Take care of the insulation materials as homogeneous, Whilst in serious programs, rigid materials have joints and in some cases cracks. Thus, the U-value is often better than what It might be When the insulation behaved to be a homogeneous substance. Reference 2 gives guidance on calculating U-values. Timber is https://beaugyqdp.frewwebs.com/35324326/the-fact-about-secondary-window-that-no-one-is-suggesting