A lot of people have heard advice about vapor barriers and vapor retarders. Many of them have walked away confused. A big part of the problem, I think, is that they've been told what to do—"Put it on the warm-in-winter side," or "Never use one"—but they haven't had the physics of what happens explained to them.

In this article, I'm not going to get into the details of vapor barriers or all the possible scenarios of different wall assemblies and moisture loads. I'm simply going to explain what happens in a wall cavity with and without a plastic vapor barrier installed.

1. Hot Humid Weather

I'm writing this article because one of our HERS raters came across a house in Charleston, SC that had poly under the drywall, on the interior side of the wall assembly. If you're at all familiar with the climate in Charleston and understand moisture, you know that can't be a good thing.

I was there one day in June a few years ago and saw condensation on the outside of a window...at 1 o'clock in the afternoon of a sunny day. The dew point of the outdoor air was 78° F. The window had a single pane of glass. They were running the air conditioner, so the indoor temperature was probably 75 or below. Humid air hits cool surface. Condensation results.

Now, imagine that pane of glass is actually a sheet of polyethylene. Next, imagine that a layer of drywall separates the poly from the indoor air. Then build a wood-frame wall outside the poly, complete with cladding and air permeable insulation in the cavities. Will that poly be protected from the outdoor humidity? Or will it, like the window I saw, be dripping with condensation

If it's a typical wall, chances are good that water vapor in the outdoor air will find its way into the wall cavity, eventually finding the sheet of poly, pressed up against the drywall. If that wall allows outdoor air to infiltrate and the poly is below the dew point, condensation is the likely result. If those conditions last long enough, the condensed water will run down the poly, get the wood framing wet, and begin to rot out the wall.

The truth, though, is that the water vapor in the outdoor air is rarely the source of moisture that rots out a wall. More likely is that moisture from a wet foundation makes its way up into the wall by capillary action, or bulk water from leaks around openings gets into the wall cavity. The presence of an interior vapor barrier makes drying out the cavity harder to do, though.

Without poly beneath the drywall, water vapor hits the drywall and diffuses through to the drier (in summer) indoor air. By installing a sheet of poly there, you cut off that drying mechanism and water that finds its way into walls can stay there longer and do more damage.

2. Cold Weather

In cold weather, a sheet of poly on the interior side of a wall probably won't cause any problems. The humid air is indoors, and the dry air is outdoors. The sheet of poly still cuts off drying to the indoors, but it keeps the water vapor in the humid indoor air away from the cold surfaces inside the wall. This is what building scientists proposed as the solution for walls that wouldn't hold paint back in the early days of insulation. It didn't solve the paint problem, though, because water vapor from the indoor air wasn't the main source of moisture.

3. Cold Weather

Plastic on the outer surface of a wall in cold weather could cause problems. The humid air is indoors. The cool surface is the sheathing, assuming no exterior insulation. If water vapor diffuses or infiltrates into the wall cavity and finds the cool surface, moisture problems can occur.

Of course, you can have moisure problems here even without the exterior vapor barrier because of what Bill Rose calls the rule of material wetting. That is, warm materials dry more quickly than cold materials.

4. Hot Humid Weather

The problem occurs with a vapor barrier when it prevents drying to the drier space. In a building with air conditioning during hot humid weather, the drier space is indoors. The humid air is outdoors. The wrong place to put a vapor barrier is on the inside because any humid air that gets into the wall cavity is blocked from drying to the inside.

If the vapor barrier is on the outside, it prevents the humid air from diffusing into the wall cavity and finding the cold surface on the other side of the cavity, the back side of the drywall. So, like a vapor barrier on the inner surface in cold weather, putting one on the outer surface in hot weather isn't likely to have a moisture problem due to vapor diffusion.

