SIPs and timber frame notch system – does this create a significant thermal bridge?
BeckyBorton
| Posted in Energy Efficiency and Durability on
We are seriously considering a home from Trinity Post and Panel (www.trinitypostandpanel.com) and are impressed by their energy efficient patented system. It consists of a SIP panel inserted into a notched timber frame. We are looking for some opinions from outside sources on their concept. Specifically, would the large timber (seen from both the inside and the outside of the home) be a significant source of thermal bridging?
I should note that we live in Northern Canada and experience true northern winters (-40C, snow etc) and our home is expected to be about 2000 sq ft.
Thank you for your time.
Becky
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Replies
I looked at the Trinity Post and Panel website and videos. The homes are visually appealing and being able to build quickly is a big plus. Here are a couple things that I wondered about though:
1. They say: "By doing this Trinity virtually eliminates thermal bridging across walls." Those big timbers, don't look at 'em. I would look for calculations that show the overall energy efficiency of the home including thermal bridging loss through those timbers. (Reminds me of why we didn't build a log home.)
2. What kind of wood are they using for those timbers and what will be the maintenance requirements for it outside? This is a particular concern if you happen to be like us and live in or next to the woods where animals don't make any distinction between trees and homes.
3. I wonder if there might be an Achilles heel outside at the bottom corners of the panels where they notch into the timbers. Seems like you would need some sort of special flashing there to kick out any water that got behind the siding else you could end up with rot at that intersection.
Becky,
You are right to be concerned. This approach to enclosing a timber frame -- using infill panels instead of wrapping the entire timber frame with an airtight layer of continuous exterior insulation -- has been associated with many failures.
There are two problems to the "infill panel" approach:
1. There is thermal bridging through the frame, as you correctly guessed.
2. There is air leakage at the joints where the infill panels meet the frame. Although this company has attempted to address the air leakage problem with rabbets (channels) and gaskets, these methods won't stand the stresses caused by the movement of the posts and beams. The timber frame will change dimensions with changes in humidity and temperature, and the timbers with check and twist as they dry. These stresses will result in air leakage.
.
Funny thing, am reading a book about Pennsylvania German Architecture and Landscape and suddenly realized why this Trinity Post and Panel system resonates; its appearance is evocative of the Fachwerk building style that one might see in Europe.
http://www.dreamhouse-group.eu/en/products/post-and-beam-houses/fachwerk-houses/
Martin, do you think if they were to utilize kiln dried or laminated timbers that your expansion and contraction concerns would be satisfied and the walls would be stable enough that they wouldn't leak? (We used red maple glulams inside our house, one reason being that my wife doesn't like the appearance of wood that has checked.)
Andrew,
It's certainly possible to minimize problems by careful species selection, by selecting trees with as few knots as possible, by kiln drying, and by laminating. But these are still large posts and beams that will inevitably move with changes in temperature and humidity.
The problems I predicted may be big problems or small problems; it's hard to predict. But from a building science standpoint, infill panels make no sense. What you want is a continuous air barrier and a continuous layer of insulation on the exterior side of the timber frame.
I design the most beautiful houses in the world and then build them in a week. They are so efficient John Straub calls daily for advice. My work with the fire department has saved so many lives I often can't get to my house without sweeping all the flowers and thank you cards off the front path. Once inside the smells from my cooking causes so many people to knock and plead for the recipes that I have to ignore the door or I would never get any peace.
How much simpler it would be to evaluate all these systems and products if the manufacturers offered an honest description rather than just hyperbole. and nonsense.
Malcolm,
You have a future in public relations and advertising! Embrace your talent -- don't resist it.
Thank you for all your time and thoughtful responses! Lots to consider here. The company explained to us that as the large timbers contract/shrink that these seams would actually get tighter. For us, our other options are standard stick frame build with batt insulation or a spray foam upgrade. Labour is actually a HUGE issue and concern in our particular area (adding huge costs to a relatively inexpensive and energy inefficient project). Will try and track down some more information on thermal bridging calculations for these timbers :)
Thanks again.
Just eyeballing it, I'm not sure the thermal bridging is any worse than the cumulative thermal bridging in a regular stud wall. One 6x6 timber conducts the same heat as four 2x6 studs. But why bother with this special construction if the result is not particularly good?
The story about the seals getting better when the wood moves is cute--they want to claim it gets tighter either way, whether the wood shrinks or expands. Kind of like a Bernie Madoff investment that's worth more when the stock market goes up and worth more when stocks go down. Ask to see a blower door test on a house they built three or more years ago.
Andrew's last concern, about water at the bottom of the panel is also a good point. I can imagine how one might attempt to mitigate that flaw, but I don't think it can be solved completely.
One of the guys in my office lived in a post & beam house with R25-ish stress-skin polyurethane panels for about 25 years. After about ten years he was constantly fixing air leaks and fighting ant infestations (!). It still looks great, has a lot of character, but it isn't particularly comfortable.
He recently sold it, moved into a 1920s 2x4 framed bungalow. Before moving in he retrofit air-sealed and dense-packed the walls, bumped up the attic to about R50 (but still has no foundation insulation- are houses ever really "done"? :-) ).
His new old-house is more thermally efficient, and noticeably more comfortable than his old new-ish (circa 1990) post & panel house. He's heating the tightened up antique with 3 mini-splits (could have done it with 2), and has no regrets.
Maybe they have solved some of the long term air leakage issues of post & panel construction or maybe they haven't but I'm dubious. From a risk point of view my inclination would be to take another approach with walls that can be know-ably-reliably air sealed, especially in a climate that regularly sees -40 weather.
Rebecca,
One of the changes that characterized modern architecture and engineering from its predecessors was the separation of the load-bearing elements of the structure from its "skin". You see this most easily in skyscrapers where the load-bearing elements are often situated either all inside or outside the cladding (and insulation). As Martin said, it's a step back to design a house where the walls act as infills and the structure spans from the inside to the exterior.
We built a timberframe home 3 years ago and chose to stick frame around the timbers and insulate with closed cell spray foam instead of sip panels. We do have several timbers that have twisted a bit and checked however since we have wrapped the timbers from the outside it has not caused any issues for us. We only have two large timbers that penetrate the building envelop from inside to outside. I would agree with others about wrapping the outside of the timbers as opposed to infills. Add additional timber accents on the outside of the home to carry the look outside.