Houses built out of hemp instead of traditional building materials leave a ‘better than zero carbon’ footprint, according to new research out of the UK.

Aside from helping to combat global warming, building homes from the cannabis plant could also give a boost to struggling rural economies. That’s good news almost everywhere except for within the U.S., where industrialized hemp is still illegal to grow under federal law.

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This is no more a black and white issue than most other green building questions. This can depend on the particular situation and the specific needs of a particular project. I'm not going to give you a definitive answer, because I don't think that there is one, any more than I do for most green building topics (other than greener is better).

First, there is the issue of material content. On the one hand, the manufactured products often use some combination of wood fiber (which is often sawdust and other scrap that would otherwise go to waste) and plastic (sometimes incorporating post consumer recycled material). On the other hand, wood is a natural material. It is sustainable, in that wood can be grown and harvested. There are some deck materials that have natural rot-resistant properties, but these tend to be more expensive. There is also the question of whether or not they are sustainably harvested, as well as the issue of shipping these materials.

Maintenance is another consideration. Wood decks need to be stained and cared for on a regular basis. Even with regular maintenance, a wood deck will wear out over time, and pieces will need to be replaced eventually. Maintenance for the manufactured products is supposed to be lower. However, I have heard of more than one architect I know who put in Trex decks for their own homes and now have some problems with its performance. Other manufacturers are competing in this field now, as well, and there may be better products available.

End of life is another question to be addressed. Making a material like Trex is a downcycling use, and there isn't much that can be done with an old manufactured deck other than to landfill it. Wood decking, if it is untreated, can be allowed to decay naturally. But, if the decking has been treated, it is not going to break down readily. Decking treated with preservatives (whether it was the older CCA — chromated copper arsenate — which is now outlawed because it was leaching arsenic, or the newer ACQ — alkaline copper quaternary compounds — or one of the other current alternatives) is meant to not break down. In the long term, it will eventually decay, but it will be a much slower process. According to a Wikipedia article, "A report published in Wood and Fibre Science (Vol 36 pp 119-128, 2004) concluded that soil contamination due to the presence of CCA-treated wood after 45 years is minimal." In the interim, however, you probably don't want it in your compost.

Finally, the workability and durability of the material factor into the question. Contractors and installers like the manufactured products because they are easier to use. The boards are straight and regular. Warping, twisting, and cupping defects are not an issue. However, many of the manufactured deck boards have a noticably fake appearance, or do not look like real wood at all. I think a case can be made for either product, as long as one takes a careful look at the material sourcing, with FSC certification for real wood or high recycled content for a manufactured product.

The most obvious improvement offered by using ICFs is the addition of insulation. Concrete has a very low R-value (an 8" thick concrete basement wall would typically have an R-value of approximately 0.75; even less than a single-glazed window with an average R-value of 1.0). So concrete walls offer very poor thermal performance. Even in the summertime, a concrete basement wall will be cool to the touch, because of this. Adding even a small amount of insulation to the concrete wall makes it better, and ICFs provide a good way of getting an insulated concrete wall.

This article is going to be somewhat less useful to owners of existing homes in general, since installing foundation walls is something that is usually only done in new construction. But understanding a bit more about how foundations work can help when it comes to improving the performance of an existing basement space, as well.

Contractors like them because they reduce labor costs. In typical construction, concrete walls are built by setting up reusable forms which contain the concrete and mold it into its finished shape. The forms are heavy and hard to install because they have to be strong enough to support the weight of the wet concrete and hold it in place for a period of several days to a couple of weeks, until the concrete has cured. Then, workers must be sent back to remove the forms and carry them to the next jobsite. With ICFs, the blocks that are used are much lighter and are easily carried by one person, making the preparation of the forms easier and less labor intensive. ICFs also save after the concrete is poured, because the additional step of removing the forms is omitted. The ICFs are left in place as part of the building.

Early ICFs were basically two pieces of rigid insulation foam held together with plastic ties. This would result in a full thickness concrete wall with rigid insulation on both the outside and the inside. They were originally developed for the speed of assembly and the labor savings, and the insulation was largely an added benefit.

Eco-block is an example of this kind of ICF, although they also offer configurations with thinner concrete cores (as little as 4", which is still structurally sound). More recent versions, such as Insulock, use a block that looks like an oversized Lego made of polyurethane foam. These closed cell foam blocks are lightweight, and can be easily carried and quickly installed. The open cores of these blocks are then filled with a steel reinforcing rod and poured concrete to create a structural wall with properties much stronger than a wood stud wall, much less concrete needed than a traditional poured concrete wall, and much better insulation than either one. Depending on the required wall strength, it may not be struturally necessary to fill all of the cores with concrete, leading to even more savings in materials.

Having insulated foundation walls makes for a more comfortable and more energy-efficient basement. Even for concrete slab-on-grade construction (where the concrete slab floor sits directly on the ground), having insulated foundation walls helps keep the concrete floor at a more comfortable temperature, and keeps the cold from conducting through the concrete as readily.

ICFs are not only for basements. They can be used for walls above ground level as well. The Portland Cement Association has a website with further information about homes built with ICFs, and showing examples of very conventional looking homes that are built with walls constructed in this fashion.