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Article Summary: In this article, I’m going to be looking at windows and doors in relation to mold. More specifically, I’m going to discuss how to install windows and doors properly coming from the perspective of a former building contractor who has spent a lot of time studying what it means to build a quality home. Along the way, I’ll point out common errors builders make that lead to water intrusion and subsequent mold growth. Quite often builders and inspectors fail to build homes that will remain water tight beyond the first few years of their lives. Later, I’ll look at why regular window cleaning is important to those with Chronic Inflammatory Response Syndrome (CIRS). May 27, 2016

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Flashing Overview

When it comes to windows and doors, it’s all about the flashing. In fact, whenever you have an intersection between to different building components, there is a risk of water intrusion and flashing is needed. When a chimney or sidewall intersects a roof, you need flashing. When a window or door is installed into a wall, you need flashing. When you break up courses of siding with trim boards, you need more flashing. It’s all about flashing.

So what do I mean by flashing? Very often we’re talking about sheet metal that is bent and installed in such a way so as to keep the water out at building intersections. However, for this article, flashing can also be various water impervious building membranes/papers that are overlapped and installed in such a way to keep walls and roofs dry. You’ll see what I mean when we get into some details later. Flashing is a thin, water impervious, material used at building intersections to keep water out.

When my grandfather was a builder, they knew all about flashing. Carpenters formed up copper sheet metal and soldered them together to make all sorts of custom door and window flashing. Caulk really wasn’t an option back then. If there was a leak-prone intersection, they installed tar paper and metal flashing to keep the water out. Unfortunately, times have changed. Today, builders seem to think that a bead of caulk is just as good as flashing. It’s not; it’s not even close.

Granted, they make some awesome caulks nowadays. You know it’s good when after getting some on your hands, it’s nearly impossible to get off. You have to let it “wear” off. Some high quality gutter and siding caulks are like this. They’re great for helping to minimize water infiltration, for reducing the burden on flashing, but they are not a substitute for proper flashing. I repeat; caulk is not a substitute for flashing.

The reason is that all caulk eventually fails. The ultra-violet light from sun degrades darn near everything. Add to this movement from changes in humidity and temperature, and it’s no wonder that every few years I have to go around and touch up the caulking where our siding intersects corner and window trim boards. Knowing full well that caulking should not be relied upon to keep walls dry, I installed wide strips of tar paper flashing under all the trim boards and lapped it over the building wrap. In so doing, what little water does weep through gaps in the siding where caulking has failed is kept out of the walls.

Window Flashing

So let’s look at how to flash a window properly so no water gets in. To start, we need to look at how windows are set into walls. Basically, the carpenter frames out the wall so there is an opening 1/2″ wider and taller than the actual window itself. This 1/2″ gap is important because it’s too time consuming for the carpenter to get the rough window opening perfectly plumb and level. The extra gap allows the window installer to use shims to true up the window in the rough opening.

If you’re new to all of this, you may be wondering about how air and water is kept out of that larger (1/2″) gap. The answer is various forms of flashing. To start, all new windows have a flange that runs around the window and covers over the rough opening gap. Many years ago when they first started making window flanges, many of them leaked. Today, all but the cheapest windows have integral window flanges that can be expected to be water-tight and last the life of the window.

Given that window flanges cover the rough opening gap, and are well sealed to the window itself, the real concern is how to keep any water that gets behind the siding from slipping underneath the window flange and getting into the walls. I’ll talk about siding in another article, but suffice it to say that all siding leaks. Siding was never intended to be water tight. I don’t care if it’s wood, cement board, vinyl, brick, or otherwise. Siding sheds the bulk of rain water but some water always gets through. Water always gets in around window, door, trim, and various other gaps.

Given the fact that all siding leaks, let’s take a brief look at how to properly flash window flanges. Now I could spend another two pages writing in detail on how to flash a window properly. However, there are many good articles on the subject. I particularly like the articles from The Journal of Light Construction and Fine Homebuilding. I was an avid reader during my building years. Although the technique varies a bit dependent on an east versus west geographic location, the overall approach is the same.

Window Flashing

In brief, the building wrap is cut and tucked in along the sides while the upper flap is temporarily lifted up out of the way. Next, the window sill gets extra heavy, wide, butyl rubber based, self-adhering tape applied. After that, caulk is applied to the backside of the window flange and the window is installed. The sides are then taped with more heavy window tape before the head flap is folded down over the upper window flange and taped in place. The idea is that as you move from the bottom of the window toward the top, each successive layer of tape and building wrap laps over the lower layer so water flowing down the face of the building stays on the outside of the wrap.

