This article explains what exterior and interior grade plywood is, how each is made, and compares the two of them based on several factors.
As the name implies, Interior grade plywood is well-suited for indoor use in homes and offices. So much of the plywood furniture that we use inside our homes is actually made from this grade of plywood.
Exterior grade plywood is a better material and is considered to be suitable for outdoor use such as for making garden tables and chairs that will be kept outdoors, exposed to the natural elements.
Plywood is made from individual plies (slices of wood veneers) that are glued together using resins/adhesives. Individual wood veneers are obtained by rotary-cutting the logs of wood obtained from trees, to get thin slices of the wood. These slices (veneers/plies) are then stacked over one another, a glue is spread over their surface, and all these veneers are then firmly pressed together using a hot-press machine, to make plywood sheets.
In the case of interior grade plywood, Urea Formaldehyde glue (known in the industry as UF) is the most common type of glue used for joining the plies together, while in the case of exterior grade plywood, Phenol formaldehyde (known as phenolic resin) which is a superior quality glue is used.
Exterior grade plywood is stronger (has better glue shear strength) when compared with interior grade plywood.
Differences in Cost:
In the plywood shops in India, exterior grade plywood is also known as BWR (Boiling water resistant) plywood and it costs more than the interior grade plywood (which is known as Commercial ply).
Resistance to water:
The phenolic resin used in exterior plywood is actually a plastic resin. Phenol is the substance from which plastics are made. This resin imparts better water-resistance properties to plywood. Quality tests of exterior plywood, often involve submerging the plywood in boiling water for upto 4 hours, and then taking it out and checking its strength and other qualities.
For all practical purposes these types of plywood are considered adequately water-proof and hence can be used outdoors or are often used in making kitchen cabinets and the like.
UF resin on the other hand is not as good as phenolic resins, and hence interior grade plywood can get damaged if exposed to water. This is the reason why its used indoors (kept away from rain).
The best way to improve on the water resistance of interior plywood is to laminate it using any of the decorative laminates available in the markets such as Sunmica, Formica, GreenLam or any other brands. These laminates are made using papers impregnated with phenolic and melamine resins, and hence have good resistance to water.
A word about Marine ply:
In India, shopkeepers and some others in the industry often refer to exterior grade plywood as 'Marine ply'. This is not fully accurate, since Marine grade plywood is a different and superior grade of plywood (Indian Standards code IS: 710) that is used in marine applications such as for making the woodwork in boats and ships.
The Indian Standards code (IS code) for both Interior and Exterior grade plywoods is IS:303, which you can see clearly marked on all the plywood sheets that have an ISI mark.