Case Study for Strawbale Construction Methods
Introduction
In this essay I will report on the strawbale construction methods that have been used in times gone by and in the use of strawbale in modern buildings in Ireland and throughout the world. It is a relativity new method of construction here in Ireland. Another method of construction is ModCell® (modcell.co.uk, 2010) which is prefabricated method of building which looks a quality way of manufacturing. I will discuss the different methods of construction, load bearing (Nebraska Style) as described in the book (Clarke Snell & Tim Callahan, 2005, p343), and post and beam infill methods. There are pros and cons to both of these methods, and with effective detailing from a technological perspective both can be effective.
Hstory of Strawbale Construction
According to Barbara Jones in her book ‘Building With Straw Bales’ (Barbara Jones, 2007 p13), straw bale was first used as a construction material in the USA in the 1800’s. It was built in Bayard Nebraskain 1896. It was a load bearing strawbale structure with a sod roof. The building was a schoolhouse for the local community. The idea for this structure at the time was that it was meant to be a temporary building, but when the community used and found it to be an effective insulation product; it was adapted as a permanent schoolhouse. In the book Building Green (Clarke Snell & Tim Callahan, 2005, p342) it makes the discussion of straw vs. hay as a construction material. They state that hay is dried grass which is a food supply for livestock and is not suitable for building as it will also be attacked by rodents and insects and is also more vulnerable to mould growth. The straw we use for construction is by product of wheat mostly, which has hard hollow stems and is often used as a bedding material for cattle and horses. So they have made the point in that straw is the best choice of material as it is more resistant to decay. This is a quotation with reference to Strawbale capacity:-
“It is estimated that enough straw is incinerated each year in the U.S. to build 5 million 2000 square foot homes “ (whygreenbuildings.com 2010)
In the book Building Green (Clarke Snell & Tim Callahan, 2005, p347) they make the argument on whether straw or timber is the most sustainable building material for construction. It is said that straw is an annual crop from agricultural food production and will be produced each year as a requirement with a short life cycle, trees will have a longer life cycle depending on the species and whether softwood or hardwood.
Post and Beam
The first straw bale post and beam building was in Alabama in 1936 and a number were built in the following years. In the years after WW 2 there was a decrease in the use of strawbale as made man made synthetic insulation products had arrived on the market. An interesting point that should be noted,regarding financial backing and insurance is that, “The first permitted (bank-financed and insured as well) straw bale structure in the United States was built in 1991 in Tesuque, New Mexico by Virginia Carabelli” (whygreenbuildings.com 2010)
Construction methods of Strawbale buildings Loadbearing (Nebraska Method)
There are two general methods of traditional Strawbale construction. They are loadbearing straw bale structures and post and beam timber frame structure, with bale infill between the posts to act as the insulation for the building envelope. Barbara Jones considers that the loadbearing structural Strawbale building would have been the first used method, because it is the simplest form of building and could be done by the relativity inexperienced construction workers/ labourers. This method is called the Nebraskamethod of building. The bales are laid side by side and the rows staggered so as to tie them together like using concrete building blocks. The rows are pinned together with hazel rods which give the bales lateral support within the wall. These rods will be fixed to the foundation to locate the bales in the wall. A box beam wall plate is laid on top of the bales to carry the roof. The window and door reveals are formed with a similar section box beam. They are pinned to the wall with hazel rods. The window frame can then be fixed into the box beam with standard timber frame fixings. (Barbara Jones, 2007 p63)
The main structural concern for this method is the compaction of the Strawbales when they are subjected to the load of the structure above them including the roof. The amount of compaction depends on the density of the bales .The common practice is for 75 mm of clearance to be left between the structure and the bales (Barbara Jones, 2007 page 64). The setting out of the structural grid of the columns should be set by the bale’s dimensions so as to minimise bale cutting or adjustment. The door reveal verticals are fixed to the soleplate in the normal timber frame manner.
