra 105 boiler fired by LP, but they rarely use it as the cost of propane is much higher than pellets. Upstairs floors are insulated from basement with R-19 batts currently.) Basement walls are 10” ICF with ~2” of EPS on each side, so I figured about R-16, plus they’re adding 1 1/2” between studs, so another R-6; call it R-23 walls. Outside walls are 28’ by 48’. Wall height is 9’, with about 2’ above grade on south & west sides, and 4’ above grade on north & east sides. Three double pane egress windows in wells (one south, two north). Using an outside design temp of -25, I get a heat load of about 7700 BTU - which seems low to me for a 1300 sq.ft. basement. Anyone willing to offer an estimate? Thanks in advance.


  • Jamie Hall
    Jamie Hall Member Posts: 23,649
    edited March 2023
    If I assume that the basement is square, I get a total wall length of 144 feet, more or less. So an area of 1300 square feet for the walls. Now if I take that area with an R value of 23 I get a heat loss of about 56 BTUh per degree F, or only 5600 BTUh through the walls at -25 F. The rest of your heat loss is through those windows, for all practical purposes, and based on these scribbles on the back of the napkin, your figure doesn't look too bad.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • hot_rod
    hot_rod Member Posts: 22,476
    Curious why the framed wall inside the ICF. Typically you attach sheetrock or interior finish to the ICFs?

    I come up with R- 24 for that wall. 7167 for the load, assuming the window size 36 sq ft total

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • pecmsg
    pecmsg Member Posts: 4,961
    Basements have a low heat loss being below a stable earth temperature. 1 issue is the center of the slab. Insulation should have been laid before the slab was poured. Too late. 
    Perhaps elec radiant can be considered.