Building Science-Existing Construction Improvements

This three-part series first appeared on the Green Building Advisor website and has been condensed into one post.

An unconditioned and uninsulated crawlspace, an unsealed and uninsulated forced air heating system, and an uncovered dirt floor, which by the way has a sewage leak.  If this were your home and you wanted to make improvements, where would you start?


In the typical year, I’m in about 100 homes conducting energy audits and assessments, many of the homes are older.  The work I do is to help a customer form a plan to deal with their high energy costs in operating the home, in my area, that is usually heating related.  The first thing most homeowners will look to do is add insulation or air seal the building.  Those steps will improve the cost to operate the home, but the changes may have a detrimental effect in other areas.  Before we form a plan to air seal and insulated, there are other considerations that must be addressed.

Step 1-Correct any exterior bulk water issues.

Water is the number one killer of buildings, and it moves into and through buildings in four ways, in bulk, usually from exterior wetting events but can also be from interior plumbing leaks, capillary which is wicking through porous materials under tension, through air leaks and by vapor diffusion.  Bulk water intrusion must be delt with before addressing any energy concerns.  There’s no use in improving energy performance if the building is going to rot or affect the health the occupants.  Many of these bulk water issues are hidden, but if you know where and what to look for, you may be able to find some of the problem areas without disassembling the building.  If you do find one issue, often there are more.

The basement or crawl space rim joist, if there is one, is where I like to start.  Are there abandoned or over-sized holes for wiring, other utilities or vents?  If you can see daylight, there’s a high probability that water is leaking into the home.

Another place to inspect at the rim joist area is under the location of any exterior doors or deck rim boards.  Exterior doors with old cement steps poured close to the home are often areas with water intrusion and rot.  These photos are from my home, a 1952 Cape built with a concrete step poured up to the home.  There was also a deck rim with improper flashing that hid damage under a patio door.

After inspecting the rim joist area, my next step is to look at the exterior walls.  It’s not often there is evidence of bulk water damage on the interior side of an exterior wall, but sometimes it can be found.  Moldy drywall or water stains below windows are one indicator, but this mold may also be from a high interior moisture load (we will get to indoor humidity in a bit) or an air leak.

Damage from bulk water leaks on the exterior can be hard to detect, exterior claddings will often hide the damage.  Peeling paint and rotting windows are something to look for.  If there is loose siding, especially around a window or door, or a roof intersecting a wall without a kickout flashing, chances are there will be some damage, unfortunately part of the building may have to be disassembled to see these issues.  Much of this damage can be eliminated with proper flashings.  Window and door head flashings and kickout flashings, are some of the most important details in keeping water out of our walls.

Much like holes through a rim joist, utility and mechanical penetrations through walls above the basement or crawlspace can also be areas of bulk water leaks.  Slab on grade homes will have most of these penetrations through the exterior walls.  Multiple pipes or wires through one-hole, oversized holes, and improper flashing can all lead to water entering the wall cavity though these planned holes.  I see this often in both existing homes and in new home construction.  Sealing and flashing these utility wall penetrations is often neglected.

Attic spaces are also areas that needs to inspected.  Water leaks through roofs are usually noticed quickly, but water entering though a gable end may not be evident.  Much like the rim joist area, if you see daylight, you’ve probably got a water leak.  Back in the early days of house wrap, it was common to only install the water resistive barrier (WRB) to the exterior walls opposite the conditioned space of the home.  This left the gable ends of vented roofs unprotected.  Most builders today know to install the WRB above the soffit area and shingle lap and tape all seams.  Gable end vents can also be a problem, they are a planned hole and need to be treated as such, flashed just like a window opening.  The vent also needs to be designed to not allow water to enter the attic space during a driving rain.

This of course is not a complete list of where and how water leaks into buildings from the exterior.  There are many good resources on the subject.  One of my favorites is “Building Don’t Lie” by Henry Gifford

How do we know if a building material is wet?  Test!  The best tool to test moisture content in building materials is a moisture meter.  I would recommend both a pinned and non-contact meter.  There are methods to test inside wall cavities or at the bottom of a concrete slab without taking apart a building.  A good moisture meter and a properly trained technician are a must.