Post and Beam Infill Method
This is a timber framed method of construction that uses tried and trusted methods of construction that designers and craftsmen are familiar with. The building can have its roof complete before the bales are put into place. This will protect them during the construction phase if any rain on the site is suffered. The main benefit of post and beam is that if moisture gets into the bales through the structure, that the bales can be removed and replaced or refilled with another traditional natural insulation and be reclad externally with timber cladding or something similar. This method could also have offsite fabrication of the timber frame which will make sure that the timber frame is of high quality and fit (Barbara Jones, 2007 p21).
Hybrid method
This is a variant method of loadbearing and post and beam (Barbara Jones, 2007 p22). This is the method I used on a studio
project (KindergartenSchool) in my final College year. It uses the post and beam method on the wetter north elevation, and a loadbearing Strawbale wall on the south elevations which is drier.
ModCell® Strawbale Panels
This is a prefabricated method of construction for making Strawbale panels as described on their website (www.modcell.co.uk, 2010). The company supply a group of crafts men for the labour and source the Strawbales and timber locally to the jobsite. ModCell then set up a temporary factory at a suitable location near the jobsite to fabricate the panels. This method will ensure that the prefabricated panels will be of high quality. The panels are approx 3000 high x 3200 wide and 490 mm thick. The walls are finished with lime render which is spray applied and trowel finished. The other variant of this system is breathable plasterboard on the inside and a plywood finish to the outside of the panel. The exterior can then be finished with a traditional timber rain screen cladding. The wall has to be kept breathable so as to prevent moisture build-up in the wall. The panels are fitted together with a 20 mm tolerance. The gap is filled with non compressible tapes. The weight of the largest ModCell panel is 1.7 metric tonnes. These are a few examples of u values for the wall panels: - ModCell u Value data (www.modcell.co.uk, 2010) ModCell® Straw 490 mm 0.13 w/m2 k, ModCell®Lite 390mm 0.14 w/m2k, ModCell® Hemp 490 mm 0.14 w/m2 k.
Detailing of Straw bale Buildings
This is a quotation about how strawbale should be protected:
“Care must be taken to assure that the straw is kept dry, or it will eventually rot. For this reason it is generally best to allow a strawbale wall to remain breathable; any moisture barrier will invite condensation to collect and undermine the structure “(greenhomebuilding.com 2010)
The Building Green (Clarke Snell & Tim Callahan, 2005, p348) book makes the statement that the biggest problem we have in straw bale building is the effects of water on our building. They say that straw is a hygroscopic material, which is a material that will dry out after the material has gotten wet. The outer render, which usually is lime render allows water through it, this is a part of the materials properties working together to form a solution to allow the wall to breathe and dry out. In the building with Straw bales book (Barbara Jones, 2007 page 60) she discusses the detail of roof overhang. It is common practice to have 500 mm or larger eaves to help to protect most of the wall from driving rain.
Fire protection is a concern for building with straw bale and the The Building Green Book (Clarke Snell & Tim Callahan, 2005, p349) reports that tests have been performed on straw bale walls around the world and the results are good. They say the straw needs air to burn, so when it is contained in a wall section and rendered with lime on both sides the straw is starved of air and will not readily burn. It is most in danger in the construction phase of the building, so due care has to be taken by construction workers. The Straw bales book (Barbara Jones, 2007 page 87) expands more on this topic and informs us of a test by Amazonails in 2004 (Amazon nails 2010) who had the Building Research Establishment (BRE) whose website can be found at (Building Research Establishment 2010) conducted a fire test on a lime rendered straw bale wall, and the result was that it had a minimum resistance of 2 hrs 40 minutes. They made the point of how much better this is than a standard plaster boarded stud wall rating of 30 minutes. They also go on to confirm that the most at risk of fire phase is during the construction of the building on site.
Conclusion
Straw bale construction is a new method of construction for Ireland. It has not been proven to be a viable method of construction here as of yet. Tests can be done in the laboratory or on small test outdoor buildings, but the real test has to be on normal sized residential or commercial buildings and tested in real world conditions and the extremes of weather that we get in Ireland at times. The effect of moisture penetration in the walls is a serious concern , and maybe the ideal situation would be to not allow the water to penetrate the wall at all and use a rainscreen and breather membrane outside the straw bale walls and do away with the lime render on the outside of the building.
References - Attached to document.
