Energy retrofits aim to improve the thermal performance of buildings’ external envelopes. With buildings of traditional construction there exists the risk that these improvements may lead to interstitial condensation and moisture accumulation. For historic timber-framed buildings, this potentially exposes the embedded historic timbers to conditions favouring fungal decay and insect infestation. Hygrothermal digital simulations can assess this risk, however these have limitations, especially regarding the study of historic and traditional materials, due to a lack of accurate data. The research presented in this paper therefore utilizes the monitoring of physical test panels to examine the performance of four replacement infill details. The paper focuses on the design and construction of the test cell and presents initial results from the first year of monitoring.