Explain muntins speaks to what are muntins, why use muntins, types of muntins.
The fact that in earlier times, because of the difficulty of manufacturing large lights of glass that were free of significant defects, windows were necessarily divided into smaller lights or panes of glass which were known as muntins.
These are then wooden bars in which the glass was mounted within each sash.
A typical double hung window had its upper sash and lower sash each divided into six lights and wad referred to as a six over six. Muntin arrangements changed with changing architectural style and improvements in glass manufacturing. Todays windows, glazed with large, virtually flawless lights of float glass, need no muntins at all, but many building owners and designers prefer the look of a window utilizing muntins.
This desire for the use of muntins is greatly complicated by the necessity of using double or triple insulated glazing to meet building code energy conservation requirements. These building code energy conservation requirements are quite specific and detailed as to what must be incorporated into the window design requirements.
Some manufacturers offer the option of individual small lights of double glazing held in deep muntins. This is relatively expensive and the muntins tend to look thick, heavy and very cumbersome.
The least expensive option of those offered utilize grids of imitation muntin bars, made of wood or plastic, that are clipped into each sash against the interior surface of the glass. These are designed to be removed easily for washing the glass.
Other alternatives are imitation muntin grids between the sheets of glass, which in reality are not very convincing replicas of the real thing. It should be noted that grids, either removable or permanent type, are bonded to the glass on both the outside and inside faces of the window.
Another option is to use a prime window with authentic divided lights of single glazing and to increase its thermal performance with a storm sash. Of all the options, this one looks best from the inside, but reflections in the storm sash largely obscure the muntins from outside.