Building Performance

Traditional framing and building practices, using wood, stone, straw, clay.
Matt J
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Building Performance

Sat Mar 07, 2015 12:05 am

I thought I'd start a new thread for this topic as we stray farther from the original...
When it comes to a wall, how might one reduce or eliminate thermal bridging? I mention this at the outset as a reduction of thermal bridging is one of the cornerstones of Passiv Haus.
Finally, an area where I have some experience! Not that I consider myself an expert, but through my work (remodeling in the Boston area) I have access to many bona fide experts in the field of building performance, or building science, or whatever you'd like to call it. And I've heard this topic debated from many angles.

In a long winded answer to Chris' question above... The theoretical goal is to have a continuous layer of insulation that surrounds the conditioned space on all six sides. Since most common (read: affordable, mainstream) insulation materials have little structural integrity, the challenge is to integrate the structure and the insulation allowing each to do its job without getting in the way of the other, and without exposing the materials to conditions that make them fail. You can put all the insulation on the inside, but then the structure is left cold, which can lead to durability problems, you sacrifice interior space, and interior partitions are difficult to support (especially floors). You can put all of the insulation on the outside, and the challenge becomes supporting and protecting the insulation. In practice we usually end up using a combination of approaches and living with some thermal bridging which we try to minimize. Tough spots are penetrations like windows and doors, and to a lesser degree the intersections between foundations and walls, and walls and roof.

To minimize thermal bridging in conventional wood framed walls (rather than mass walls like brick, stone, etc.) the approaches I'm familiar with include double stud walls- basically two walls built next to each other but not touching, with insulation in between; truss walls (larsen truss, TJI, open web)- which are like double stud walls but the two walls are tied together with minimal amounts of material for strength and low conductivity; and staggered stud walls, where wide top and bottom plates (like 2X6 or bigger) are used with narrower studs (say, 2X4s) which are offset so that the studs carrying the exterior sheathing and cladding are not in direct contact with studs carrying the interior finishes. There are various ways to put the insulation outboard. The approach we end up using the most in our residential work (though I'm not saying it's the best!!!) is insulating the cavity then attaching polyisocyanurate foam board outboard of the structural sheathing, secured with wood strapping and long screws.

Sadly I have almost no experience with "alternative" insulation techniques. Antoine- I'm eager to hear how your hempcrete project goes! Please take lots of photos!
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Chris Hall
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Re: Building Performance

Sat Mar 07, 2015 12:11 am

Here's a short article looking at the performance of double walls, comparing having the studs staggered or not staggered:

http://www.google.com/url?sa=t&rct=j&q= ... 1401,d.eXY
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Paul Atzenweiler
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Re: Building Performance

Sat Mar 07, 2015 10:22 am

So many factors to consider. I do have a question about the offset studs (I was surprised from the article you posted, there was so little difference in straight/offset performance), is there a vapor barrier on the inside of the outside row of studs or on the outside of the sheathing - or on the interior of the structure. I know there are several camps when it comes to vapor barriers of any kind since vapor barriers are relatively new to the constructions process. Houses that are many centuries old (I am sure with good maintenance) have been able to "breath". I know every little bit helps, but one could have a 10-pane window and it will still be the source of heat loss compared to everything else. And when there are whole walls that are windows . . .
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Re: Building Performance

Sat Mar 07, 2015 10:50 am

I'll keep you aware of the developpements. Just so you know, the hempcrete still needs a very light framing in between the timber posts. Since the hemp wall is 10'' or 12'', it completely covers the frame from the outside. It's free of thermal bridges. The small framing needed is made of 2x4 every 32'', and it's only there to install the formwork (is that the right word?). It can be made from recycled wood and even 2x3... Hempcrete is also known to stiffen the frame. While it can't support heavy vertical loads, it it supposed to be solid enough to brace the building. You still have to add simple braces (again, recycled wood, forence... etc) to the frame while the hempcrete is installed. I am trusting our hempcrete specialist on this for now, since our project is an experimental one, but I can't wait to see if he's right on every point. It seems a bit too perfect for me at the moment.

