Expert: Ted Barnhill (Principal, Design45 LLC) - 10/21/2008

Question
QUESTION: Hi Ted, Sorry to bother you with this, but hoping you could help with your expert advice. I'm an American living in Bhutan and my husband and I are in the process of building a small, approximately 1,200 sq ft house. Currently they're nearly finished with the foundation (stone/concrete) and now we're in a toss up on how to finish the house. I'm familiar with construction practices in the States, but here they are not and contractors are tough to find (the reason we're attempting this ourselves). First thought was to do traditional wood framing, and have the exterior buckel (or the outer bark part of the tree - approx. 2" thick). Then the inside just plyboard. Between the ply and the buckel we have 6" but insulation is only available in 4". So my question is would insulating this be a waste of time and money because of the extra space? It does get cold here in the winter. Now we're considering using "hollow brick" as they call it (cinder block) but I'm just concerned how much better (or worse!) this would be. I've read many ways to insulate these, but most of the materials aren't available here. Another thought to help in insulating was to use one of the traditional methods here for constructing homes which is, between the framing, use woven bamboo as a "frame" and coating that with mud to create a mud pack. Some people use this simply as their interior and exterior, but I guess I'm a bit spoiled as I'm hoping for a house which is of tolerable temperature in the winter. Again, I apologize for this somewhat juvenile question, but any thoughts/suggestions you may have would be most appreciated. Thank you again! Jamie

ANSWER: Jamie,

We designers always get excited about new (to us) techniques, especially traditional "vernacular" construction.  My interest and curiosity draw me to recommend that you look further into the bamboo/mud concept, which sounds like a variant of "wattle and daub."  This technique has been used for thousands of years, often with great success, and can be incorporated into a well-insulated wall.

The keys to keeping a home comfortable and efficient are, in order:

1. Eliminate air infiltration.  The air barrier can be an applied membrane such as homewrap, or can be an element of the wall assembly such as the exterior finish (plaster, say, or wattle-and-daub) if properly sealed.  Windows and doors should be high quality and should seal tightly.  In many homes, air infiltration causes more heat loss than having poor insulation.

2. Insulate well.  What kind of insulation is available in your area?  I'm not sure why you can only get 6" framing and 4" insulation, but it shouldn't be a problem if you are able to keep the insulation in place.  A 2" air gap would allow convective currents, and might be a place for moisture to condense under just the right (wrong) circumstances, but wouldn't nullify the benefits of 4" of good insulation.  Still, it would be better an adequate insulation depth for your climate (probably 4"), and use framing and insulation of that depth.

3. Build with thermal mass to modulate temperature swings through the night.  Using CMU ("cinder block", "hollow brick") would do this, but the cost and buildability tradeoffs might not be worth it.  Whether to pursue thermal mass depends on your budget, heating source, local climate, etc.

You have an exciting chance to build a comfortable and sustainable home using both modern and traditional techniques.  As a general approach, I would recommend trying to start with a local, customary building technique, then critique/improve on it.  What does it do well?  Are the local materials durable?  Are they responsibly produced?  What drawbacks does the technique have?  Is it drafty or poorly insulated?  Is there a way to incorporate modern air barriers or insulation without compromising the assembly's ability to "breath?"  (That is, to safely get rid of excess moisture.)

The details of the wall assembly are numerous and will require more research and design; one source could be books on (western) alternative/sustainable/natural home building.  Good luck, and feel free to post followup questions.  -Ted

---------- FOLLOW-UP ----------

QUESTION: Hi Ted! Thank you so much for all the thoughtful and helpful advice. So I think we're going to go with the traditional wood framing and insulation technique. If I may, just a few more questions...

I think the layer of materials will be as follows:

From the inside:
1) wood framing with the bamboo/mud technique in between - probably only a total of 1 to 2" thick. This will actually also serve as the interior wall.
2) then insulation (2" or 4" depending upon what you suggest) - would it be better to cover the whole area with insulation, or better to just jam the insulation in the remaining space behind the bamboo/mud layer? This would mean essentially insulation in squares behind the bamboo/mud frame so there would not be a "wall" of insulation.
3) an air barrier as you mentioned between these levels and the outside buckel - how important is this and what material would we need to use? Just basic plastic or something more specific? Is this the same thing as a vapor barrier?
4) lastly, the buckel with grooves/joints in the back to allow for a better seal (similar to how wood paneling snaps together). Would it be advisable to use something else to fully seal the space?

And last two questions,

1) What is the best way to seal around windows? The windows will not be of great quality - just done with a single glass pane (or double?) and carpenter-made wood frame. Are there any relatively easy improvements you would suggest to help in this area? Proper techniques for sealing around the windows and/or glass?

2) With construction as mentioned above, do you think rats will be an issue, or will the fiberglass (or rock wool) insulation possibly keep them at bay?

That's all for now. I can't tell you enough how helpful you have been. Any tips for improvement or changes needed would be most appreciated. Thank you again. You're wonderful!

Jamie


Answer
Jamie,

Your broad description of the wall assembly sounds pretty good, but there are many places where the specific installation details could "make or break" the success of your scheme.  The devil is indeed in the details, and you'll have to learn more about the local techniques than I can help with.  On the basic wall assembly:

1. I suggest you look into whether it is practical to install the "wattel and daub" as a continuous surface on the inside of your timbers, rather than in between.  The aesthetic of the exposed timbers could be great, or not, depending on the size/spacing/configuration.  The functional reason to use a single smooth surface is to help act as an air barrier.  [The building science behind air barriers and vapor retarders is too detailed to recount here; in short, an air barrier (anywhere in the wall) is a must, and a (warm-side) vapor retarder is a great idea if properly implemented, which it often isn't.]

2. Insulation will, ideally, be: as thick as the framing, installed so as not to settle down over time, and not compressed (which reduces it's insulating value).  Both fiberglass and rock wool, which not a source of food, could be used for rodent nesting, so keep the rats out.  

3. Don't use plastic for your air barrier, as it will not allow moisture within the wall to dry out.  This is a problem in winter as heated interior air tries to escape, and condenses within the wall, causing rot/mold.  You may consider using the typical western method: plywood sheathing over your frame, covered with with building paper (asphalt-impregnated paper) in lapped-horizontal strips.  The plywood is in this case structural, bracing the wood frame against racking.  The building paper acts as a secondary drainage barrier for any incidental moisture that gets past the siding.

4. I don't have experience with buckel, but your local craftsmen will undoubtedly have techniques for installing it tightly to keep wind, bugs, and rodents out.  Talk to several suppliers and installers about the options.  Most exterior wood products should be sealed on all sides before installation; check into this.

Windows will ideally have two layers of glass to provide basic insulating value.  This can be a sealed, manufactured double-pane unit if available, or can be single-pane windows with storm windows.  Your carpenter should be able to produce windows in which the sashes fit tightly in the frame (with some weatherstripping).  Pay close attention to the gap between the outside of your window frame and the inside of the opening in your timber framing.  If you can get expanding foam sealant, this works well.  Otherwise, ask around; horsehair is used to seal gaps in log building here.  When everything is in, be sure to go back and caulk any remaining gaps on both interior and exterior.

Good luck building your home.  Keep asking all of the experienced local builders; there are many details to each step of the way that you'll want to get right!  -Ted