So that’s all well and good. The trouble is that it’s only been within maybe the last 15 years that carpenters started using this practice in earnest. It was quite common for builders to slap a bead of caulk on the back of the window flange and then nail it in place. There wasn’t any layering of flashing, there was no tape. All it takes is a bad siding job combined with a pinhole in the wrong place around the window flange for the walls to get really wet. The last house I built was in 2010 and I was giving the carpenters lessons on how to properly flash a window.

To make matters worse, carpenters often do not flash the finish trim and siding properly around windows. Remember, the trim and siding is supposed to keep most of the water out leaving small amounts to be handled by window flanges, window tape, and properly lapped building wrap. To give you an example of what I’m referring to, take a look at this set of double-hung windows with an arched top above. It looks great. The house is in an expensive suburb of Chicago. However, when you look more closely, you realize that the trim and siding details are poorly executed.

To begin, these windows are trimmed out with cedar boards. Inevitably, there is always some small gap where the board abuts the side of the window. To minimize water infiltration, the carpenter should have sealed all cut ends of trim boards with good primer paint and then laid a bead of quality caulk along the window side. In so doing, the caulk fills the entire depth of any gaps between the window and the trim as the trim board is pressed into place and nailed. In comparison, filling gaps between the trim board and window after the fact with a thin bead of caulk will fail within a few years. Deeply embedded caulk as I described will maintain its seal for the life the trim.

Moving along, remember how I said successive layers of flashing should always lap over one another as you move up the building. This holds for all building materials. If we look at the tops of the window set, there are two serious mistakes. First, on the horizontal sections of the window tops, they did not install metal Z-flashing to keep the water out where the siding meets the top trim board. Water cascading down the face of the building reaches the top trim board and some of it can easily seep through this intersection and get behind the siding.

Normally, Z-shaped metal flashing is tucked behind the siding, laps over the top of the trim board, and turns down over the trim boards face – left picture. Notice how the gap above the Z-flashing is not caulked to allow water a way out. Z-flashing keeps water out of intersections between siding and trim boards along with various other intersections. In this case, if there wasn’t a trim board, the window flange itself would serve the purpose of Z-flashing. However, the trim board is much wider than the window flange.

To make matters even worse, look closely and you’ll see that the upper trim overlaps the siding. You can see that the siding has shrunk over the years leaving the bead of caulk that once kept this intersection dry now sitting a good 1/4″ above the existing line of intersection – middle picture. Lots of water is going to get behind this trim and really tax the underlying flashing. We can only hope they did a better job sealing the window to the building wrap.

Shifting out attention, let’s look at that arched window. This window suffers from the same lack of Z-flashing. Making metal flashing for curved windows used to be a laborious task. Nowadays, you can buy vinyl solutions that work great – right picture. I don’t particularly love vinyl, but if it’s thick enough, it’ll last. Just like with all flashing, the building wrap should be cut and lapped over any Z-flashing.

So enough about this window assembly, let’s talk a bit about vinyl siding and windows. In terms of keeping water out at the tops of the windows, vinyl siding trim works great. Typically, a length of J-channel the width of the window is cut and nailed above the window. This channel serves to catch water cascading down the building face and direct it out to the sides of the window.

Assuming the installer knows what he’s doing, the ends of this upper channel were cut long and a portion was bent down over the J-channel nailed to the sides of the window frame. In so doing, water is kept out the intersection between the upper and side J-channels. The water flows downward in the side channels. Everything is good so far.

Unfortunately, it’s the lower corners of vinyl windows that are almost always improperly flashed. Typically, the same sort of detail is used on the lower corners as the upper corners. J-channel is installed underneath the window and the side channel is cut a bit long and bent over the lower channel. This keeps water out right where the channels meet but where does all the water go flowing down the side J-channels?

I’ll tell you. A lot of it slips right behind the vinyl siding at the lower corner. There is no flashing to direct this substantial amount of water back out onto the outer face of the siding. Now you may be shaking your head in disbelief, but I’ve install a fair amount of siding. I’ve read manufacturer installation directions. All of them direct the installer to simple cut the siding to fit into the channels with zero flashing. It’s crazy.

Once again, I’m not going to drag you through all the details on how to handle this situation. Other guys have done a nice job describing how to address this weakness. Basically, there are two approaches. One is to slit the siding at the lower corner and run the side J-channel out onto the face of the side. The other is to slip a square of metal flashing under the window flange at the lower corner. This metal sheet is cut just long enough to direct water into the joint where the vinyl siding overlaps.

