I am having a house built, and after the subcontractors put in the drywall and left for the weekend, they left all the doors and windows open. We then had terrible storms and the house was soaked. I'm worried about the potential of mold/mildew in the drywall, insulation, and OSB. The contractor said he looked at it and "it's good. No replacements needed." Are my concerns valid?
TJ, your concerns are very valid - at least about the possible damage to your insulation and sheetrock. Any reputable homebuilder knows that once sheetrock is in a home being constructed, care needs to be taken that the structure is weatherproofed. While windows and doors may be opened on a nice day to help the drywall compound cure, at the first hint of rain everything needs to be closed up. That includes ensuring every house with sheetrock, even if it's just stocked for hanging, is closed up every evening before the construction personnel leave the job-site.
All that being said, accidents and oversights do happen as construction sites can be hectic at times. Even the best construction managers and superintendents occasionally forget to close a few windows and doors. However, when that happens and sections of sheetrock and insulation are subjected to a drenching rain such as you describe, they should know that the material needs to be replaced.
If the builder refuses to replace the sheetrock and insulation that got wet with new material, insist on being allowed to have a mold remediation company inspect the house. The builder should pay for the inspection and you should be furnished with a certified report that shows what they found. If the report indicates that everything is fine, then you shouldn't have a problem with mold or mildew in the future. However, any findings that show existing mold or the possibility of a future occurrence should cause the builder to take action.
This is still a buyers' real estate market, so if it comes to a threat of legal action or the chance of you breaking your contract, I can't imagine the builder taking a stand - even if it means having to replace half the sheetrock and insulation in the house.
I wouldn't be as concerned about possible damage to the OSB floor sheathing. Most OSB floor and roof sheathing used these days is designed to hold up to the moisture often encountered as a house is being framed. Depending on the size of the house, it could be two weeks or more from the time framing begins until it's weathered-in with roof felt and windows and doors. During that time, the OSB can be subjected to a few rain storms.
As the construction on your house progresses, take some time to walk across the OSB floor sheathing whenever possible. If it starts feeling spongy or soft in certain areas, then those spots should be replaced before the finished flooring is installed. However, I would consider having removed any sheetrock and insulation that was drenched or you could have future problems.