The Case for Knockdown Sealant Application

Everyone knows builders need to improve their window installation methods to avoid water intrusion problems. In fact, builders need a fool proof method of window installation that protects them from losses associated with water intrusion. The problem becomes: what is the “fool proof” method that is easy and cost effective?

First, we need a method that uses the same products we already use; the moment we add new products to the mix we run into the problem of not having the new products during the installation process. Next, we don’t want a process that requires hours of training; some ideas simply cannot be used on today’s jobsite. Splitting the atom is not something to attempt on our jobsite. Last, we need a process that provides a solid window installation PLUS protects the window from subsequent trades that will be working around the window after it has been installed.

Plenty of sealant is the answer.

TLS Laboratories believes that when sealant is applied under the nail fin in a manner that creates squeezeout, we have the start of a great process. Many windows used by today’s builder have prepunched nail holes in the nail fin. Squeezeout must ooze through these prepunched holes. Squeezeout must easily be seen to ooze out around the entire nail fin. Once this oozing sealant is seen, the next step is essential: the sealant must be buttered or knocked down. If this does not happen, then the sealant will harden in the shape it is left in. Many times this shape is a detriment to subsequent layering of building paper and other flashing steps. Have you ever seen the little white worms that squeeze through the prepunched holes in a nail fin? They cannot be left in that shape because they will hold the building paper away from the vertical surface of the wall. Bad, Bad, Bad. (Why is this bad? Because water should be guided “down and out”. Holding the paper away from the wall encourages water to travel “side to side”.)

High quality window flashing products are an important part of window installation. These flashing products must be installed using AAMA method “B” or AAMA method “A”. Additionally, flashing products may be layered on top of the nail fin of the windows. If a product like Moistop Next™ is used, simply press the Moistop Next™ into the wet sealant on top of the nail fin. This will provide a perfect sealant application and a bulletproof flashing condition. The beauty of Moistop Next™ is it has no compatibility problems with any of the sealants used today. Some peel and stick membranes have compatibility problems with sealants on today’s jobsites. If a peel and stick membrane is applied on top of many polyurethanes a liquefication may occur to the adhesive side of the membrane. If a peel and stick membrane is applied on top of a nail fin that has been prepared as we described above, problems may occur due to liquefication of the membrane. Also, wrinkles in the membrane are hard to avoid. Wrinkles are not good. Wrinkles create straws that allow water to travel in directions that often results in trouble. (Remember “Down and Out”.) So the answer is to avoid peel and stick membranes layered on top of nail fins that have a knock down sealant application. Either use plenty of sealant and be done with it, or apply Moistop Next™ on top of the nail fin and press into place.

The final beauty of both of these procedures is they protect the nail fin from damage by the lath contractor. Furnails and staples are notorious for damaging brittle nail fins. When sealant is used as we describe and recommend, furnails are far less likely to create damage to the windows. Either of these applications is fine. If the budget exists to layer Moistop Next™ on top of the nail fin, then, Great, do it.

Gene Summy is the owner of TLS Labs and may be reached via e-mail at

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