Both methods should work well. I’ve used the metal flashing technique. I’d make one addition to the article I’ve linked to. Namely, the weep holes at the siding overlap joint need to be enlarged in the area of the metal flashing. That’s a lot of water and the typical, factory cut, 1/8” holes aren’t going to be able to handle that amount of water.

So that’s the basics. There are a lot of nuances depending on the materials used and trim details. Nonetheless, the important points related to windows and fungi have been made. They are; it takes real attention to detail to keep walls dry around windows and builders often fail to do this work correctly.

One way to tell how your windows and doors are doing is to rent/borrow an infrared camera the next time it rains hard. Point it at the corners of your windows from inside. You’ll know you’re in trouble if the area around the corners looks blue/darker indicating they’re colder. They’re colder because they’re wet and the evaporating moisture cools the area. The window in the picture is fine.

Installing and Flashing Windows Correctly
Installing Flanged Windows: Two Strategies Compared
Rethinking Window Flashing
Install and Flash an Arch Top Window with Field Applied Flanges
Vinyl Window Flashing How-To Tutorial
Vinyl Siding Done Right
Flashing A Flanged Window (login)

Door Flashing

When it comes to flashing, doors are a lot like windows with a couple exceptions. You can read all about door flashing details in one of the articles below. In general, you want to flash the top and sides of the door trim just like you would a window. Namely, seal the door flange using heavy tape and overlap it with the building wrap.

The only trouble is that, unlike windows, most doors don’t have flanges to help cover over the rough framing opening. I know; it’s stupid. Instead, most doors simply have “brick molding” nailed to the jambs (sides and top) of the door. You know, the brick molding is the decorative trim board that runs along the sides and top of the door on the outside. Most installers simply lay a bead of caulk along the backside of the brick molding before pressing the door into place.

This approach relies on caulk alone. You know from the window discussion that caulk alone is not a good solution. Furthermore, larger gaps are often not sealed with a single bead of caulk. Just like with windows, a much better approach is to use heavy butyl rubber tape to create your own door flanges as described in the articles. These flanges made on site are then sealed and lapped in with the building wrap just like a window. It’s a good solution.

The other difference between doors and windows when it comes to flashing is at their bottoms. Doors have thresholds that get a lot of foot traffic. It is very common for doors to leak at their thresholds. This is often a hidden, insidious leak. It’s only months or years later when the house is full of mold that owners discover that water has been seeping in under the threshold with each rain and feeding mold growth in the sub-flooring.

So here’s why threshold leaks are a very frequent problem. Commonly, builders simply lay down a couple beads of caulk on top of the floor where the threshold is to rest before dropping the door into place. As you’ll soon see, this is so wrong for many reasons.

First, just look at the bottom of a threshold. Thresholds are typically made of aluminum that has many structural channels underneath. In addition, a portion of the aluminum threshold is often in-filled with a wood board for extra support. In other words, when the door is dropped into place, much of the caulk the builder laid down will remain untouched by the threshold in the areas where there are channels. A good portion of the underside of the threshold will not be sealed by the caulk. The bottom of the threshold is not flat wherein you might expect the caulk to squish out and seal the entire underside.

So what happens to the water that seeps in under the threshold when the wimpy bead of caulk fails at the outer edge of the threshold? I’ll tell you. It seeps underneath and drenches the sub-flooring. It’s bad because you won’t know its happening. The finish floor (carpet, wood, vinyl, etc.) often hides the leak below. Some day later on, you’ll look up in the basement and notice blackened sub-flooring in the area of the front door. Ugh.

The solution is to use a door pan. When my grandfather built, they bent up copper sheet metal and soldered it together. Today, you can get PVC door pans that you glue together. Although made of different materials, the idea is the same. Before setting the door, a pan with a lip on three sides is set in place. This pan directs any water that does manage to get under the door back out onto the face of the building. It’s a simple and effective solution.

Unfortunately, very few builders use door pans. It’s crazy. By the way, some builders are getting smarter and laying down heavy building tape over the sub-floor in the area of the door before setting exterior doors. That’s nice but thresholds are made of metal that I suspect will cut through this protective layer of tape with foot traffic. I’d much rather use a metal or heavy PVC door pan.

Before we wrap up this discussion on doors and mold, I’d like to make two additional points. One relates to the materials used on the door brick molding and door side jambs. The other relates to using protective storm doors and entryway roofs.

