Securing Straw Bales to the Foundation

Here is a response to a student doing a case study of straw bale homes in North Carolina. He had a good question about how the bales of an existing structure in Alabama might be connected to the foundation. I think the details of the connection are worth mentioning here as well.

I am glad to see that you have chosen such a cool topic for your case study! Perhaps the most important distinction to make is that the homes are made with straw bales, not hay bales. Hay would be a food source for many critters and would also run the risk of fire because of the higher moisture contents within hay bales. Straw on the other hand has no food value and is extremely dry, typically around 8% moisture content during installation.

The old style of construction has changed significantly in recent years so how the bales were attached to the foundation in the building you are studying is hard to say. In today’s homes, we use a system that bolts 4×4 sills to the foundation. Those sills are place on the interior and exterior faces of the bales so each bale is held up n both sides at the bearing points. The space between the 4x4s is filled with gravel and/or rigid insulation. 20 penny nails are then driven into the sills at 4″ on center staggered from one side of the sill to the other (both sills are nailed and treated as separate entities when measuring the layout of the nails). The nails are only driven in until firmly secured in the sills and about 2″- 3″ of nail is still exposed above the sills. The bales are then placed on the nails and held tightly in place by the “grab” of the nails. That is the easiest and most effective way of connecting to the foundation.

4 Responses to Securing Straw Bales to the Foundation

  1. anonymous
    anonymous Thu, September 13, 2007 at 1:51 pm #

    Roof Ventilation Systems.
    I haven’t seen this topic anywhere here and so wanted to post my question. I’m building a straw bale house in a very cold, wet climate—Estonia. Am experiencing one serious problem at the moment: we have no adequate roof ventilation system. Basically, we’ve built an eco-house with a euro-standard roof, and we need to install roof ventilation. We have such a problem with condensation that water is literally running into one wall of bales—runs along the roof line inside the attic when outdoor temperature cools. Measured the temperature difference with an infrared camera, and we have -4 Celsius (frozen water, obviously) on the top bales, while the lower walls match the room temperature (roughly 16 degrees Celsius when we don’t actively heat).
    Construction experts have suggested we hire an engineer to tell us exactly how big the roof vents should be (tubes inserted from the roof which penetrate into the attic and allow air to circulate), but an engineer is overkill, and we don’t have the money for it.
    Can someone who’s built with straw bale before tell me how many vents and what sizes I should install? I could send a pdf file of the house, if necessary, but it’s 6 x 13 meters, two floors, with a single-slope roof. There’s maybe a meter of air space on the high end of the attic, zero on the low end. I also have a second house of similar dimension (8 x 13 meters, 2 stories, same roof structure).
    Big thanks for any help. Please write me directly: scottdiel@gmail.com

  2. Andrew
    Andrew Thu, September 13, 2007 at 1:51 pm #

    Scott,
    I ventilate all my roof lines with a continuous vent in the soffits and ridge. If possible, the best option would be to cut a slot vent about 5 cm wide the full length of the roof at the bottom of the roof where it meets the wall. You can then do the same thing at the top of the roof line. Be sure to use screened vents so no bugs get inside. In general, you want enough ventilation to satisfy the following criteria: ventilation to area ratio of 1:150. For example with 150 square feet of area in the attic, one square foot of cross ventilation, like described above, must be provided. If you use vent stacks, the ventilation is not as effective. If you must use those, then place enough at the high end of the roof and some at the low end of the roof to draw air from throughout the attic space. Good luck.

  3. gene
    gene Thu, September 13, 2007 at 1:52 pm #

    Hellow Andrew,
    I was watching you non-loadbearing dvd and have was curious as to the length of anchor bolts you used to tie the p.t. 4X4’s to the concrete floor. With 7 inches in the concrete and at least another 4 above, it would seem thay must be about a foot long or so. I have never seen any greater than 10″ at my local home center. Thanks for your time,
    Gene
    Spokane, WA

  4. Andrew
    Andrew Thu, September 13, 2007 at 1:52 pm #

    Gene,
    The bolts used depend on what is called out on the plans. like you mention in your post, I call out for a minimum embedded depth of 7″ in the concrete. That is a standard depth requirement for anchor bolts in Oregon. Your math is correct and so if one had to meet the requirements listed as such they would need to use a Simpson SSTB16 which is a 16″ special anchor bolt. They are more expensive, so be sure that your local code requires the same depths. Another option is to use a rated dry bolt that can be installed after you pour the concrete and it has cured. These work well for the interior toe ups especially as the interior toe ups don’t get installed for some time if building post and beam (not as big of a concern with load bearing). These bolts often only need to have an inch or so of depth into the concrete.

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