Efficient House Design and Construction, Part 2

The Bruhl-house living room. Note the "clothes dryer"--- the rail around the woodstove. An energy-efficient method, but also a source of indoor humidity.

The Bruhl-house living room. The very-tall interior flue (barely visible on left of photo) drafts exceptionally well, and we’re very happy with the overall performance of the woodstove. Note the “clothes dryer”— the rail around the woodstove. An energy-efficient method, but also a source of indoor humidity.

(Note— This is a continuation of a post from last year, “Another Tough Cookie: Efficient House Design and Construction“.)

Ok, a longish and slightly technical post here. But, it might pertain to, say, all those people out there who live in houses. To wit—the “topic-of-the-week” between me and Mr. X seems to have revolved around indoor air quality, and the problems that arise as buildings are made more efficient and air-tight. Particularly, the air-related problems the Bruhl house is having, and what exactly the best design going forward might be to remedy those problems. And all of this was triggered last week when I picked up the latest “Energy-Smart Homes” edition of Fine Homebuilding. It contains quite a few articles about air leaks and building durability, one which includes a photograph of a house in Minnesota with the siding off and the house wrap pulled back, which reveals enormous areas of rot and mold caused by air leaks.

energy smart magazin cover

The Winter 2016 edition of “Energy-Smart Homes”; well-worth the cover price.

To catch you up if you haven’t read the post from last April, our house is fairly tight, but has no dedicated ventilation system. Adding one has been on my “to-do” list for years, but it has become more of a priority as I have come to better understand this topic. We currently have fascia and trim boards in at least three places that are rotting from the back side due to air leaks, and the photo in the magazine really made me wonder how much damage is being done that isn’t visible.

So off I went on a reading-binge. And guess what? No real surprise; it’s complicated. In some cases, really complicated. But for the sake of clarity, let me skip rather quickly to some of my preliminary conclusions, and mention the complications briefly as I go.

Ok, the short version—our house is built with stress-skin panels around a post-and-beam frame, and is fairly tight. I’d like to be more specific, but I haven’t had a blower-door test done, so I don’t know the actual air-changes-per-hour (ACH) numbers. Without a dedicated air-exchange system, though, this tightness causes the humidity in the house to run on the high side during the winter. In colder climates (of which Vermont qualifies) this causes problems when this humid air leaks out of the house (generally in the upper half of the building, where the pressures of the warmer air inside, which is less dense, are higher.) The humidity condenses into water when it hits the cold exterior parts of the house (or into ice, which later melts into water), and causes rot and mold, often behind siding and sheathing where it can’t be seen. Ideal indoor humidity levels (in terms of building durability) vary depending on how cold it is outside (good info here), and these ideal levels drop as it gets colder. For example, about 40% relative humidity (RH) is ideal when it is 32-degrees Fahrenheit out, but only 20% RH is ideal when temperatures are -10 (F). Our house, in general, runs 10 to 20 points higher than this “ideal” in the winter, and was probably higher still in years past, when the construction materials were new (and moisture-laden) and the bathroom vent fan wasn’t yet vented to the exterior. In some ways these levels are comfortable—we don’t get dry skin or chapped lips, and we never have static-electricity problems. But, the downsides that affect building durability probably outweigh these good aspects.

A photo looking up under the eaves, the edge of the eaves is at the top of image. Obvious rot is visible in a place where rainwater never hits, caused by an air leak from the interior.

A photo looking up under the eaves; the edge of the eaves is at the top of image. Obvious rot is visible in a place where rainwater never hits, caused by an air leak from the interior.

Now, I could (and will) find and fix the air leaks. BUT—to do so is a Catch-22. As I seal up the leaks we will have even less ventilation, and the  the indoor humidity levels will go even higher, absent some additional air flow. And, these humidity levels are in some ways proxies for general air-staleness, as well. The bottom line—we need some additional ventilation.

Here’s where it gets complicated, because there are a number of ways to proceed. Let’s start with a short primer on something that’s around us all that time but that we don’t often think about—interior air.