Photo by Peter Yost

Step 2-Deal with the interior sources

After dealing with any bulk water issues on the exterior of the building, next we need to address any interior moisture problems.  One of the questions I always ask a homeowner during an energy audit or assessment is do you have any moisture on the windows during the heating season?  Often the answer is no, but occasionally window condensation and frost are a problem.  In my climate, the windows can act as dehumidifiers during the heating season.  The window glass can be cool enough to condense water out of the air during periods of elevated humidity, I’ve seen this happen even with triple pane windows.  In hot and humid climates, sweating ductwork and uninsulated pipes can also condense water out of the air.  In both cases, warming the cold, condensing surface above the dew point temperature (insulation), or lowering the indoor humidity level is recommended.  The best way to lower indoor humidity is by finding the moisture source.

An area of a home that can lead to high indoor humidity levels is a dirt floor basement or crawlspace.  I see several of these every year.  The photo is an unvented and conditioned crawlspace in a home with only spot exhaust ventilation present in the home, bath fans and a range hood.  The heat in the home is radiant under floor which also conditions this crawl space.  When I entered this area during a recent building diagnostic inspection, the temperature in the crawlspace was 80°F with 65% relative humidity and there was the unpleasant odor of musty wood.  This moisture was moving up and into the home.  The fix is to add a 6-mil polyethylene vapor barrier to the dirt floor and seal to the exterior walls and any penetrations in the dirt floor.  I’ve been in some homes where these spaces that are not readily accessible.  I was recently in a home where someone cuts an opening into a crawlspace to add forced air ductwork, and then blocks entrance with the ducts.  Gaining access to these spaces and adding the polyethylene sheeting should be completed before any improvements to the building shell are made.

Other sources of indoor humidity can be foundations and basements that leak water from the outside or basements and crawlspaces with no sub-slab polyethylene sheeting.  These can be difficult to fix, a good recent article on the subject is by Martin Holiday, Dealing with Basement Water from the Inside.

Sometimes the moisture source is from something the occupants have or store in the building.  I occasionally see basements used to dry and store firewood, houses with a lot of house plants or even an aquarium without a cover can be a source.  The use of an indoor humidifier, either tied to a forced air system or portable unit will increase humidity levels.  Simply living in the home will also increase humidity, breathing and perspiring, cooking and bathing are all sources, the number of occupants of the home are also a factor.  Homeowner education is important.

How do we know if a home has a bulk water or high humidity problem? Test or monitor.  The least expensive method is to monitor humidity levels by using an inexpensive hygrometer or moisture meter.  I have several scattered around my home, this requires a homeowner to track the data manually.  Another option is to use some sort of data logger or recording device.  These are more expensive but will supply the best information.  Monitoring should continue after any energy improvements are completed on the home.


Air movements can affect both wetting and drying of a material, air moving in and out of a home also influences the energy usage and can affect indoor air quality.

Air sealing is a big deal in the building science community.  Codes now require testing in new construction with air sealing mandates at 3 or 5 air changes per hour at a test pressure of 50 Pascals depending on your location in the country.  There are many articles on GBA, FHB and other publications on blower door testing with regards to both new and existing construction.  Older homes utilize blower door testing for many reasons.  I test many as part of an energy audit and I also recommend testing if any changes are planned to the building shell.  A personal experience I learned the hard way, simply replacing a few windows can change the building tightness, which in turn can affect other conditions in the building.  Using a blower door to find and seal any air leaks is critical if the goal is to decrease energy usage and increase comfort.  The locations of holes and the natural ventilation rate will be unique to each home.

The place I like to start looking for air leaks in my cold climate is in the attic.  Vented, unconditioned attics are common in my market, air sealing wasn’t a consideration in most older homes as they were being built, sealing this location usually has the best improvement opportunities.  Air sealing an attic space should always be performed before any additional insulation in installed.  It may even be necessary to remove existing insulation to expose potential problem areas.  There are a few strategies on how air sealing an attic can be accomplished, they mostly depend on how the attic is constructed.  There are several very good articles on GBA covering this topic, rather than me paraphrasing those articles, here are links to a couple.

Air Sealing an Attic – GreenBuildingAdvisor

Blower-Door-Directed Air Sealing – GreenBuildingAdvisor

Cold-Climate Attic Air-Sealing – GreenBuildingAdvisor

This thermal imaging photograph is an air leak, but the air isn’t directly entering the home in this location.  Instead, the air is affecting the insulating properties of the fiberglass batt insulation by reducing the effective R-value, a phenomenon called wind washing.  Cold outside air is moving through the insulation due to improper blocking and sealing of the eave.

