We may earn a commission from links on this page.Did you know you can customize Google to filter out garbage? Take these steps for better search results, including adding Lifehacker as a preferred source for tech news.Renovating projects can actually sometimes make your home worse—like when mold suddenly appears post-renovation. Even though your house never had mold before, and you’ve just put a lot of brand-new materials into it, mold can suddenly spring up after a house renovation project for a variety of reasons. That makes it crucial to monitor your contractors (or your own DIY work) during the project to make sure you’re not turning your house into the ideal breeding ground for post-renovation mold. Inadequate ventilationOlder homes are often pretty “leaky” when it comes to air. They’re difficult to heat and cool in part because your climate-controlled air keeps leaking out into the world, and they allow allergens and dirt to infiltrate your living space. So when folks plan a gut renovation, they often seek to make the house as airtight as possible—which isn’t a bad idea on its own. But you need to ensure the house has adequate ventilation, or that airtight seal will trap moisture in the house, and that extra humidity will almost always lead to mold growth.Renovated bathrooms can be a specific concern. Bathrooms are naturally humid rooms, and they almost always require a properly calibrated ventilation fan to pull moisture out of the space. Not only does the fan have to be the right capacity for the size of the room, it needs to be run long enough after every shower to ensure the room dries out (some models have a built-in humidity sensor that will trigger the fan any time the rooms gets damp), and it needs to be vented to the outside of the house, not into the attic or walls.Water intrusionIf your renovation plan meant any area of your home was exposed to the elements—lacking windows, roofs, or doors—there may have been water intrusion at some point. This is normal for extensive renovations, and harmless as long as your contractor took the time to dry the place out before closing up walls and floors. It doesn’t require a heavy rainfall inside the house to sow the seeds of a moisture problem weeks or months down the road—just letting a space get swampy and then sealing it up is all it takes.Damp spotsMishaps will happen during renovations, including plumbing emergencies, water spills, and drain problems. As long as the problem is fixed and it was an isolated incident, these kinds of accidents aren’t a big deal—but they can lead to damp areas on walls, ceilings, and floors that need to be addressed. Slapping a coat of primer over a damp spot on your wall does nothing to prevent mold, and may even encourage it because sealing up that damp spot prevents the moisture from evaporating.You should also never assume that water stains from a repaired leak accident are only unslightly. Covering water stains on a wall with cabinets or paint without ensuring they’re completely dry is a recipe for disaster. Bottom line: Any damp areas need to be dried out thoroughly before they’re covered over, no matter how small or unobtrusive they might be.Moldy lumberSometimes you can do everything right in terms of construction practices and still bring mold into the house, by using moldy or even damp materials. Just because you’re buying your lumber, drywall, and other renovation materials brand new doesn’t mean they’re clean and mold-free, especially if they’re stored outside for a while. If the framing lumber being used to create your walls or the joists supporting new floors come into the house too damp, they become breeding grounds for mold once they’ve been sealed up behind drywall or subflooring. And drywall is porous and can easily absorb moisture while looking superficially clean.It’s vital that all of these materials be inspected prior to installation—both visually (for visible signs of mold) and using a pinless moisture meter. Wood should show a moisture reading of around 6%.Damp drywallDamp drywall is an almost perfect host for mold, so ensure that all the drywall being installed during your renovation has a moisture reading of 0.5% or less and shows no signs of visible molding. This is especially crucial if the drywall was delivered and left to sit outside; even if it doesn’t rain or the drywall is covered, it can absorb moisture from the air. Priming and painting over damp drywall will look good for a while, but that trapped moisture will eventually give mold spores everything they need to grow.HVAC disruptionOne unexpected way a renovation project can cause a mold infestation involves your HVAC system, even if you’re not actually doing anything to it. HVAC systems are often disturbed during renovations—ducts have to be shifted out of the way, registers temporarily removed, compressors temporarily displaced. And that can be a problem because mold can grow in HVAC ducts undetected, and all that jostling and movement can disturb it, releasing spores that are then circulated all over the house when everything is hooked back up. If those spores are blown into a damp area—like a poorly ventilated bathroom—mold will happily take root.It’s always a good idea to think about the cleanup after a renovation to make sure that dust and debris don’t plague you after the workers are gone—and a thorough cleaning of your HVAC system’s ducts should be part of that plan, even if they weren’t directly affected by the project.