We can summarize the vapor barrier issue like this:

If you're in a place like Miami where it'll almost never be colder outdoors than indoors, a vapor barrier on outer surface of a wall assembly may be OK. If you're in Maine and never use an air conditioner, a vapor barrier on the inner surface may be OK. If you're in a cold climate, however, and do use air conditioning, you need to be careful with interior vapor barriers like polyethylene. You could be creating the kind of problems I described in scenario 1 above.

Understanding moisture is one of the most important aspects of making buildings do their jobs properly and not fail prematurely. We know now that mid-twentieth century building science incorrectly ascribed magical properties to vapor barriers. Water vapor from indoor air wasn't the source of most moisture problems. Bulk water from deficiencies in drainage planes, flashing, and other moisture management details caused most of the problems.

Building science has progressed since then. We know that vapor barriers can cause problems, but we still have homes like the one in Charleston with poly in the walls. And we have $4 million dollar homes with poly on the walls, too. I saw the one below when Martin Holladay came to Atlanta last year. That's in the basement, but the kneewalls in the attic also were covered with poly.

Our understanding now is that it's generally more important for wall assemblies to be able to dry than it is to block water vapor with materials like polyethylene. Here's what Bill Rose wrote in his book, Water in Buildings:

"Given the fact that a very small percentage of building problems (1 to 5% at most in the authors experience) are associated with wetting by water vapor diffusion, the argument for enhanced drying potential becomes much stonger."

Those wet concrete blocks in our page top photograph are diagnostic: water was still entering the crawl space through the foundation wall, ponding on to of the gravel-covered plastic "moisture barrier". Here we explain why the good idea of covering the floor of a dirt crawl space may not be enough to stop a building moisture and mold problem.

How to Place Polyethylene Ground Cover in a Crawl Space

Heavy polyethylene plastic sheeting works well since it resists deterioration by mold. Covering the poly with one or more inches of sand or smooth rounded gravel will protect it from occasional trampling.
In new construction, where the polyethylene overlaps on the ground it should be overlapped by at least two feet, or sealed using a caulk or sealant that will adhere to the poly, and the poly should be carried up the walls several inches or more, at least to grade line - a height equal to the height of soil outside.
Some installers use a sealant caulk or furring strips to secure the poly to the building foundation wall. We do not like to staple the poly to the sill plate on top of the foundation wall as doing so can in some areas provide a ready path for termite attack. For this reason we exercise similar care when insulating a crawl space foundation wall interior.-- DF
In building retrofit installations of crawl space moisture barriers, Jennings and Moody recommend leaving about 20 percent of the ground uncovered so that the structure is not subjected to undue shrinkage and movement. In particularly wet spaces, they suggest first covering 50 percent, then finishing up to 80 percent of the ground area in 10 percent increments every 4 to 6 weeks to reduce "moisture shock".
[This was 1984 vintage advice. Our building inspection and testing experience in the ensuing decades indicates that covering 100 % of a dirt crawl space floor with 6-mil poly, sealed as we described above, is the most effective practice and can substantially reduce unwanted building moisture and mold problems.

Should Crawl Space Ventilation Be Included in a Crawl Space Dryout Scheme?

In the original 1984 Solar Age article, the same experts were recommending what was conventional crawl space ventilation wisdom - specifications that were consistent with building codes:

1984 crawl space advice: "A ground cover should be used in conjunction with ventilation. The HUD standard [1984] typical of others, recommended four crawl space vents with a total minimum free vent equal to 1/150 of the crawl space floor area if there is a ground cover, 1/1500 with the ground uncovered. For best results, place two vents each on opposing walls."

 Experts observed that crawl space venting was not effective in many instances, for example depending on wind direction as well as the source and amount of crawl space water or moisture, crawl space vents were simply ineffective.
In some instances, such as blowing warm high-moisture laden air into a cool crawl space in summer months in some climates greatly increased the level of crawl space moisture and condensation, making crawl space moisture worse rather than better.