The photos are of a detail I like to use in new construction.  I like to run the exterior sheeting up to the bottom of the top cord of the rafter energy heal, (old houses probably do not have an energy heal), then cover the remainder of the opening with a cardboard vent chute and use closed cell spray foam to seal and add additional R-value to heal area.  This will work well in new construction or during a deep energy retrofit when the home is completely gutted, addressing this issue in an existing home can be difficult.  You’ll either have to squeeze into the eave space in the attic or remove the exterior soffit to gain access.  How you approach this problem will depend on the type of roof and attic present.

After addressing any issues with the eaves and attic ventilation, next are any penetrations leading from the conditioned space of the home into an unconditioned ceiling or attic.

The yellow line on the left of the thermal image shows a plumbing vent that extends from the basement and moves through the attic and out the roof.  The yellow to the right side is an unsealed wire penetration from the attic to the switch and continues down to the next level of the home.  The photo was taken during a blower door test during hot weather.  It’s common in older homes for the holes of plumbing vents, electrical wires and even ducts that enter the attic from the space below to be unsealed.  Holes in the finished ceiling are even more common.  These are mostly lighting electrical boxes and light fixtures, such as recessed lighting.


Air sealing penetrations into the attic from holes in the interior and exterior wall framing will require either attic access or removal of the wall finish.  Air sealing recessed cans is best accomplished from the attic, ceiling electrical boxes may be able to be sealed either from the attic or from the conditioned space.

Other holes in the ceiling air control layer can be from chimney chases, soffits over kitchen cabinets, a soffit used to conceal ductwork, supply and return boots for air handling systems or an attic hatch. 

This is common when ductwork is in an unconditioned attic space.  This duct boot will leak air from the unconditioned attic if an air sealing strategy isn’t thought through before the ceiling finish is installed.  Sealing this boot in a home that is finished is usually best done from the inside using a caulk or quality tape from the boot to the ceiling finish.

I’ve also seen almost an entire finished ceiling leak air.  It’s common in my market for both ceilings and walls to have tongue and groove paneling.  Wall to ceiling transitions are often leaky.  Both ceilings and walls that have tongue and groove paneling need to have proper air sealed before the paneling is installed.  Air sealing after the fact is nearly impossible without removing the wall and/or ceiling covering.  The thermal image is of a cabin where tongue and groove paneling was installed throughout.  Based on what I was seeing with the thermal image during a blower door test, I think the ceiling was insulated using craft faced batts, the kraft facing was unsuccessful as an air control layer. 

I’m always leery of false or dropped ceilings in upper levels of a home.  Often these are hiding something.

A remodel of this home included some structural repairs.  Plaster was removed, the “improvements” completed, new drywall installed but the air control layer was not continuous.  Problems were hidden above a drop ceiling.

After air sealing the attic and ceiling areas, holes in the lower areas of a home are the next most important address.  Rim joists in basements and crawl spaces along with the bottom plate framing are areas I often see issues.

Both these thermal images are of homes with full basements.  Air is leaking under the wall, not the rim joist.  The photo lower was taken during a very windy day when outside temperatures were below -20°F, no blower door needed to produce the effects of the wind.  The upper photo was taken during a blower door test.

There are many utilities that need to move through rim joists and lower wall cavities.  HVAC, electrical, exterior water faucets, gas lines.  All these holes need to be addressed for bulk water management, but often addressing water will also solve for air.  The photo below is one I took of Jake Bruton’s home while it was under construction, Jake has a good strategy for dealing with holes in the air control layer, one hole, one wire or pipe, then seal using a fluid applied flashing, caulk or tape.

The water and air sealing for these penetrations should be done at the sheeting or WRB, not at the siding or cladding in most cases.  When only dealing with air, the sealing can be completed on the interior.

Fibrous insulation does not make a good air barrier anywhere in a home.  Older homes often used fiberglass at rim or band joists, around windows and doors, and even stuffed between rafters at the heal of truss.  If you need proof fibrous insulation can leak air, here’s an older home that had a poorly installed air barrier.  The fibrous insulation did little to slow the movement of air through this home.

