Build Your Home on the Green Gauge

Asphaltic fiberglass, least expensive grade, approximately 215 PSF

5
points

$ - While this is the choice of many developers and spec home builders, because of its intial low cost, these shingles do not last very long, sometime 7 years, others 10 to 12 years, which means you will have to replace your roof multiple times during the life of some of the other options for roofing, which means that ultimately this choice will actually cost More than other choices and this choice involves a petrochemical (asphalt) as a main ingredient, which, as we all have seen during the last several years, can be costly in a shortage situation and we have been hearing that the free ride of oil is about over and that we have to find a different source of energy and product substitutes that are not fossil fuel based. Despite manufacturer's claims, we have never seen a fiberglass asphaltic roof shingle that did not grow mold and mildew. When this happens, people pressure-wash the roof. That in turn, blows away a huge percentage of the shingles' embedded coating of mineral granules, which will expose the asphalt to the sun, which will make them age even faster. Some contractors say that these roof shingles might be in the $0.30 to $0.60/sf range for material, and perhaps $0.40/sf for installation labor, = $0.70 to $1/sf installed price. All costs will vary for your location (up or down) for all items and you need to verify all costs with your contractors and material suppliers

Asphaltic fiberglass, 340 PSF

25
points

$$ - See information above for comments about fossil-fuel based oil derivative building materials. Also, it is interesting to note that we assigned a "greener" value to this choice, for reasons having nothing to do with its asphalt nature, but rather, that these thicker shingles will simple last longer. Lasting longer means that there will be less energy involved in replacing them, because you won't have to have replacement materials as often. These shingles may last as long as 25 years +/- -see manufacturer's warrantees. These shingles are more expensive than the thinner ones. These roof shingles might be in the $1.50 to $2.50/sf range for material, and perhaps $1/sf for installation labor, = $2.50-$3.50/sf installed price +/-.

Cedar Shingles, which are thinner than shakes

13
points

$$$ - We have ranked this choice surprisingly just below the thicker asphaltic fiberglass roof shingles, mainly because thinner cedar roof shingle typically don't last more than about 7 years (at least in our area of the country) and we have seen instances where ridge shingles have been replaced on neighbor's homes within 4 years. They look great. However, the roofers that install them have even told us that cedar shingle crack and that there are many hard joints through which water can pass, however, when water hits them, they sell up fairly quickly, meaning that they seal many of the cracks, making it harder for water to pass through them. Although we provide these for clients who want a rustic roof, I personally don't want this on my own home. Also, there is a matter of probable increased fire insurance, due to the combustibility of your roof, which can be a real problem if there are burning embers being transported through the air from forest fires or neighbor's fireplaces or even your own (even though spark arrestors are supposed to contain this hazard). Costs vary, perhaps from $1.50/sf for material and $1 to $2 installation = $2.50-$3.50/sf installed price.

Pressure Treated #2 pine roof shingles

20
points

$$$ - Not many people know about these. It is the same wood that you might use for an outdoor deck. The preservative chemicals embedded in the wood are safer than those used in the past, according to manufacturers, and the wood lasts much longer. We have heard 15 years+. Even though we believe this to be a longer-lasting choice than untreated cedar shingles, it is still a wood roof and under the right circumstances, can still burn. We like green and we like wood, but we also want our clients to be as safe as possible and to have as little maintenance as possible.

Cedar Shakes

18
points

$$$$$ - These are thicker than standard shingles, which means they will last longer. They can still burn and can still crack, and will. While I love the look, a roof, in our opinion as luxury home architects, is no place to place temporary aesthetics above safety and ease of maintenance (as in reducing leaks). We use them where clients want them, because it is their house. We do like the fact that wood has less embodied energy, which means that there is less energy used to manufacture the product. We believe this will likely cost at least $1 to $2/sf more than Cedar Shingles, depending on options.

Copper

50
points

$$$$$$$ - this is an excellent choice and should last longer than you do, or your grandkids. This is essentially deemed a permanent "forever" roof. I have seen original copper roofing and flashings on buildings in Europe that are hundreds of years old. This is, however, very expensive for intial cost.