There are many ecological ''autoconstruction'' (self built?) homes around here, and a couple of them used the double wall system. The feedback I get is that it's longer that you'd think, but worth it. It's very cold here (in fact february 2015 was the coldest ever recorded with an average of -15C!) and people who are using the double wall usually make them very large, wich is logic since it's not much more work or wood for a lot more (cheap) insulation.

Paul, it's important to mention windows. It often feels so strange entering a modern ultra efficient house built with R30 walls and 16 feet tall R5 windows... It feels unbalanced to me, but I don't know much about passive house and sometimes these designs are more logical than we'd think.
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Chris Hall
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Re: Building Performance

Sun Mar 08, 2015 4:02 pm

Paul Atzenweiler wrote:So many factors to consider. Houses that are many centuries old (I am sure with good maintenance) have been able to "breathe".
That is an interesting topic in and of itself. Old houses were far from air tight, and could definitely 'breathe'. Some cite this as a plus, perhaps without thinking the matter through in detail.

There are a few aspects to this 'breathing':

-heating is not cheap and thus a leaky house is either going to be expensive to live in and wasteful of energy, or drafty and cold and 'hard to heat'. Kinda like a traditional Japanese house actually. Wasteful thermal performance is not only hard on the pocketbook but means using more energy than necessary, which mean more environmental load for heating than otherwise.
-breathing involves an in and an out aspect. A house that breathes easily also can bring in surrounding air easily, and in a lot of settings, more particularly urban ones, the air quality is not so good so the uncontrolled intake of that air will be a health hazard to occupants. This, even more so if the occupants are sensitive to allergens in the surrounding area. Cars these days have filters on the external air intake for this very reason - so, why not on houses?
-for wood in general, even air circulation on all sides is a benefit. Trapping moisture in a structure is bad for the wood and is a road to ruin all around. A house with a carefully controlled regulation of moisture and air can fail spectacularly if details are not done right. In contrast, a leaky breathing house doesn't control much very well at all, and in some ways is more forgiving of poor detailing.

There are the 5 key points to Passive House design:

1. Exceptionally high level of thermal insulation
2. Well-insulated window frames with tripe low-e glazing
3. Thermal-bridge-free construction
4. Airtight building envelope
5. Comfort ventilation with highly efficient heat recovery

No one wants to live in a plastic bag, so to speak, so if you are going to make a house well-insulated and airtight you must also provide some means of bringing fresh air in and taking the stale air out. Thinking simplistically, this may appear to require no more than opening a window or door from time to time. However, this has many disadvantages. It is way less controlled, for one thing - you may forget that a window was left open and inadvertently let in rain or, worse, burglars. When it is cold outside and the house is being heated, opening a door or window throws dollars out the window, so to speak. Opening and closing windows to control interior air and heat is only controllable during the occupant's waking hours.

Studies have shown that to obtain good indoor air quality, the windows would have to be opened fully about every 4 hours, and all the air in each room completely exchanged at that time. It's not going to work well to arrange things on such a system.

In a Passive House, the basic principle is that moist air is extracted from the kitchen, bath and toilet, and the air draw brings fresh air into the living areas of the house. The air to be expelled from a house, in the colder months, is going to be warm air. Putting a heat exchanger into the system allows the heat to be removed from the exhausted air and transferred to the intake air, which again will reduce the heating load. Some heat exchangers are really efficient, drawing 90% of the heat from the exhaust air, meaning incoming air is going to almost be at room temperature. Heat recovery systems require power, however they save something like 10 times the energy that is required to operate them.

To ensure a good air flow within the house, typically there are air transfer openings above the interior doors. Supply and exhaust air ducts are fitted with silencers that prevent sound transmission between the rooms.