First, contaminants and humidity. In decades past buildings weren’t built with much air-tightness, and air flowed through them fairly easily. Modern energy-efficient homes are much tighter, however, and the air inside these buildings gets “stale” over time. Occupants breathe in oxygen and exhale CO2, levels of dust and other particulates rise, and cooking adds humidity, tiny grease particles (and, in the case of gas stoves, combustion byproducts), and odors. Interior air in tight houses generally gets more humid, too, if it isn’t exchanged. In addition to cooking and people breathing, water vapor comes from house plants, putting clothes out to dry, showers, and a number of other sources.

To deal with stale air, energy-efficient houses need some way for air to be exchanged. We’ll come back to this.

An inexpensive indoor/outdoor thermometer with humidity readout.

An inexpensive indoor/outdoor thermometer with humidity readout. In this case, 45% RH indoors is ok for the exterior temp of 42, but wouldn’t be ok when/if the temperature outside was in the 20’s. I have several of these gizmos, in different parts of the house.

Next—air pressures in a house. Ideally, interior air pressures in a house should be as close as possible to neutral—neither positive or negative when compared to the exterior. In reality, a building that is heated always has some degree of “stack effect”, however, where the heated air is buoyant and is trying to rise up, which causes slight pressures high up in the interior, and slight suction down low (short overview here). (The reverse can happen in an air-conditioned building.) Stack-effect pressures are unavoidable (and generally slight, unless you live in a very tall building), but pressures beyond those should be avoided. Excessive positive pressure can push warm, humid air out through the building envelope, exacerbating condensation problems, and negative air pressures (depressurization) can prevent woodstoves from drafting properly, pull radon up through basement slabs, pull flue gases backwards above atmospherically-vented water heaters and other such appliances, and can contaminate infiltrating air by pulling it through moldy or dusty areas, or from attached garages.

To deal with all of this, modern houses often employ balanced ventilation systems that keep pressures neutral while pushing stale air out of a building, and pulling fresh air in. As I discussed in that previous post, those two air streams are often run through heat-exchangers in a Heat Recovery Ventilator (HRV) or Energy Recovery Ventilator (ERV), in order to capture as much heat as possible from the outgoing air.

This sounds simple, but in real life it can get complicated. In my case, let’s look first at what provides wintertime ventilation in the house as it is set up now (in the summer this is all a moot point—we keep the windows open).

Current wintertime ventilation in our house–

1) The woodstove. We heat with wood, and most woodstoves draw 10-15 cubic-feet-per-minute (CFM). When we installed the stove I did a bunch of reading about makeup air, and decided to install a 1 1/2-inch PVC pipe that runs from the outside, under the foyer, and lets fresh air in just behind the stove, where the heat from the stove would temper the cold draft created. It seems to work great; this make-up air pipe is at about the neutral-pressure level in the house, and seems to allow fresh air be sucked in at a slow and steady rate.

Note PVC make-up air pipe behind stove. The is much disagreement about make-up air for wood stoves, but in very tight modern houses some form of makeup air is likely a good idea. The devil is in the details on this one, though. Ours works well, but it might partly be a happy accident; the same inflow at a different level of the house could well be a huge air leak.

The PVC make-up air pipe on the short wall behind the stove. There is much disagreement about make-up air for wood stoves, but in very tight modern houses some form of makeup air is likely a good idea. The devil is in the details, though. Ours works well, but it might partly be a happy accident; the same opening at a different level of the house might well function mainly as a huge air leak.

2) The bathroom vent fan. In the main bath we have a 50 CFM vent fan that exhausts steamy air from the shower to the outside.

3) The whole-house vac. Our whole-house vac exhausts to the exterior, but doesn’t run for long durations in the big scheme of things, so it can be discounted in terms of ventilation (though not in terms of depressurizing the house).

4) The air leaks. In general, I was happy with the quality of work done by the timber-frame contractors who built the house, but they were a bit negligent (or they didn’t understand the importance of air-sealing) when they used spray-foam to seal up the gaps between the stress-skin panels, particularly where the roof panels meet the wall. The leaks have caused problems, but have also helped the house to “self-ventilate” to some degree at wintertime temperatures.

That’s it. We don’t have a clothes dryer, the heat-pump water heater is self-contained, and we have no range hood in the kitchen. The good news—the house as it is set up now allows for some limited air exchange, but, as discussed above, the current setup is in no way ideal.

