As you know I’ve been doing a second story addition myself over the last six months. I’m working on siding and windows now. I have tar paper over the sheathing. Recently I was reading about mold and how some usually comes on lumber when its delivered from the lumberyard and is a harmless type of mold. I was wondering what causes toxic mold growth. I’m sure some moisture gets behind the tar paper when it really pours so I was wondering if mold could grow behind the tar paper before I get to finish the siding. If anyone knows more about this please give me your thoughts. Thanks.
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"Toxic mold" seems to be a creation of bored lawyers ... mold is mold, too much of it is bad, and it's presence can be an indication of poor practices.
Simply put, your home is made up of 'mold food.' Mold exists everywhere. For mold to thrive, though, requires darkness and moisture. If you have light and moisture, you have an algae or moss issue, not a mold issue.
Since moisture is essential for mold to grow, controlling that moisture is the key. You want to avoid trapping moisture anywhere. Even in the dryest desert, moisture exists - so make sure you don't trap it.
This is where everybody went astray under Carter .... high energy prices led to efforts to excessively limit the air leaks in homes. Many of these efforts resulted in moisture - which had formerly been vented - being trapped instead. Now, 30 years later, we're finding the mold this moisture fostered.
Often overlooked is the role moisture plays in construction. Some products, like concrete, actually create moisture as they cure. Others - silicone caulk and spray foam are examples - require moisture to cure.
Many traditional construction methods had, as part of them, a means to control this moisture. For example, the 'weep holes" in masonry walls, as well as the spaces left between layers, allowed excessive moisture to drain / vent.
"Mold control" really is better thought of as "moisture control."
There are particular types of mold that are worse than others, because they either are strong allergins or because they do indeed contain a toxin. And some individuals, due to their genetic makeup, are far more sensitive to mold toxins than other people.But, as you imply, the quantity of mold is as much an issue as the type -- small amounts of mold are always present, but large amounts occur where conditions encourage it.Mold needs basically four things: The mold spores (always present), food (wood, paper, etc), moisture, and an appropriate temperature. The easiest of these to control is generally moisture. And with regard to moisture it's not so bad (though not necessarily good) if things get occasionally damp, but if they remain continuously damp then you'll almost certainly have mold problems.
The mark of the immature man is that he wants to die nobly for a cause, while the mark of a mature man is that he wants to live humbly for one. --Wilhelm Stekel
Thanks for the answers. So is it possible to have mold growth on the plywood if a heavy rain wets the tar paper to the point that areas of the plywood shething get wet underneath (obviously it would be dark cause the tar paper is on top)? I'm working as fast as I can but by myself it takes a loooong time (like months). Also, if it can grow and if a little does get trapped under the siding on the outside of the sheathing can it affect the inside of the house or the wood itself? Thanks.
Edited 12/13/2008 8:44 pm ET by Richie921
Proper tar paper installation assumes that the material under the paper WILL get wet. That's why it's lapped the way it is ... what gets past layer #1 is diverted by layer #2.
Even if water makes it past all the paper layers, the lap pattern again helps divert it away as it flows down the wall.
For similar reasons, we don't caulk the bottom edge of lapped siding.
What you want to avoid is sandwiching a 'food' layer between two moisture barriers. For example, drywall between tile and insulation vapor barrier paper. Do that, and you're trapping moisture there ... and mold will follow.
Yeah, I started at the bottom of the walls overlapping as I went up. We had a major storm roll through and the one side of the house that the wind blasted rain into got wet to the point that in two of three spots the water soaked the paper so much that the plywood underneath got wet (I could feel the wetness in the plywood from inside the house). Normal rain seems to be stopped by the paper but in this case it was ridiculous amounts of rain with HIGH wind. So if the plywood gets wet occasionally through the tar paper until I finish the siding is mold growth likely? Mind you its also Dec. in NYC so its cold (30 to 50 degrees). Thanks.
You know whats funny, I've torn into a lot of old houses (you know plaster walls and not a lick of insulation) and have never seen mold growth. It seems we make houses so tight today we've stirred up a whole host of problems to go along with the energy efficiency.