Current best practice in controlling crawl space moisture involves:

• Seal the crawl space from outdoor air - close off those crawl space vents - and convert the crawl space to conditioned space, providing a small amount of heat where climate dictates, to help keep the area dry and above freezing. 
• Identify and cure sources of crawl space moisture, such as roof drainage spilling around the foundation. In roughly 90 % of inspections performed by experts, we find that wet or moldy crawl spaces or basements that had been blamed on "high water table", "rising damp", or "built over a spring" were actually being caused by gutter and downspout defects, perhaps combined with in-slope grade that concentrated roof drainage right against the building foundation.
• Use a moisture barrier such as 6-mil poly continuously over the crawl space floor and lower crawl space walls, up to grade level, sealed as we described above. In new construction the poly may be installed under a crawl space slab or gravel. In crawl spaces that are rarely entered, placing sand or gravel over the poly is probably not necessary, and its use can hide depressions in the poly that may actually be holding ponding water on top of the poly in some cases.
• Inspect the crawl space periodically, at least once a year, to be sure that the poly moisture barrier is working as intended. As we just suggested, an outside water leak, such as roof spillage entering through a foundation wall, or an inside water source such as from a leaky plumbing supply or drain pipe, can place water on top of your crawl space poly moisture barrier, leading to a costly building moisture and mold problem. 

Required Materials 

System Overview and Summary 

 Set-up and Site Preparation 

Roof/Attic/Ceiling Applications 

Exterior Wall Applications 

One of the best home improvements I’ve made in a while, is one you’ll likely never see (unless you read this blog). And at about $10.00, it was also one of the cheapest projects I’ve done.

When the temperatures started to drop, I noticed that the second floor of my house was quite a bit colder than the first. So, I did what my mother had always done. I went out and bought window wrap, and applied it to all the windows in the house.  You know the stuff. The plastic wrap, that shrinks with a hair dryer.

After spending a day installing the stuff, I noticed it was still quite cold upstairs.  As I investigated further, I found that I could still feel a draft near the windows.  So I thought to my self “The air must be coming in around the window, not through it.”.

The next day I removed the casing from one of the windows, and found a puny bit of fiberglass batt insulation stuffed in around the window.  I couldn’t believe that in a newer home, in the northern United States, this was the way they insulated around a window.  I knew what had to be done, so I hopped in my truck and headed to Home Depot. Where I picked up two cans of Great Stuff™ Window & Door, for about $10.00.

If you’re going to do this, make sure you get the stuff designed for windows and door.  The “regular” Great Stuff™ Gaps & Cracks may expand too much and/or too quickly, causing the window or door frame to bow out.  If this happens, the door or window may not function properly.  The window & door formula is created to expand with less force, so it will not bend the frame of a door or window.

Installing the stuff is dead easy.  Once you have the window trim removed, and the gap around the window frame revealed. You simply gently pull the trigger, and fill the gap about 50% of the way (that’s 50% of the depth, you want to fill the entire width). Run a smooth continuous bead in all the gaps, and sit back an watch it expand.

Don’t worry about being super neat, you can trim off anything that expands out of the crack later.  Use a hack saw blade, or utility knife to trim off any excess, once the foam has cured (usually about 8 hours).

Once the foam has cured, and you’ve trimmed off any overflow. There’s nothing more to do except, install the casing, and celebrate a job well done.

I found that one 16 oz. can, was enough to do two standard sized windows. You’ll also want to be aware that once you start using a can, you have to use  the whole can.  If you don’t use all the product in the can, you cannot save the remainder for later. So before you start spraying, make sure the trim is removed from all the windows you plan to insulate.

Considering an attic sealing project?
Air sealing in the attic is generally a challenging DIY project, but the benefits can be substantial. If you are doing a major home renovation project, now may be a great time to tackle this project too.

The good news is that, even if you're not comfortable taking on this project yourself, there are many qualified contractors who can help you get the work done. EPA recommends that you complete the attic insulation project after this project.

Does my home need this project?

