We present the results of long-baseline interferometric observations of the classical Cepheids Polaris and delta Cep in the near infrared K' band (1.9-2.3 mu m), using the FLUOR instrument of the CHARA Array. Following our previous detection of a circumstellar envelope (CSE) around Car (Kervella et al. 2006), we report similar detections around Polaris and delta Cep. Owing to the large data set acquired on Polaris, in both the first and second lobes of visibility function, we have detected the presence of a circum-stellar envelope (CSE), located at 2.4 plus or minus 0.1 stellar radii, accounting for 1.5 plus or minus 0.4% of the stellar flux in the K band. A similar model is applied to the delta Cep data, which shows improved agreement compared to a model without CSE. Finally, we find that the bias in estimating the angular diameter of delta Cep in the framework of the Baade-Wesselink method (Merand et al. 2005b) is of the order of 1% or less in the K band. A complete study of the influence of the CSE is proposed in this context, showing that at the optimum baseline for angular diameter variation detection, the bias is of the order of the formal precision in the determination of the delta Cep pulsation amplitude (1.6%).