Biblography - Attached to document.
In this essay I will report on the strawbale construction methods that have been used in times gone by and in the use of strawbale in modern buildings in Ireland and throughout the world. It is a relativity new method of construction here in Ireland. Another method of construction is ModCell® (modcell.co.uk, 2010) which is prefabricated method of building which looks a quality way of manufacturing. I will discuss the different methods of construction, load bearing (Nebraska Style) as described in the book (Clarke Snell & Tim Callahan, 2005, p343), and post and beam infill methods. There are pros and cons to both of these methods, and with effective detailing from a technological perspective both can be effective.
Hstory of Strawbale Construction
According to Barbara Jones in her book ‘Building With Straw Bales’ (Barbara Jones, 2007 p13), straw bale was first used as a construction material in the USA in the 1800’s. It was built in Bayard Nebraskain 1896. It was a load bearing strawbale structure with a sod roof. The building was a schoolhouse for the local community. The idea for this structure at the time was that it was meant to be a temporary building, but when the community used and found it to be an effective insulation product; it was adapted as a permanent schoolhouse. In the book Building Green (Clarke Snell & Tim Callahan, 2005, p342) it makes the discussion of straw vs. hay as a construction material. They state that hay is dried grass which is a food supply for livestock and is not suitable for building as it will also be attacked by rodents and insects and is also more vulnerable to mould growth. The straw we use for construction is by product of wheat mostly, which has hard hollow stems and is often used as a bedding material for cattle and horses. So they have made the point in that straw is the best choice of material as it is more resistant to decay. This is a quotation with reference to Strawbale capacity:-
“It is estimated that enough straw is incinerated each year in the U.S. to build 5 million 2000 square foot homes “ (whygreenbuildings.com 2010)
In the book Building Green (Clarke Snell & Tim Callahan, 2005, p347) they make the argument on whether straw or timber is the most sustainable building material for construction. It is said that straw is an annual crop from agricultural food production and will be produced each year as a requirement with a short life cycle, trees will have a longer life cycle depending on the species and whether softwood or hardwood.
Post and Beam
The first straw bale post and beam building was in Alabama in 1936 and a number were built in the following years. In the years after WW 2 there was a decrease in the use of strawbale as made man made synthetic insulation products had arrived on the market. An interesting point that should be noted,regarding financial backing and insurance is that, “The first permitted (bank-financed and insured as well) straw bale structure in the United States was built in 1991 in Tesuque, New Mexico by Virginia Carabelli” (whygreenbuildings.com 2010)
Construction methods of Strawbale buildings Loadbearing (Nebraska Method)
There are two general methods of traditional Strawbale construction. They are loadbearing straw bale structures and post and beam timber frame structure, with bale infill between the posts to act as the insulation for the building envelope. Barbara Jones considers that the loadbearing structural Strawbale building would have been the first used method, because it is the simplest form of building and could be done by the relativity inexperienced construction workers/ labourers. This method is called the Nebraskamethod of building. The bales are laid side by side and the rows staggered so as to tie them together like using concrete building blocks. The rows are pinned together with hazel rods which give the bales lateral support within the wall. These rods will be fixed to the foundation to locate the bales in the wall. A box beam wall plate is laid on top of the bales to carry the roof. The window and door reveals are formed with a similar section box beam. They are pinned to the wall with hazel rods. The window frame can then be fixed into the box beam with standard timber frame fixings. (Barbara Jones, 2007 p63)
The main structural concern for this method is the compaction of the Strawbales when they are subjected to the load of the structure above them including the roof. The amount of compaction depends on the density of the bales .The common practice is for 75 mm of clearance to be left between the structure and the bales (Barbara Jones, 2007 page 64). The setting out of the structural grid of the columns should be set by the bale’s dimensions so as to minimise bale cutting or adjustment. The door reveal verticals are fixed to the soleplate in the normal timber frame manner.