Common Symptoms

• Drafty rooms
• Hot and cold rooms; uneven temperature between rooms
• High heating or cooling bills
• Ice dams in the winter
• Dry indoor air in winter
• Dust (especially in rooms directly below the attic)

As much as half of the energy used in your home goes to heating and cooling. So making smart decisions about your home's heating, ventilating, and air conditioning (HVAC) system can have a big effect on your utility bills — and your comfort. Take these steps to increase the efficiency of your heating and cooling system. 

Check your filter every month, especially during heavy use months (winter and summer). If the filter looks dirty after a month, change it. At a minimum, change the filter every 3 months. A dirty filter will slow down air flow and make the system work harder to keep you warm or cool — wasting energy. A clean filter will also prevent dust and dirt from building up in the system — leading to expensive maintenance and/or early system failure.

Just as a tune-up for your car can improve your gas mileage, a yearly tune-up of your heating and cooling system can improve efficiency and comfort. Learn more:

A programmable thermostat is ideal for people who are away from home during set periods of time throughout the week. Through proper use of pre-programmed settings, a programmable thermostat can save you about $180 every year in energy costs.

Ducts that move air to-and-from a forced air furnace, central air conditioner, or heat pump are often big energy wasters. Sealing and insulating ducts can improve the efficiency of your heating and cooling system by as much as 20 percent — and sometimes much more.

Focus first on sealing ducts that run through the attic, crawlspace, unheated basement, or garage. Use duct sealant (mastic) or metal-backed (foil) tape to seal the seams and connections of ducts. After sealing the ducts in those spaces, wrap them in insulation to keep them from getting hot in the summer or cold in the winter. Next, look to seal any other ducts that you can access in the heated or cooled part of the house. 

If your HVAC equipment is more than 10 years old or not keeping your house comfortable, have it evaluated by a professional HVAC contractor. If it is not performing efficiently or needs upgrading, consider replacing it with a unit that has earned the ENERGY STAR. Depending on where you live, replacing your old heating and cooling equipment with ENERGY STAR qualified equipment can cut your annual energy bill by more than $115. But before you invest in a new HVAC system, make sure that you have addressed the big air leaks in your house and the duct system. Sometimes, these are the real sources of problems rather than your HVAC equipment.

Replacing your old heating and cooling equipment with new, energy-efficient models is a great start. But to make sure that you get the best performance, the new equipment must be properly installed. In fact, improper installation can reduce system efficiency by up to 30 percent - costing you more on your utility bills and possibly shortening the equipment's life

Air that leaks through your home's envelope − the outer walls, windows, doors, and other openings − wastes a lot of energy and increases your utility costs. A well-sealed envelope, coupled with the right amount of insulation, can make a real difference on your utility bills.

Sealing leaks and adding insulation can improve the overall comfort of your home and help to fix many of these common problems:

• Reduced noise from outside
• Less pollen, dust and insects (or pests) entering your home

• Better humidity control
• Lower chance for ice dams on the roof/eves in snowy climates

Most homes in the United States don't have enough insulation and have significant air leaks. In fact, if you added up all the leaks, holes and gaps in a typical home's envelope, it would be the equivalent of having a window open every day of the year!

1. Work from the perimeter of the attic toward the door or hatch so that you don't trample all over the insulation you just put in.
2. Always cover the tops of the ceiling joists to make sure the insulation is deep enough to reach your target R-value and to prevent thermal bridging, the heat loss that occurs through the wood framing.
3. Shower thoroughly after you're done for the day to remove fibers from your skin, and launder your work clothes after one wearing.

For loose fill
• Fasten blocking around the hatch or door to allow the material to be installed around this area without escaping.
• Make sure the fill's depth is uniform across the attic. To make it easy to eyeball how level the material is as you blow it in, screw depth guides to joists throughout the space.
• To get the target R-value from the product you chose, use the number of bags your calculations showed that you needed to insulate your attic—never fewer. If you've reached your target depth but still have a few bags left over, keep adding the material at an even depth throughout the space until all the bags your calculations called for are empty.
• To help the material achieve the right density as you install it, hold 
the blower hose parallel to the floor and the floor joists. Blow in the fill between and over the joists rather than across them.

