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I'm currently interviewing contractors for exterior basement waterproofing. The difference in price is not significant, but there is one major difference.

The contractors will:

  • Trench down below the footings,
  • Parge the foundation
  • Apply AquaBlock Rubberized adhesive sealant.
  • Contractor A will apply an 11 mil plastic membrane here.
  • Apply Dimple board.
  • Weeping tile replacement etc.

Generally I am of the opinion that more stuff means a better job, but I need an opinion as to the 11 mil plastic. Is it 

 a) Harmful, 
 b) irrelevant, or
 c) beneficial?
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More stuff doesn't always mean better. Sometimes it means more expensive, or sometimes it means overcompensating for other weaknesses in the structure. Or sometimes it means old contractors who just don't want to give up their stubborn ways.

In your case, the waterproofing should be the parging layer. That should be some form of rubberized sealer that forms a complete barrier. The dimple board is there to protect that layer from backfill and to provide a drainage plane to the french drain.

Provided those are all done properly, I can't see the plastic adding any value to the equation. Getting a foundation wrapped watertight with plastic sheets doesn't seem viable either from an installation POV.

So, my vote is that it's unnecessary.

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  • The contractor who is applying the plastic has a 25 year warranty, and is 2000 cheaper than the one who is not, and is only offering a 20 year warranty. The question then becomes "Is there any reason to avoid Contractor A, who appears to be doing more for less". (Both contractors come highly recommended from many different sources.)
    – Chris Cudmore
    Commented Sep 26, 2011 at 15:30
  • I wish I could say there was a direct correlation between cost and quality of work, but in my experience that isn't the case. That said, if it seems implausibly cheap, do be wary. In your situation, I'd find out exactly what product both contractors are using to parge the exterior. If one of them says something like 'tar' then maybe be wary.
    – DA01
    Commented Sep 26, 2011 at 16:08
  • Both are using a product called Aquablock. On is coming in at 15k, the other at 17. So neither one sounds cheap.
    – Chris Cudmore
    Commented Sep 26, 2011 at 18:48

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