Understanding LP Siding

The 1990’s was not our finest hour of residential construction. In many respects, this decade represents a transition decade as we went from the way we used to build houses to the way we
 build houses today and as often happens when innovative technologies are unveiled into the building industry, the laws of unintended consequences prevail and some new products and inventions do not live up to their expectations.

Louisiana Pacific’s Inner Seal siding, commonly referred to as “LP” fits this bill. Inner Seal siding exploded onto the market in the early 1990’s and at least in the Western Washington area, many of t
he developments from this era were sided with LP. The siding is made from chips of wood glued together to make a building material commonly called OSB – or oriented strand board. OSB is widely used today as a substitute for plywood. If you watch modern houses being constructed, nearly all of the wall sheathing, sub-floor and roof decking is done using OSB. Louisiana-Pacific used this OSB material and formed it into siding panels and lap siding with an embossed covering meant to look like the natural grain in real wood.

LP siding information from Dylan Chalk Hose Tlak

Modern houses are all sheathed in OSB

LP was not the only player. The 1990’s saw other “hardboard sidings,” or sidings made from a composite of wood fibers emerge onto the market. Weyerhaeuser had their product, so did Masonite. Technically, the other products could be accurately labeled as a hardboard siding, whereas LP’s Inner Seal is an OSB siding product. Technicalities aside, the whole wood products industry was racing headlong into the composite wood siding business and nearly every one of these 1990’s era wood siding products has had problems with premature failure and product defect litigation.

This shows classic failure of the Weyerhaeuser product – the siding is known to buckle.

LP’s Inner Seal siding was the subject of a class action lawsuit. The settlement for that lawsuit has been paid, so if you are reading this, you have missed the opportunity to file a claim.  The problem with LP’s Inner Seal, generally, is that it fails where exposed to the weather. Inner seal was sold as both lap siding and in panel-like 4’x8’ sheets with the T1-11 profile. The most vulnerable profile is the lap siding, where water can cling to the drip edge and wick into the bottom edge of the siding as shown in the photo below. Once water starts to penetrate the drip edge, the siding saturates, swells and delaminates. Where Inner Seal has been exposed to the weather for years, I would expect to see the siding in some stage of failure.

This shows water beading up on the drip edge of the lap siding. You can see the siding starting to crack and swell along the drip edge – first stages of failure.

LP Inner Seal siding can be identified by the classic LP knot that is embossed into the siding – see attached photo. The timing is a little fuzzy, but to my understanding, LP stopped making Inner Seal siding around 1996, and they started making a similar and improved product called SmartSide starting around 2000. You cannot distinguish LP’s Inner Seal product from the newer SmartSide product once it is installed….at least I can’t. Unfortunately, the knots are the same and the siding looks the same. The only way I can make a guess at distinguishing the two sidings is the age in which they were installed.  Roughly 2000’s and newer should be SmartSide where anything from the 1990’s should be Inner Seal. This leaves late 1990’s in something of a gray area.

Classic LP knot

Fungus growing on the drip edge of LP siding

No siding system last forever, but I do not consider siding one of the disposable systems on a house like a furnace or a water heater. This makes the need for siding replacement a large and unanticipated expense. By contrast, there are a lot of 100-year-old houses that still employ the original siding system. In many ways, LP reflects a new approach to construction: our stocks of quality old growth lumber were starting to disappear and we went with a siding system that has a life span more akin to a composition roof than our old siding systems which could last the better part of a century.

Some LP installations have difficulty lasting even 20 years. The useful life of the siding depends a great deal on the amount of exposure and the quality of the installation. The siding will perform adequately when it is protected by generous roof overhangs and well-installed, but will fail where exposed to the weather and where sloppy installation leaves the product vulnerable.  The panel siding tends to do better than the lap siding because there is no drip edge for water to cling to.

One way to prolong the useful life of the siding is to keep it well-maintained. All penetrations, such as over-driven fasteners and site cut edges of siding need to be sealed with caulk and paint. A product commonly used to seal the drip edge is a paint primer called Permizer Plus; this can slow the delamination of the bottom drip edge. Lack of maintenance, exposure, and poor installation are common and major contributing factors to premature failure of LP siding.

I hope this blog helps shed some light on a common home inspection and house problem from the 1990’s.

This photo shows some localized failure in the panel siding at the bottom edge where it meets a head flashing. See how the siding is starting to swell here.


Classic swelling and delamination from LP siding