The dirty fiberglass insulation simply acted as an air filter, capturing the dirt and dust before it entered the home.  Notice how the dirt is concentrated at the lower end and corners on the exterior of this home.  In my very cold climate, air during the heating season will infiltrate low in the structure and exfiltrate high, at or near the ceiling.  This phenomenon is called stack effect.  I would expect the wall fiberglass insulation in this home to be dirty on the interior near the ceiling, the result of filtering the exfiltrating air near the top of the building.

Many older homes used boards as the exterior sheeting material.  Often these boards were originally covered with 15 lb. felt.  If the planned openings (windows and doors) are properly flashed, the felt paper works great as a bulk water management detail, but the air sealing of all those board seams is non-existent.  This is where a peal and stick water resistive barrier can double duty as both the WRB and air control layer.  This is a great choice if the siding is being replaced.

What if the exterior is not being removed but there is a planned renovation on the interior?  We can then move the wall air control layer to the inside of the exterior walls.  This is often accomplished in my climate by using 6 mil polyethylene sheeting installed in an air-tight manor.  I personally prefer to use a smart vapor control product as the air control product.  Installing airtight drywall is also an option.  The air control layer needs to be continuous; floor, wall and ceiling intersects need to be well thought out.

Any planned or punched openings are also important details to get right when air sealing.  As I said earlier, many older homes used fiberglass to insulate around windows and doors.  There are much better ways to deal with air leaking around these planned openings.  Minimally expanding spray foam, backer rod and caulk, or tapes can all act as good air seals.  It’s best practice to leave the exterior bottom sill of an installed windows open on the exterior to allow any water that should enter around the window or window opening to drain out.  Expanding foam may hinder draining.  Backer rod and caulk or an interior window tape might be best at this location.  It’s also important to seal the bottom of doors.  This is typically accomplished by using a bead or two of caulk or another sealant at the bottom of the door.


Two ways we improve the energy performance and comfort of a home is by reducing the movement of air and slow the movement of heat through the building envelope.  This section is about insulation and slowing heat loss or gain by adding or changing the insulation strategy of an existing home.

Much like air sealing, the attic can be the area of the home with the best return on investment when it comes to insulation improvements.  This will depend on the type and accessibility of the space.  Convoluted ceiling designs with many different rooflines and cathedral ceilings are some of the most difficult roofs to insulated (and air seal) correctly both in new and existing housing.  One of the most common questions asked on GBA’s site is how to properly insulate a cathedral ceiling.  In an existing home, the best way may be to remove the ceiling finish and start over.  Occasionally this is the advice I give during an energy audit and definitely not what a homeowner wants to hear.

The roof in the above photos would require a different approach to correct.  Improving the thermal performance can only be accomplished from the exterior.  The way the roof was constructed, the 1 ½ inch tongue and groove paneling was installed over the log trusses, tar paper was then added.  2 x 6 framing was fastened to the paneling creating a space for R-19 fiberglass insulation.  The roof sheeting and roofing materials where then installed.  It is impossible to just remove the interior finish.  This approach of removing the exterior portion of the roof may be possible in other cases when the roof shingles (or other roofing materials) are replaced.

Adding insulation to the exterior of the roof can also be an option.  There is a very good article on Green Building Advisor on how this is accomplished.  How to Install Rigid Foam On Top of Roof Sheathing – GreenBuildingAdvisor  The correct ratio of exterior to interior insulation needs to be followed based on the climate zone of the home.  Other important consideration, air sealing should be continuous and there should not be an air space between the interior and exterior insulations.  This air space is often present in vented roof assemblies.

Conditioned attics are another space in existing homes that can benefit from insulation improvements.  Often these attics are full of mechanical systems that will complicate the insulation process.  Multiple trades might need to be involved.  This may lead to other opportunities, such as improving the duct systems or adding a dehumidification or whole house ventilation system.

If you are lucky enough to have an attic with ample amounts of room to move about, adding insulation is straight forward.  The first step is to air seal the attic from the conditioned space of the home.  Air sealing the space is usually more important than simply adding insulation.  Next, add the insulation, I prefer to use blown cellulose whenever possible, but other insulations can work well.  Make sure any eave ventilation chutes are not blocked by the added insulation if the attic space is vented.