Some considerations going forward–

A large range hood in a modern kitchen. Range hoods can cause significant negative pressures in houses.

A large range hood in a modern kitchen. Range hoods can cause significant negative pressures in houses.

Problem #1- The future range-hood. We cook, a lot. We don’t currently have a hood over the range, but we really need one, especially if I make the house even tighter. With multiple skillets and pots going, the air in the kitchen gets decidedly contaminated. Range hoods are difficult for both aesthetic and mechanical reasons, but the latter is our focus here. Vented hoods draw between 150 and 1200 CFM, and their exhaust streams cannot be run through an HRV; they must be directly vented to the exterior (grease particles would clog up an HRV, and/or the volume of air is too high). Because range hoods are not balanced ventilation systems by themselves, they can work to depressurize a house when they run, which can cause problems. In our case, the big problem is the woodstove—it has a tall interior flue which currently drafts extremely well. If a range hood was running, though, it might diminish or reverse the draft, or cause smoke to spill, or, if the stove was cold, could pull the smell of ashes and the flue backward into the house. When you add the dangers of carbon-monoxide to the mix, I really don’t want any negative pressure pulling on the stove.

There is a great deal of controversy about the issue of makeup air for range hoods in energy-efficient houses, and I find some of the “solutions” to be less than ideal. Some recommend using recirculating vent hoods that filter the air and release it back into the house, but I have serious doubts as to their efficacy (and I don’t want the expense or trouble of replacing more filters). Others recommend, drum roll—not cooking. This also seems like a really bad “solution”. Some recommend opening kitchen windows when cooking rather than installing a hood, but I don’t like this idea, either; it seems to be a partial solution, at best. So, after much consideration, I think we will need a vent hood, and, because we also have a wood stove, I think we will need to combine it with a dedicated makeup-air source. Some sources say that makeup air is needed if exhaust flows exceed 400 CFM, but with the wood stove, I think we will need provisions for makeup air regardless of the size of the hood.

Broan vent damper

A makeup air vent damper, this model by Broan.

There are a number of ways to provide makeup air; in houses with central-air the air can be fed into the supply ducting. In our case, though, I think the best solution would be to add a 6-inch duct that would go from outside to the wall behind the wood stove, and put a motorized damper on it. The damper would automatically operate in tandem with the vent hood, using a low-voltage motor. So, when the range hood is turned on, the damper would open and allow large quantities of makeup air to enter the house, which would keep the house from depressurizing. (The dampers are also available in 8 and 10-inch versions, and can also be “ganged”, so the sky’s the limit here, but you wouldn’t want to purposely oversize the makeup-air portion of the system.)

Problem #2- Can the exhaust air in the bathroom be run through a HRV? More differences of opinion exist on this one, too. Ideally, exhaust air from bathrooms would run through an HRV, rather than dumping all of that heat straight outdoors. Some HRV manufacturers recommend this, and some even provide a “boost” mode that temporarily increases exhaust rates. But it seems to me that very moist air from when someone is taking a shower, if run through an HRV on a day when it’s very cold outside, would cause near-instant frost problems in an HRV. Most models have automatic defrost modes, but you wouldn’t want the ventilation to stop just when you need it the most, when someone is showering. I’ll have to look into this. It might be a moot point, though, because of a new style of HRV that doesn’t use ducts at all. Which brings me to…

Problem #3– Should I use an HRV or an ERV, and/or what type? This question led to more research, and yet more differences of opinion.  The best I can figure— ERV’s, which transfer humidity as well at heat, are generally used in hotter, humid climates, to reduce loads on air-conditioners (it makes AC’s work really hard if outdoor humidity is pumped into an air conditioned space). Here in relatively-cool Vermont, an HRV would seem to be a better choice. And there’s another option to consider—the new, ductless HRV’s from a company called Lunos. Traditional HRV’s have ductwork that pulls air from bathrooms and kitchens, and puts fresh air into bedrooms and living spaces. But Lunos has made models that consist solely of short 6-inch diameter tubes with fans that are inserted through exterior walls, but that push the incoming or outgoing air through a ceramic media. It’s quite ingenious—a controller synchronizes each pair of fans, so that while one is exhausting, the other in pulling fresh air in. Every 70 seconds, the controller reverses the flows. The ceramic media imparts room heat to incoming air for 70 seconds, gets cold, and then soaks up room heat as the flow reverses. A nifty idea, and one that eliminates both ductwork and defrost cycles.

