DIY Builds
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How much do you overlap vapor barrier?

6 inches Seal all vapor barrier seams with the tape provided by the barrier manufacturer. Seams should overlap 6 inches.

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Vapor Barrier Installation Tips for installing vapor barriers under a concrete slab to prevent vapor transmission

W. R. MEADOWS

Form stakes sealed to the barrier prevent vapor intrusion. VaporStake in Chino Hills, CA

Install boots around all penetrations and blockouts. Fortifiber in Reno, NV

To be effective, a vapor barrier must be installed in a way that prevents any water vapor from getting to the slab.

Find out where to buy vapor barriers and other problem solving products.

Most exterior slabs do not need a vapor barrier. If you are going to seal an exterior slab, find a sealer that transmits water vapor (breathes). For some tips on this, see Sullivan's Corner. In general, place the concrete slab directly on top of the vapor barrier, with the subbase below. If the subbase is sharp angular gravel, a thin layer of sand can be placed on top of the gravel subbase next to the vapor barrier. See below for more discussion on this. The vapor barrier can be placed under the subbase, if the building is closed in when the dry subbase material is placed.

Never punch holes in the vapor barrier to allow bleedwater to escape.

Do not pound form stakes through the vapor barrier. Some supports come with wide bearing pads. Another solution is the new VaporStake that seals to the vapor barrier and is then cut off and left in the slab. Seal all vapor barrier seams with the tape provided by the barrier manufacturer. Seams should overlap 6 inches. Seal around all pipe penetrations and blockouts; Howard Kanare's book (available from the Portland Cement Association) provides more details. Run the vapor barrier up onto the footing or seal it to the foundation wall—or both. Protect the vapor barrier as much as possible during construction and repair any damaged spots with the manufacturer's tape. Where to position the vapor barrier has been argued about for years. For a time in the 1980s and 1990s, ACI recommended using a "blotter" layer, on top of the vapor barrier. In 2001, the recommendation changed to typically placing the concrete directly on the vapor barrier. The argument is that putting the concrete directly on top of the vapor barrier prevents water from leaving through the bottom, which leads to longer bleed times and to slab curling. While this is true, we've shown in other parts of this article that the subbase will end up damp and a continuous source of water vapor into the slab. Today, most slabs experts recommend placing the slab directly on the vapor barrier unless the subbase is protected and can be assured of staying dry. Curling can be controlled with well-graded aggregate and a little more reinforcement in the lower part of the slab.

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Are there carcinogens in Styrofoam?

In the case of polystyrene, tiny amounts of styrene may remain following manufacture and it's this substance that may migrate. In 2014, the National Research Council in the US reviewed the evidence and concluded that styrene is “reasonably anticipated to be a human carcinogen”.

An employee complained that offering tea and coffee in polystyrene cups is dangerous as hot liquid in polystyrene cups causes cancer. Is this true?" All food packaging, whether it’s plastic, paper, glass or otherwise, contains substances which can “migrate” in very tiny amounts to foods or beverages. In the case of polystyrene, tiny amounts of styrene may remain following manufacture and it’s this substance that may migrate. In 2014, the National Research Council in the US reviewed the evidence and concluded that styrene is “reasonably anticipated to be a human carcinogen”. However there is more to consider. Firstly, the evidence was limited because alternative explanations in these studies, such as chance, bias or confounding factors, could not be excluded. Secondly, the Plastics Foodservice Packaging Group found that exposures to styrene from the use of polystyrene food contact products is extremely low – with the estimated daily intake calculated at 6.6 micrograms per person per day. This is more than 10,000 times below the safety limit set by US Food and Drug Administration. Regulatory bodies around the world, such as the US Food and Drug Administration and The European Commission/European Food Safety Authority, who undertake rigorous studies to evaluate a product’s safety, have not changed their view that polystyrene is safe for foodservice packaging. When it comes to reducing our risk of developing cancer, not smoking, minimising exposure to UV, reducing alcohol intake, being physically active, maintaining a healthy body weight and eating healthy foods are the best things that we can do.

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