Aluminum, factory painted enamel

26
points

$$$$$ - We like aluminum, however, any course in sustainability marks down aluminum because its manufacturing process requires a lot of embodied energy. In other words, in the factories that refine and process the aluminum, there is a lot of fossil fuel burned to heat the mined materials and refine it to become the aluminum we see as consumers. Also, while the aluminum will likely last a long time, the cheaper paints, like enamel, will probably not and will fade, chalk and even peel and could require field painting several times during its lifespan, to keep it looking good. You can also obtain an architectural grade of paint called Kynar, which most commercial architects like, because it is softer and flexible, hence stays bonded for a longer time to the surface on which it is factory-applied. Of course, this costs more. Some middle ground paints would be in the siliconized polyesters.

Galvanized steel, factory painted enamel

30
points

$$$$ - Factory painted galvanized steel is typically less expensive than aluminum, with the same factory paint. For this reason, the steel is often used in many commercial metal roofing applications and for some residential roofs. This is a reasonable quality long-lasting roof, depending on your exposure (not by salty ocean sprays which will rust them swiftly). Same comments about the surface paints as for aluminum. Some people actually use the exposed galvanized panels, which is rare, because of the industrial silver appearance that most people do not appreciate. This roof should typically last longer than most low to mid-grade asphalt shingles. Cost: $2.50 to $3/sf material cost + $1.30/sf labor = $2.80-$4.30/sf installed +/-. We have seen some finished options throw this into the $6.50/sf range for the really nice metallic choices.

Slate faux, recycled rubber and plastic

65
points

$$$$ - In our opinion, this is a very green choice and looks great. The rubber and plastic, while at first glance may appear to be using petrochemical products, in fact, these roof shingles are manufactured from Waste rubber and plastic created from the primary manufacture of commercial "single-ply" roofs and other incidental waste products that would otherwise end up in a landfill. Rather than polluting our environment in a gigantic mound somewhere in the country, these roofing tiles grace our roofs. We have one of these ourselves and it has a 50-year warrantee from the manufacturer. For many of us, that is just about the rest of our lives and in a resale, should be very appealing to the buyers, because the manufacturer certified installations can have a transferable warrantee. The fact that only enough embodied energy is used to remelt the waste rubber and plastic to mix them and mold them (these tiles can resemble slate and wood shakes and other faux materials) only enhances their appeal. Costs: ours cost about $5/sf installed price

Slate faux, fiberglass reinforced concrete

70
points

$$$$$ - This gets a rating just a bit higher than the choice above, because fiberglass and concrete are the main ingredients and neither is a petrochemical product. You can't get much more environmental than concrete, because not much energy goes into its manufacture and those ingredients are found nearly everywhere, which is why, when the famous 20th century Swiss-born architect LeCorbusier designed his facilities in Chandigarh, India, they were out of cast in place concrete, which is labor-intensive (which they had in abundance) and low-tech. These concrete roofing tiles might carry a 20 year warrantee, but are likely to last much longer. They are brittle and can possibly be damaged by large hail. They are non-combustible. Costs: the contractor on one of our projects said this cost about $5.50/sf in place. I think today, it might be a little more.

Slate genuine

80
points

$$$$$$$ - This is an extremely green choice, because it is a natural material that doesn't pollute in any known context. This is much more expensive that just about any other roofing choice and should actually last longer than even copper, although copper is the logical choice for a real slate roof, as why would you select a forever roof with joint metal that will need to be replaced frequently? Our green rating for this material assumes that you are obtaining the slate roofing tiles from sources within about 500 miles of your project site. If you obtain them from farther away, the energy required to burn in the trucks will offset the "greenness quotient," meaning that you would be using too much fossil fuel to transport your slate from afar, which reduces the environmentally positive aspects of this otherwise outstanding and centuries-long durable roof. Are you beginning to see that for some green options that the initial cost can be higher? Green quite often is synonymous with quality. Do it once; do it right, takes less energy in all senses of the term. Also, slate tiles of standard thicknesses can weight 7 to 8 pounds per square foot, which is a higher dead load on your roof than standard asphalt roofing shingles (which might be from 2.25 pounds to 4 pounds per sf) which means that your roof decking sheathing and the roof trusses and beams and joists will likely need to be upsized to accommodate the greater roof load, which increases their cost somewhat. Slate roofs can run between $10/sf to over $40/sf depending on roofing complexity and slate selection and shipping. Typical Costs: this can vary: $5-$8+/sf for material + $2+/sf labor = $7 - $10+/sf installed price.