How about walls?  I’ve seen evidence on many energy audits of homes that have had their walls dense packed with cellulose.  This was common practice if a home received a weatherization improvement from the 1980’s through the early 2010’s, at least in my market.  Several of the insulation contractors in my area will no longer perform dense packed cellulose work on existing homes, mostly due to issues they have had in the past.  Not being able to see into the wall while they are blowing the insulation can be a risk.  Again, dense packed cellulose is not common in my market, other areas of the country use this method in both existing and new construction with success.

I have been involved in several building diagnostic inspections on recently renovated homes where closed cell spray foam was the choice for wall insulation.  This typically requires either an interior gut remodel or removal of the exterior siding and sheeting.  I’ve seen both successes and failures to this method.  The failures I’ve seen have been the result of an air leak that the CCSF was not able seal.

This usually presents as water dripping out the siding high on the wall during the winter months.  There is an insulation method that can prevent wintertime condensation issues such as the one in the photo, and it happens to be my favorite way to improve wall insulation levels.

Adding exterior insulation to a wall can have many benefits, but also creates a few challenges.  The main benefit is the first condensing surface, typically the wall sheeting can be warmed above the dew point temperature.  There is no longer a concern with warm, moist air contacting a cold surface, that is as long as there is enough exterior insulation for the climate.  The recommended ratios of interior insulation to exterior insulation for walls are found in table R702.7.1 of the IRC.  This table shows the ratios needed to move from a class I or II vapor retarder to a class III retarder, which happens to be the point where the exterior insulation level is thick enough to prevent the issues experienced in the above photo.  There are a lot of considerations when choosing to add exterior insulation to an existing home.  The vapor control product present, the drying potential of the insulation choice, the presence of a venting rain screen, the type of cladding, and how to detail window and door openings all need to be thought through. 

The above sketch is an example I use to show customers how both exterior insulation on the wall and roof would work on their homes.  The window door openings will present a challenge. 

Photo by Chris Laumer-Giddens

Having to deal with this assembly will give most contractors nightmares, though once you’ve done a couple, they really are not as hard as they look.  There are several different methods, having a conversation with an insulation and siding manufacturer and working through the details with the installing crews can help.

On a side note, manufacturers of many products have services available to make sure a builder is successful in using their products.  Whether that is a site visit by a factory rep or hygothermic analysis of an assembly by a building scientist on staff, many manufacturers have help available.

The rim joist area, both in basements/crawlspaces and between two floors are another place that is often a missed opportunity in older homes.  A lot of the homes I visit have no insulation (or air sealing strategies) at the rim.  This can be a tough area to properly insulate.  First, you need to have access to the space.  Finished ceilings in basements and between floors or heating or other mechanical equipment placed against basement or crawlspace walls may limit access.  Penetrations leading to the outside, like electrical service conductors, ducts and outdoor faucets all need to be insulted around.  This is one area where I like to use closed cell spray foam, though a cut and cobble method using rigid foam can also be successful.

Insulating the foundation or walls of a basement is another question that is often asked at Green Building Advisor, the topic has many blog postings dedicated to the subject.  How you approach insulating this space will depend on several factors.  The climate, foundation type and condition, and how the space is used will all affect any strategy.  A couple examples, if you have a stone foundation, using a sheet foam probably isn’t a good choice, or if you have a basement or crawlspace that leaks water, you want to deal that issue before insulating.  As I said, there are many good posting on the GBA website, another very good resource is at Building Science Corporation: BA-0202: Basement Insulation Systems | Building Science Corporation   

One of the tricks to improving a home’s performance is to first address water, both in bulk and vapor form before making any changes to how energy moves through a home.  Peter Yost likes to say, “energy and moisture must be managed with equal intensity.”  That statement is so true in both new and existing buildings.   Next we want to tackle air sealing.  How do we know where to concentrate our air sealing efforts?  Test.  A blower door test will not only indicate how leak a home is by the information it provides, but it can also help to identify where the leaks are located.  Lastly, we look at improving insulation to slow the movement of heat through the structure.  Each of these control layers need to work as a system.  Often these upgrades will involve a team of contractors and manufacturers working with each other.  Success will depend on formulating a good plan based on inspecting and testing the home, and then implementing that plan using methods and materials that bet suit objectives.


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