A short video about Lunos HRV’s–
(Another Lunos video here, with graphics about air flows, and a discussion of different models and sizes.)

I like the idea of the Lunos HRV’s—they appear to be highly efficient and easy to install. What I don’t know is whether they would work well if the pairs were well above or below the neutral pressure plane in a house. If they need to be near this line, and my guess would be that they would function better if they were, then it would preclude putting one in the downstairs bath, whereas a traditional HRV with ductwork could easily have an intake in that bathroom. It would be nice to have air continuously drawn from that room, as it is probably our largest source of humidity, between showers and damp towels. If I went with the Lunos models, then the standard bathroom exhaust fan would have to remain (I think?) to cope with large volumes of humid air generated while showering. The Lunos idea is something worth considering, though. The units are expensive (about $1,000 a pair), but could pay off in terms of ease of installation, and the costs saved by eliminating ductwork.

Problem #4— how to distribute air from top to bottom in our house. Even with proper air exchange and ventilation, it would be good if somehow air was circulated within the house, specifically from top to bottom. The wood stove is on the main floor, but the bedrooms are in the basement, and temperatures are slightly too cool down there in the winter. A year ago I experimented with a high-efficiency (17-watt) inline fan that Mr. X gave me, which moves large volumes of air from the main floor to the basement, but I need to rework it to make insulated ducts that will move air from even higher in the house. Needless to say, this is also on the “to-do” list. If we used Lunos HRV’s, and if those units needed to be near the neutral-pressure plane to work properly, then this system would move that fresh air throughout the house and into the finished basement.

My trial version of ducting air into the basement. It works, but I might modify it to make it more efficient, and to filter the air.

My trial-version of a system that moves air into the basement. It works, but I might modify it to make it more efficient, and to add a particulate filter.

So, here’s where Mr. X’s solutions differ slightly from mine. He feels that simplicity is a virtue in these situations, and that adding a larger bathroom fan, running it longer, and installing and using a clothes-dryer would solve nearly all of the problems without expensive additional equipment. I pondered this, but I’m leaning toward a slightly more comprehensive solution. I decided that whatever design I end up constructing should meet three criteria— it should be efficient, it should be automatic, and it should be optimized. To explain:

Efficient– Blowing cold air outside without capturing heat is wasteful, and so air streams that don’t go through heat exchangers should be kept to an absolute minimum. Likewise, equipment should draw as little electricity as possible when operating, and systems should be set up so that they aren’t fighting against static pressures that exist in the house, where possible.

Automatic– A house should function even if the people who are in it don’t understand all the nuances involved in the design. So, visitors should be able to use the house without worrying about “messing up the air”. HRV’s should be controlled automatically with timer units or humidistats, makeup air ducts should open and close automatically, and at no time should windows be required to be opened or cracked in order to keep the woodstove drafting properly, or to provide additional ventilation for a bathroom or kitchen. By the same token, the systems should be unobtrusive—visitors shouldn’t be able to hear or see them, they should just be there, functioning, in the background.

And finally, optmized. If I’m going to put time and effort into improving these systems, the finish product should fully address all the current shortcomings. It seems crazy to work toward partial solutions, especially in a house that I might live in for the rest of my life. I might even add an ionizer unit, just to top it all off when I’m done.

So, to conclude: I don’t quite have all the answers, yet. But, I will have most of them soon, and I will begin to put these changes into place. I’ll fix the air leaks when the weather gets nicer (much of it will require work high up on the exterior of the house), and will put the new systems in. Next winter, when it gets cold, I’ll have the Vermont “Heat Squad” come by and do an energy-audit on the new systems, which would include a blower-door test and a look around with a thermal-imager (and likely more air leaks to seal!).

In the end, it looks like we’ll need a “Part 3” for this topic; I’ll keep you posted!


Image credit, vent damper: Broan Model MD6TU.
Range hood: Goedeker’s, “Summit SEIH4636CV4 Professional Series Range Hood”, Flickr Creative Commons. Image has been cropped.