Post and Beam Infill Method
This is a timber framed method of construction that uses tried and trusted methods of construction that designers and craftsmen are familiar with. The building can have its roof complete before the bales are put into place. This will protect them during the construction phase if any rain on the site is suffered. The main benefit of post and beam is that if moisture gets into the bales through the structure, that the bales can be removed and replaced or refilled with another traditional natural insulation and be reclad externally with timber cladding or something similar. This method could also have offsite fabrication of the timber frame which will make sure that the timber frame is of high quality and fit (Barbara Jones, 2007 p21).
Hybrid method
This is a variant method of loadbearing and post and beam (Barbara Jones, 2007 p22). This is the method I used on a studio
project (KindergartenSchool) in my final College year. It uses the post and beam method on the wetter north elevation, and a loadbearing Strawbale wall on the south elevations which is drier.
ModCell® Strawbale Panels
This is a prefabricated method of construction for making Strawbale panels as described on their website (www.modcell.co.uk, 2010). The company supply a group of crafts men for the labour and source the Strawbales and timber locally to the jobsite. ModCell then set up a temporary factory at a suitable location near the jobsite to fabricate the panels. This method will ensure that the prefabricated panels will be of high quality. The panels are approx 3000 high x 3200 wide and 490 mm thick. The walls are finished with lime render which is spray applied and trowel finished. The other variant of this system is breathable plasterboard on the inside and a plywood finish to the outside of the panel. The exterior can then be finished with a traditional timber rain screen cladding. The wall has to be kept breathable so as to prevent moisture build-up in the wall. The panels are fitted together with a 20 mm tolerance. The gap is filled with non compressible tapes. The weight of the largest ModCell panel is 1.7 metric tonnes. These are a few examples of u values for the wall panels: - ModCell u Value data (www.modcell.co.uk, 2010) ModCell® Straw 490 mm 0.13 w/m2 k, ModCell®Lite 390mm 0.14 w/m2k, ModCell® Hemp 490 mm 0.14 w/m2 k.
Detailing of Straw bale Buildings
This is a quotation about how strawbale should be protected:
“Care must be taken to assure that the straw is kept dry, or it will eventually rot. For this reason it is generally best to allow a strawbale wall to remain breathable; any moisture barrier will invite condensation to collect and undermine the structure “(greenhomebuilding.com 2010)
The Building Green (Clarke Snell & Tim Callahan, 2005, p348) book makes the statement that the biggest problem we have in straw bale building is the effects of water on our building. They say that straw is a hygroscopic material, which is a material that will dry out after the material has gotten wet. The outer render, which usually is lime render allows water through it, this is a part of the materials properties working together to form a solution to allow the wall to breathe and dry out. In the building with Straw bales book (Barbara Jones, 2007 page 60) she discusses the detail of roof overhang. It is common practice to have 500 mm or larger eaves to help to protect most of the wall from driving rain.
Fire protection is a concern for building with straw bale and the The Building Green Book (Clarke Snell & Tim Callahan, 2005, p349) reports that tests have been performed on straw bale walls around the world and the results are good. They say the straw needs air to burn, so when it is contained in a wall section and rendered with lime on both sides the straw is starved of air and will not readily burn. It is most in danger in the construction phase of the building, so due care has to be taken by construction workers. The Straw bales book (Barbara Jones, 2007 page 87) expands more on this topic and informs us of a test by Amazonails in 2004 (Amazon nails 2010) who had the Building Research Establishment (BRE) whose website can be found at (Building Research Establishment 2010) conducted a fire test on a lime rendered straw bale wall, and the result was that it had a minimum resistance of 2 hrs 40 minutes. They made the point of how much better this is than a standard plaster boarded stud wall rating of 30 minutes. They also go on to confirm that the most at risk of fire phase is during the construction of the building on site.
Conclusion
Straw bale construction is a new method of construction for Ireland. It has not been proven to be a viable method of construction here as of yet. Tests can be done in the laboratory or on small test outdoor buildings, but the real test has to be on normal sized residential or commercial buildings and tested in real world conditions and the extremes of weather that we get in Ireland at times. The effect of moisture penetration in the walls is a serious concern , and maybe the ideal situation would be to not allow the water to penetrate the wall at all and use a rainscreen and breather membrane outside the straw bale walls and do away with the lime render on the outside of the building.
References - Attached to document.
Biblography - Attached to document.