SIG owns 50.6% of Ice Energy which specialises in ground and air source heat pumps.
SIG will take an £8m hit on the deal.
The firm said: “This disposal follows challenging market conditions resulting from delays to, and the subsequent slow uptake of, the UK government’s Renewable Heat Incentive (RHI) scheme.”
SIG is selling its stake to Ice Energy’s existing management team.
The Ice Energy team could have to pay an extra £5m dependent on the performance of the company between now and 2019.