Gas oil, solar, and wood may be used for heating and hot water in a Passive House.
Matt J
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Re: Building Performance

Sun Mar 08, 2015 10:42 pm

Two of the most confusing concepts in building science seem to be the "houses need to breathe" thing, and vapor barriers.

When a client asks or tells me that their house needs to breathe, my reply is usually, "Well, the occupants need to breathe. The house needs to be able to dry."

As Chris points out- old, leaky, poorly insulated houses are more forgiving of moisture than tight, well insulated ones. In the leaky house, warm moist air is driven through the assembly, but isn't a problem because there is so much energy transfer to dry it out. Thermal images of such houses show the whole structure glowing hot in cold weather. If you increase insulation levels but don't take steps to control indoor humidity and air movement, parts of the structure will be colder than before and that warm, moist air leaking through will dump its moisture on any surface below the dew point, causing mold and rot.

"Vapor barrier" is a term that is often misunderstood and misapplied. The 'barrier' part implies a material that is "vapor closed," or "vapor impermeable," which is usually undesirable because it can trap moisture inside the assembly. Materials that are low permeance, but not completely impermeable are called vapor retarders. Air can transport moisture in vapor form around 100 times more quickly than vapor diffusion through building materials, so what we usually want is an air barrier to keep moisture out of walls and roofs. Once the air leaks are controlled, it is usually a good idea to have a vapor retarder on the interior side of above grade walls and ceilings to control vapor diffusion (in heating climates) but vapor diffusion gets way more attention than it really deserves. Materials that are somewhat, but not totally impermeable will let the (hopefully small) amounts of moisture that enter the assembly to dry out in one direction or the other. Walls that contain a true vapor barrier can be successful in certain cases in certain climates, but you definitely do not want to have a double vapor barrier, as this would prevent drying in either direction.

Controlling air movement through a building is a good thing, but you also need to control moisture and pollutants (like radon) when you drastically reduce air leakage in a house. The place for the true vapor barrier is below grade, to keep moisture in the ground from entering the house. Heat recovery ventilation is then a great way to control moisture and pollutants generated by the house's occupants.


Great information on this stuff can be found here:
http://www.buildingscience.com/document ... %20barrier

and here:
http://www.buildingscience.com/document ... %20barrier
Matt J
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Re: Building Performance

Sun Mar 08, 2015 10:51 pm

Gas oil, solar, and wood may be used for heating and hot water in a Passive House.
Don't forget about electricity! Air-source heat pumps (for space heat and hot water) are some of the most promising technologies coming out today.

And even inefficient ol' electric resistance heat can be used to heat a Passive House, they're designed to consume as much energy as a hair dryer!
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Chris Hall
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Re: Building Performance

Sun Mar 08, 2015 11:11 pm

It's not that I had forgotten, it's simply that the list was not exhaustive.
ernest dubois
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Re: Building Performance

Mon Mar 09, 2015 4:11 am

These vapor control materials, they come out of the sport world, mountaineering in particular. Acquaintances of mine who did some of the early product testing on this stuff always warned me that its effectiveness fades fast, breaking down and becoming blocked in time with particles and dust... though I do love my polypropylene sock liners in the winter.

I do go for the breathing concept but not in isolation. On the techno side, and I didn't see it mentioned yet, the energy saving can more accurately be called energy use displacement when the embedded costs, money, energy, not the least, social, get accounted for.

A breathable construction out of local materials with low embedded energy, and maybe I can add wearing your sweater indoors during the winter. Some might call it a small sacrifice, or not.
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Steve
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Re: Building Performance

Mon Mar 09, 2015 6:22 pm

In the mild climate of the NW our newly built house - 1350 sq ft living space - costs around $90 a month to heat during the coldest month - heat being natural gas, radiant floor. Conventional framing, double insulated windows - old school 15 lb felt vapor barrier. Around $130 sq ft to build... I am just wondering how extra money spent on insulation upgrades would have paid off with energy costs in our climate?

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