More cost-effective to put in than double glazing, as you don’t need to wholly clear away the present models. Timber can rot with moisture and warp after some time, creating gaps which could leak h2o and air. This may defeat the goal of secondary glazing which requires a solid seal. https://andersonsmdvm.link4blogs.com/51663377/the-fact-about-secondary-glazing-that-no-one-is-suggesting