Standard Conventional roof trusses with gangnailers + R49 fiberglass batt insulation

95
points

$ - This is the bargain of the century and very green. This is integral to our "Going Green 4 Less" Green Design Initiative. Why? It uses skinny, economical pieces of wood to make the roof trusses in a standard, local lumber plant truss warehouse. These trusses are extremely strong, yet downright cheap and they can clearspan over most homes, from 24' to nearly 60'. Amazing. The 15" thick single layer fiberglass batts cost only around $0.74 to $0.85/sf for material cost, which is only a few more pennies per sf than standard, code required 9-1/2" thick R30 or 12" thick R38 batts. We have asked local insulation installers if they would charge anymore for the slightly thicker R49 batts for labor only and they said they would not charge a penny more for the labor. The extra weight is only a few more ounces. So, you get 22%+ more roof insulation in your ceiling/attic for just a few more pennies per square foot. What a deal! Also, the fiberglass from this particular manufacturer obtains much of the material from waster by-products of glass manufacturing, so this is essentially, built-in recycling of material that might otherwise go to a landfill. This gets 95 out of 100. It does not get a perfect score, because you have to ventilate the fiberglass batts to prevent anaerobic bacteria from growing, especially in warmer, wetter months. We have actually created an attic ventilation system controlled from automated switches within your home to deal with this and give you some measure of control over this, reducing the amount of attic ventilation during the winter when cold winds are blowing. This choice has a stronger Weight, because, as any mechanical engineer can tell you, your primary heat loss and gain is through the roof of most conventional structures. Costs: about $11.75/sf +/-

SIPs 9-1/2" to 12" thick & some LVLs

60
points

$$$$ - this initially seems like a great idea, until you begin to understand that the SIPs alone can't do the job. You need ancillary supporting beams called LVLs and possibly even timber frame supporting systems and neither are free, although for this study, we are Not including the timber framing, but we are including some LVLs. Also, you have thicker roof edge trim and a host of other unexpected costs, which drive this choice to be nearly double the conventional choice above, and for a reduced R value. It appears that you may be able to control infiltration better with SIPs, but that is a matter of opinion, as all wood shrinks, and gaps could occur around the perimeter of SIPs, just like around conventional framing. We do like the fact that you do Not have to ventilate the EPS insulation in the SIPs. See Wall SIP information below. Costs: about $18.25/sf +/- for the entire impact of all related systems, not just the material cost of the SIPs.

Conventional Roof Joists and Built-Up Beams of Dimension Lumber

65
points

$$$ - many people will be surprised to see the price tag associated with this. It is due to increased labor and construction time. The material cost is the cheapest. However, in the prefabricated economical roof trusses option above, most of the roof trusses can be installed in a day or two or three, depending on the size and complexity of your homes roofs. Because they can clearspan your house. Dimension lumber for roofs is typically 11-1/4" x 1-1/2" maximum. These are roof joists cut out of solid trees, such as #2 Southern Yellow Pine on the eastern side of the USA. If you want gable roofs, you have to have center beams and girders made of LVLs or built-up sections of dimension lumber. To control horizontal thrust, you have to install tension collar ties and other members to result in a stable structure. Each piece of wood has to be custom measured, cut and fit, often without much engineering, if at all. We have noticed that some contractors prefer this option, as it means that they have to spend more time on site framing with their personnel, which means that their clients have to pay them more for their labor. We do not believe that this constitutes a good value. We believe that this approach should be avoided if at all possible. Most clients will not even realize that they are paying extra for something that could have been accomplished by simpler means. Most reputable contractors will advise their clients of this fact and we have been blessed to mainly deal with them. So, due to the much higher labor cost, which represents higher energy levels in the construction, this approach, in our book is downgraded, to be only slightly better than SIPs. Also, because you are dealing with 11-1/4" maximum dimensions for roof sandwich thickness, and that you have to ventilate conventional insulation, the maximum standard thickness insulation would likely be a 9-1/2" thick R30 batt, which these days isn't as good as it could be and is only code minimum in many locations and won't even meet code in other places.