For batts
• Always use unfaced batts, both when laying product for the first time and to prevent moisture from becoming trapped between new and old layers of insulation. You can buy them unfaced or simply remove the paper or foil backing.
• Place a new layer of unfaced batts perpendicular to the old layer, to cover any gaps in the lower layer. Adjoining batts should be butted snugly together, but not tight enough to compress them.
• Never lay heavier batts, such as cotton, over lighter ones, like fiberglass. You'll compress the lower layer and reduce its effectiveness.
• Always cut batts to fit around obstructions and penetrations. Stuffing or cramming them around ducts, piping, and the like will compress the air-trapping pockets in the material, reducing its insulating properties.
• Don't leave any gaps between batts and joists, obstructions, or abutting batts. Even narrow ones will let air escape. Instead, cut a thin strip of the insulation you're using to size.

At first it may seem odd to add insulation for warmth and then purposely allow cold air to enter the attic through vents, but this combination is the key to a durable and energy-efficient home. Here's why: in the winter, allowing a natural flow of outdoor air to ventilate the attic helps keep it cold, which reduces the potential for ice damming (snow that melts off a roof from an attic that is too warm and then re-freezes at the gutters, causing an ice dam that can damage the roof). Proper insulation and air sealing also keeps attics cold in winter by blocking the entry of heat and moist air from below. In the summer, natural air flow in a well-vented attic moves super-heated air out of the attic, protecting roof shingles and removing moisture. The insulation will resist heat transfer into the house.

Attic fans are intended to cool hot attics by drawing in cooler outside air from attic vents (soffit and gable) and pushing hot air to the outside. However, if your attic has blocked soffit vents and is not well-sealed from the rest of the house, attic fans will suck cool conditioned air up out of the house and into the attic. This will use more energy and make your air conditioner work harder, which will increase your summer utility bill.

You don't want your unfinished attic cooled by your air conditioner. To prevent this, follow the air sealing and insulation strategies in this guide and make sure the attic is well-ventilated using passive vents and natural air flow.

Laying fiberglass rolls is easiest for a DIY job. If you have any type of insulation between the rafters, install the second layer over and perpendicular to the first (again, the second layer of roll insulation should be unfaced — with no vapor retarder). This will help cover the tops of the joists and reduce heat loss or gain through the frame. Also, when laying down additional insulation, work from the perimeter toward the attic opening. Never lay insulation over recessed light fixtures or soffit vents. Keep all insulation at least 3 inches away from "can" lights, unless they are rated IC (Insulated Ceiling). If you are using loose fill insulation, use sheet metal to create barriers around the openings. If using fiberglass, wire mesh can be used to create a barrier.

Installing Rafter Vents

To completely cover your attic floor with insulation out to the eaves you need to install rafter vents (also called insulation baffles). Complete coverage of the attic floor along with sealing air leaks will ensure you get the best performance from your insulation. Rafter vents ensure the soffit vents are clear and there is a channel for outside air to move into the attic at the soffits and out through the gable or ridge vent. To install the rafter vents, staple them directly to the roof decking. Rafter vents come in 4-foot lengths and 14-1/2 and 22-1/2 inch widths for different rafter spacings. Rafter vents should be placed in your attic ceiling in between the rafters at the point where your attic ceiling meets your attic floor.

Once they are in place, you can then place the batts or blankets, or blow insulation, right out to the very edge of the attic floor. Note: Blown insulation may require an additional block to prevent insulation from being blown into the soffit. A piece of rigid foam board placed on the outer edge of the top plate works very well for this.

It is because the green building community has almost universally decided that ol' fiberglass batt insulation is bad. There are good reasons for this, and we'll look at those, but the more obvious question is, What does this have to do with mineral wool?