Although a door pan will keep the flooring dry, what about the sides of the door and the brick molding down at the bottom by the threshold? Often, these pieces are wood. If these pieces aren’t sealed perfectly, the end-grain of this wood wicks water up into the brick molding and side jamb pieces. It isn’t long before the paint starts to peel. A while later, serious cracks and rot develop.

It’s hard to seal the bottom ends of these pieces. Door manufactures are wising up and making the bottom ends of these parts out of composite material that is seamlessly spliced in with the wood. This composite material is water impervious and is a nice solution. Make sure to look for this feature when purchasing new doors.

The last point is to install a protective roof above exterior doors whenever possible. At a minimum, use a storm door. Not only will a storm door help reduce energy costs but it’ll keep water away from the intersection of the door side jamb where it meets the threshold. This is an inherently difficult joint to keep sealed. You have to rely on caulk. I haven’t ever seen any better solution to this joint.

If you’re using a door pan, this isn’t a serious of a problem but you want to try to keep the water out as much as possible. A roof or storm door serves this purpose. Make sure to yearly check caulking at the intersection of the door side jambs and the threshold, as well as, along the outer edge of the threshold and the building.

Just like windows, current building practices fall way short of keeping homes dry. We all suffer as a result – especially those with Chronic Inflammatory Response Syndrome (CIRS). The old timers knew how to keep buildings dry. Greed has replaced sound water management practices of yesteryears with cheap and leaky substitutes. Now that you know what constitutes good practice, you can go back and address any deficiencies in your own residence.

Flashing an Entry Door (login)

Slider Window Cleaning

For the remainder of this article, I’m going to cover slider window cleaning. I’m not talking about mixing up some ammonia with a touch of dish soap in a bucket of water and cleaning the glass. Instead, I’m talking about the hidden nooks and crannies in modern slider windows (move side-to-side) that hide mold. You may be thinking this is insubstantial but I can tell you that I took a big mold hit cleaning out these hidden places last year. I now pay someone else to do this work as part of a yearly maintenance schedule.

Unlike double hung and casement windows, most modern slider windows have tracks or channels below the window that are covered over by removable plastic trim pieces. These channels collect the rainwater that sheets down the face of the window and direct it out onto the face of the siding below. If you think about it, there has to be some way for the bottom groves of slider windows to empty themselves of water.

On many slider windows, the groves at the bottom of the window frame are covered with plastic trim that is press fitted in place. This trim provides a clean surface for the window rollers to move across. Dirt that washes off the window is directed into holes at the ends of these trim pieces and falls into the covered over channel below. Weep holes in these covered channels direct the water out onto the face of the building.

It’s a reasonably good system. You don’t want the window rollers mucking through a bunch of dirt. However, I can assure you that in a year’s time, most slider windows are going to start to get substantial mold growth in these covered over tracks/channels. I was surprised to see what was growing in my tracks. Remember, all it takes is water and a little organic matter for some form of mold to start growing.

Unwittingly, I proceeded to clean these dirty window tracks without protection. I foolishly just thought this was a bunch of inert dirt that needed to get cleaned out. Using a hook I bent out of a piece of aluminum, I carefully removed the two covers over the lower channels along with an additional two trim covers in side channels that hold the screen. My wife flushed out the tracks while I scrubbed the plastic covers and screens clean. By the evening, I knew I’d taken a serious mold hit that took days to recover from. What happened?

Obviously, this was more than just a little benign dirt. I’m not sure if it was the plastic trim pieces, the screens, or both, but there was a substantial amount of mold. I suspect it was mostly from the window tracks but we do live in woods. I suppose it is possible for some mold growth to get a foothold on the screens. I know when I lightly scrubbed the screens with a soft brush dipped in a bucket with QUAT that I could definitely see places I’d missed after the screens dried. Was this just stuck on “dirt” or was there mold too?

In the end it doesn’t matter. The main point is that windows with these sorts of hidden places need to be cleaned regularly. Granted there isn’t that much air movement through these tracks even when the windows are open. That’s good. Nonetheless, for someone with CIRS, it’s just a little too close for comfort to have mold growing underneath my windows. No thanks. I now remove the windows and install a sheet of plastic before having the window tracks cleaned out from outside.

I’ll finish by mentioning that a good portion of the “dirt” that collects in the tracks is comprised of dead bug bodies. The bugs love to climb in through the weep holes looking for a place to get out of the weather – especially as winter draws in. Many of them never make it back out. To remedy this situation, I cut small sections of hardware screen spray painted to match the windows and fitted them into the weep holes at the bottom of the window. The rain can still get out but the opening is now too small for all but the tiniest of critters. It helps.