2x4 Stud Walls

20
points

$ - You can't get much cheaper than this. Unfortunately, the typical R-value of fiberglass insulation in a 3-1/2" thick wall usually is R11, although some performance batts claim up to R14 or so. There are all sorts of options for wall insulation and we will examine that separately. This is Not considered a very green choice, in our opinion. The wall is just too thin. In colder climates, this not only can be the cause of frozen plumbing pipes, they can also be illegal, because you cannot install enough insulation in them to make them energy losses reasonable. Also, we have encountered much more infiltration (unwanted cold air passage) through this thin of a wall during winter months.

2x6 Stud Walls

40
points

$$ - This, in our opinion, should become the default home wall. 5-1/2" affords much more space in which to stuff insulation. With standard (i.e.: less expensive) fiberglass batt insulation, you will typically have an R19 and with performance batts to about 21. Typical 2x6 walls with insulation and wall finishes and associated air films result in combined R21 or so. Once again, there are many other insulation options, which we address in another question.

SIPs - Structural Insulated Panels - 6"

19
points

$$$$ - The sellers of these panels have done a great job in marketing them to DIY shows and to other TV networks illustrating an interesting new type of home technology. They sound really neat. Architects like ourselves have a bias in favor of SIPs. We think they are nifty. You get your insulation and your structure all in one panel. Sounds great. Unfortunately, the construction realities and costs are another matter. First of all, subcontractors typically hate them. Particularly HVAC, electrical and plumbing. If the pre-planned chase locations end up only an inch or so off from floor to floor, they can't get their conduits, pipes and wires through them. They end up wanting to cut the face of the panels with a skillsaw so that they can cut a realigned chase and you know what? That ruins the structural integrity of the SIP. Why? Because SIPs pretty much rely on the 15/32" OSB (sort of like plywood) facings on either side at the main structure. Even the EPS insulation in the core supports weight. Cut the panel face and you have destroyed the structural capabilities of the panel. If you have never actually been on a job site, I had it explained to me by an 80-year old architect in my 20s: "Son, you aren't building a watch here. You are building a building." Meaning: precision on a job site is relative and construction tolerances of much more than an inch are difficult to achieve. The systems you use need to be forgiving or you get problems. Also, SIPs are inherently more expensive. The core EPS insulation is more expensive than fiberglass and the manufacturing process makes them more expensive and there are special installation considerations that make them more expensive. In terms of greenness, they are okay, but the cost factors and problems during construction and structural stability if someone damages the faces make this, at least in our book, not as attractive, or as economical as a stud wall. There is also an implied assumption with SIPs that you will likely be using a timber frame structural system, because SIPs really aren't very effective at handling long, unbraced runs of wall. Timber posts help stabilize the wall panels. Timber frame structural systems + the SIPs in a recent project of ours was bid by a subcontractor around $61.50/sf more than conventional construction. On a 3,000 to 4,000 sf house, this means that using SIPs + timber frame could = $184,500 - $246,000 more than conventional construction. I guarantee you that the manufacturers of the SIPs and the timber frames will try to convince you that it costs less, but when push comes to shove, they will not give away their interesting products for anywhere near the price of conventional stud and standard truss framing. It becomes a matter of your taste and budget. We happen to design hybrid systems that deliver the Look of timber frames for pennies on the dollar using conventional systems.

Steel Studs - 6"

30
points

$$$ - We like steel studs. They are straight and strong and those are real pluses. They do transmit more heat loss and gain, because they are denser than wood. Carpenters generally don't know how to deal with steel, unless you get a crew used to doing office building partitions from a big city, or steel stud crews from theme park work. They cost more than wood, because of the manufacturing process and embodied energy that goes into making them. They are non-combustible and insect and vermin proof and can't rot, although, if exposed to a leak, they can rust. I actually built a home using steel framing like this in Florida in 1996. My guess is that I probably spent an extra $10-$20/sf for the privilege. Did it make any difference when I sold it? I don't think I got an extra penny. Would I do it again? Probably not, unless I was going to live there the rest of my life. You find out pretty quickly, when you are selling your home, that things you put a lot of extra money into, the buyers really don't care much about. So be careful out there. In terms of greenness, the embodied extra energy makes them not as attractive as a 2x6. Still, a 6" steel stud is better than a wood 2x4, mainly because you can put thicker insulation into the wall.

Wood siding -vertical board and batten

90
points

$ - This is another of our "Going Green 4 Less" choices, because, there is very low embodied energy in the fabrication of real wood. Also, real wood cost less than any manmade product.

