W Ratings for Buildings – The What, Where and How of Water Ratings

Posted on 11/27/2018 12:13:20 PM By Sharron Halpert

We have been talking about the four different ratings from firestop F rating for fire, T rating for temperature, L rating for air leakage and today we will talk about the W ratings for water. We will go over the following:

- where W ratings come from
- how they are tested
- what they can and can’t accomplish.

As buildings get more air tight, the moisture inside buildings create  more opportunities for the growth of mold. Water intrusion can happen during construction, before the exterior is enclosed on upper floors but drywall may be installed on lower floors. Inclement weather can cause issues during this stage of construction. Another problem can come when pipes are tested.  If pipe connections fail,or sprinkler heads are hit accidently this can cause water damage and can create concern related to mold.  I was on a project where a pipe in an unoccupied penthouse unit failed and no one knew there was an issue until the ceiling in the unit below collapsed. Water moved easily from the penthouse to the ground level lobby through openings in the floor. Investigation after the incident identified that water leaked through the small cracks in the firestop sealant. These cracks could be at the edge of the concrete floor, or at the edge of the pipe. Especially if the pipe was subject to any movement after the firestop material had cured.

Well before this incident, Insurance companies began looking at ways to reduce the risk of mold issues and water damage. They identified firestop as one critical element that can play a role in mitigating the movement of water in any case of water intrusion during construction or after completion.

This led to the creation of the W rating. Defined by UL below.

“The Class 1 W rating determines the capability of the firestop system to maintain water tightness of the penetration through a floor or wall construction at ambient air conditions under 3 ft of water pressure head (1.3 psi) for a period of 72 hours. The W rating may be applicable for building structures whose floors are subjected to incidental standing water and/or for buildings that house critical equipment as described in ANSI/NFPA 75, "Fire Protection of Information Technology Equipment," and ANSI/NFPA 76, "Fire Protection of Telecommunications Facilities."

The test works a little like this. They install the firestop and allow it time to cure, then they put a three-foot column of water over the firestop installations for 72 hours and blotter paper underneath the installation. If the blotter paper shows no water came through, then the installation achieves a W rating provided the assembly passes the ASTM E814 fire test.

This is not intended to show that firestop installations can be regularly or permanently submerged in water, but rather that if there is a water issue on a project, that the firestop can play a role in protecting the floors below from water damage. 

Architects sometimes specify W rated firestop on projects without understanding that insulated pipes or cable bundles are not capable of offering a W rating without procedures to seal them that are not likely justifiable. For example, fiberglass insulation is not going to be able to prevent water movement for 72 hours. AB/PVC (black foam rubber) insulation probably can, but not if it is cut to accommodate support brackets required by the pipes that are being insulated.

So, while the idea of an entire floor that is able to reduce water movement sounds good, it is not practical in most situations because there are likely going to be shaft openings through the floor. Different occupancy types are going to have more shafts, such as hotels, apartments or condos. Also, not all firestop penetrations are capable of achieving a W rating. A few examples would be cable bundles, insulated pipes or ducts. The interstitial space between the cables or the wound wire jacket on a metal clad cable cannot be expected to pass the test described above.  Pipes through floors are required to have support brackets (as shown in the photo below) to displace the weight of vertical installations.

                   (click to enlarge)

The insulation is often cut to accommodate the support bracket, and this would be an opportunity for water to get between the pipe and the insulation. While this type of penetration CAN be firestopped and achieve a W rating, the cost impact needs to be considered before writing a specification with a blanket statement of “W rating required for all floor penetrations.”

The other concern is that not all firestop materials have been tested for a W-rating, so the installer needs to be aware of which materials can and can-not achieve a W rating.

Come back for our next blog post when we wrap this all up and share with you an unexpected story of when W rated firestop installations performed above and beyond expectations.


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