You're not likely to get troublesome amounts of mold from rain ... however hard ... against the outside of the walls. Inside is another matter.
As you said, it's winter. You probably have some space heaters running. If they burn fuel ... well, fuel makes an enormous amount of water as it burns. Where does this water go? It often condenses against cold, uninsulated parts of the house. Then you come along, put on insulation, staple down the vapor barrier ... and have just trapped a gallon of water in the stud bay. You've made a mold farm.
I doubt the rain went all the way through the plywood. I think you saw the results on indoor moisture condensing on the wall. You really need to dry out the place before you close in the walls. The key here is, again, ventilation ... and get some electric heat in there!
I'm no fan of 'radiant' heat, but this might be a time to use it, to warm stud bays before you close them up.
Nope, no heat inside. Wind driven water soaked through the walls in 3 small areas. 2 were around windows and one was on the sidewall of a shed dormer. It was crazy the amount of rain we had and the wind blew it right at that one wall of the house. It definitely soaked the wood. It has since dried out but like I said i was curious if mold could grow and cause a problem. I'm still installing windows as I side the house and when I cut the plywood where the openings are that have been covered with tar paper for months, the wood looks brand new. That side of the shed dormer is another story. Its hard to get to (a 40 foot ladder fully extended doesn't even reach the top of the wall) so It was uncovered for a few months. Amazingly when I did put the tar paper on the plywood was still in good shape. Not like new but not delaminating either. This is the first time I've seen water get through the paper and wet the wood. Now I got curious as to the mold issue. I'm not doing any work inside yet so no heat yet. I just layer the clothes :)
As Dan said there are different types of mold. Some are definitely toxic. The "stain molds" typically seen on lumber (often a bluish slick) are generally harmless. While litigation has almost certainly exaggerated the issue, there is no question that some fungi can have powerful effects on the body while others are relatively effectless.
It takes a sustained damp (not wet) environment to foster fungal growth. So while rain is actually washing over your sheathing, mold growth is inhibited. And as noted above, if the water can find its way out at the bottom, the environment will not remain at the optimal dampness long enough for much if any mold growth.
Generally speaking, the worst cases are situations where water gets in, but can't get out. New home construction with exposed sheathing and tar paper doesn't really represent that.
k
Thanks a lot. I really appreciate it. Obviously I don't know much about mold other than that I don't want it. So I'll ask another question since u guys know more than me. When I get to doing the insulation, what is the best way to do fiberglass and not trap moisture. Do i use the stuff with the kraft paper I see all the time or do you guys recommend something else. Also if u use the kraft paper one, do you tape the seams? Thanks again.
what's your concern about "trapping moisture" with the insulation? i'm not sure i follow. if you're talking about dew points, that's one question, if you're talking about closing up insulated stud bays which have some residual moisture, (or damp fiberglass) that's a different question.
k
No not residual moisture. I guess it would be dew point moisture. Warm air passing through the insulation and condensing on the outside wall.
ah, i was afraid you'd say that. yeah, the dew point is the temperature at which that warm (moisture laden) air condenses. depends a lot on your climate and the temperature difference between inside and out. what's your general location?
in some situations you want the vapor barrier on one side, at other times, the other. and there's room for debate- as the temperature changes, the location of the dew point can move from the back of the drywall to the back of the sheathing.
i know just enough about it to get myself in trouble, so i'll hold off on advice for now. an advanced search of "dew point" would probably shed some light (or confusion). i'll look into it as well.
i think there are some folks who know quite a bit about it here. someone else can probably help out. that could be another thread topic- dew point in geographic location x.
k
one thing i like about your situation is you're using tar paper right? that has a relatively high permeability for water vapor, so the wall will breathe to the outside. personally, I would worry about quality flashing details at the windows and doors and not worry too much about sealing the stud bays. but like i said, i'm not the expert.
Edited 12/14/2008 12:08 am ET by KFC
Edited 12/14/2008 12:35 am ET by KFC