Hardiplank

75
points

$$ - Not a bad choice, but this is manmade and will cost more than real wood. This means there is more embodied energy in the product, too, so it is Not as Green. Also, in our opinion, you can start picking up the grain repetition aesthetically and know that it is not a genuine material. And, contrary to popular opinion, you Do have to paint it.

Poplar bark shingle siding

80
points

$$$ - interesting for mountain-style homes for accent areas. You can't get more natural than this! The material and labor, however are both more expensive than more conventional wood sidings.

Stone

110
points

$$$$$ - very expensive and very nice looking and it is entirely natural with no embodied energy used to make other that Mother Nature's, unless you use cultured stone (man-made). This typically cost $12/sf for the material and $12/sf for the labor around WNC, USA. If you can get it for less where you live, good for you.

Steel insulated panels

25
points

$$$ - Not a very good choice, at least as far as the Green Gauge is concerned. High embodied energy in the metal and in the exotic rigid insulation in most of these types of panels.

Aluminum siding - conventional

20
points

$$ - Cheap and looks it. But, not as economical as real wood siding. High embodied energy. When swapped out, it ends up in a landfill and stays there forever. Not green.

Vinyl siding -conventional

20
points

$$ - Yuk. Cheap and not nice. Petrochemical based. Forever in a landfill. Not as high embodied energy as aluminum. Still not as economical as real wood siding. Joints are visible and the embossed grain visible and obvious to anyone but your dog.

Brick - conventional 3-5/8" thick standard

35
points

$$$$ - greener than aluminum or vinyl siding, but a lot of energy is required to fire the bricks in a kiln. Clay is natural, but there are also some other colorants. Expensive to install, like rock. Not as good a score as rock because of the manufacturing energy required.

Log Home Walls - approx. 6" to 8" thick - prefab system

40
points

$$$$ - A lot of people do not understand the high price of this option. Many log home companies will tell you that their log home package for your house can be had for $35 to $55/sf. Sounds cheap for a home, doesn't it? Unfortunately, what many people do not realize, until much later, is that those small charges only pay for a big pile of logs dumped in your driveway. That price does Not include assembly, your foundation, electrical, plumbing, HVAC, windows, doors, roofing insulation, cabinetry, your sitework, water source, sewage and on and on! Not such a deal, once you know that. We have found that a log home package in fact Adds at least the cost of the "dumped big sticks" cost to the price of a conventional home. Has anyone else noticed the drastic attrition of log home companies these days? People cannot justify spending $35 to $55 More than conventional construction to their home construction cost. Some people do. I actually enjoy designing real log homes and we are one of the best at it in the World, just check out our Page 1 ranking on Google in that category. Just because we happen to be good at that does not mean that we believe that they are least expensive value for homes in the World today. They are not, nor are stone castles with beautiful spires. It will come down to a question of what you want and what you are willing to pay for it. We are trying to educate people and correct the common myth that a log home costs the same or less than a conventionally built home. It does not. Other than a mobile home, there is probably not much of anything that beats the cost of a conventionally built home. I do this for a living. For 40 years. Most believe will not believe me and will have to discover these things for themselves and I encourage them to do so. Okay. On to greenness of log homes. Very green, in terms of lower embodied energy, as long as they are prefabricated and shipped to you within 500 miles. Not as green as some log home manufacturers may intimate, in terms of insulation and infiltration. It takes a log about 8" +/- thick or so to start being about the same as a 2x4 wall with batt insulation +/-, depending on the species of the wood. This is not the best in terms of insulation value. Also, I have personally seen log homes where I could see through the settling logs to the outside. Of course, this admits outside air and wind into your home. All logs continue to dry and wood shrinks as it loses moisture content. A lot. I have large timbers in my own home, and for the first 2 years, during cold winter nights, when the air was very dry, we would be startled awake by what sounded like a gunshot in the dark! It was cracks snapping in the timber beams and posts, as the wood dried. I have seen some timber member changing size by as much as an inch. That's a lot! Sometimes, these cracks can go right through a log, meaning that so can air, and the logs can settle at different rates and non-uniformly, opening up gaps between them through which insect and air can travel. So, genuine log homes do not rank very high in our books as far as greenness for insulation and infiltration. But we still like how they look and appreciate that some of our clients want them, so we create them for that select clientele, doing our best to make them tight and efficient.

SEER LEVEL 13

25
points

$ - standard efficiency nothing very Green about that

SEER LEVEL 14

28
points

$ - marginally better than standard

SEER LEVEL 15

30
points

$ - a little better than standard. This is will begin to result in improved efficiencies for you're A/C unit and some modest change for slightly lower power bills.

SEER LEVEL 16

32
points

$$ - now we are starting to do something much better than just getting by.

SEER LEVEL 17

34
points

$$ - this SEER may or may not be available

SEER LEVEL 18

40
points

$$ - this SEER may or may not be available

SEER LEVEL 19

50
points

$$ - this SEER is a major improvement over standard requirements for energy efficiency and your use of less power to make your home comfortable will pay you back each month in lower energy bills

SEER LEVEL 20

75
points

$$$ - about the highest efficiency you can get these days for a residential air conditioner. An extremely Green choice, because of the reduced energy consumption and your lower power bills

Electrostatic Air Filter

60
points

$ - Highly recommended - very Green - for much cleaner Indoor Air Quality Management for a whole-house filter. No more paper filters to trouble with each month or so. Up to 99.98% of airborne allergens can be removed from your filtered air with a system like this. Highly recommended for cleaner indoor air, particularly for people with respiratory situations, per the manufacturer's testing.

Energy Recovery Ventilator - ERV

60
points

$ - Highly recommended - for improved energy efficiency and cleaner Indoor Air Quality. Exchanges stale indoor air for fresh outdoor air. Retains heat and moisture from exchanged winter air and pre-conditions and dehumidifies incoming summer air, to reduce mold and mildew situations that can occur from uncontrolled moisture-laden summer air might enter your house otherwise. Transfers energy from the exhausted air to the intake air for improved energy efficiency. Highly recommended. I used to handle "sick building" corrections for large commercial projects that I had not created in the first place. I was called in to correct the problems. One of the main things each mechanical engineer did on each project was to install a preconditioner so that incoming air was pre-treated before being run through ductwork and into the interior spaces and into peoples' lungs.

Improved Sealing of your home

80
points

$ - Very highly recommended. This exceeds most building codes and energy efficiency requirements. You need to control how air enters your home, preferably through the ERV above and have your home Positively pressurized. You do not want your home negatively pressurized, as that leads to uncontrolled "wicking" of moisture in summer and frigid air in winter, neither of which you want in your home. To control the air pressure in your home, you have to seal it well. This can be accomplished, not only with conventional batt insulation, but with strategic use of expanding foam insulations with surgical precision, sealing the cracks and crevices sparingly. Most air infiltration does not occur Through materials, but around their edges and joints. So, use of expanding foam at those locations during a single afternoon with 2 workers can help tremendously in sealing your home. Then, it will be important to have the ERV above, so that your home does in fact have a mechanically controlled fresh air intake that you control.

Double Hung windows

20
points

$ - lots of cracks and crevices for uncontrolled infiltration to enter your home, even when closed. This is because the upper pane frame (called a sash) and the lower one move vertically and in order for them to be able to slide, they have to have some leeway and this is where the air enters your house, without your permission

Casement windows

60
points

$$ - Great choice! Casement windows are like little side-hinged doors and they have excellent weatherstripping against which the sash compresses with a hand crank, and there is a little arm that you flip down to compress the seal even better. Much less infiltration. They do cost more than double hung windows, but in our opinion, are worth it, if you want to have half a chance at controlling the source in air coming into your home.

Single pane glass in doors & windows, clear

10
points

$ - Not Green at all! Bad choice. Illegal in some areas of intense cold. Heat transfer in both summer and winter is nearly transparent, which is not a good thing when it comes to glass.

Double pane insulated glass with Low-E coating that appears transparent

70
points

$$ - Excellent choice! You will not notice the clear Low-E coating, which does a really good job of controlling summer heat energy trying to enter your home, and that air or gas space between the panes of glass is a miracle worker at helping diminish heat transfer from interior to exterior and vice-versa.

Solid aluminum door and window frames

10
points

$ - Not Green. Direct heat transfer and high embodied energy in the manufacture if this metal.

Solid wood door and window frames with exterior factory painted aluminum skin

40
points

$ - Much better choice than just solid aluminum. The wood does a better job at insulating the frames and the aluminum skin protects the exterior of the wood so that they don't rot as they would if exposed (although some window manufacturers debate this issue.