Infiltration and Ventilation

Why am I devoting an entire post to the movement of air you ask? And why now, at the early design stage?

As it turns out, moisture buildup in walls is one of the biggest problems in building. Moisture can get into walls from both the outside (rain and humidity) and from the inside (bathing, cooking, house plants and breathing). Both sources of moisture often get into walls by hitching a ride with air. Once moist air is in the wall it can condense into water and degrade your insulation value and cause mold and mildew problems, not to mention structural issues if serious and left untreated.

Since we’ve decided to build a super-insulated house, the more insulation you use, the more this can become a major concern. I’ll explain this more in a future post on wall sections. For now, trust me, its a problem.

There are several ways to control moisture in the wall, by limiting its ability to get in, and then giving it a chance to dry out when it does. I’m just going to consider the first part in this post (limiting moist air infiltration into the wall), and its related topic ventilation. Because if you build a tight house to keep moisture out of your walls, you need to provide a way to get fresh air into the house.

Infiltration and tight houses

As the literature goes, old houses blow. Literally. Old houses were so loosely built (that doesn’t mean sloppy, just not sealed well) that air blows right through the house, through cracks in the wall and floor, electrical outlets, window and door frames and basement and attic openings. It may not seem like a lot, but its a bit like the dripping faucet problem. It doesn’t seem like a lot, but it really adds up over time. As outside air blows through your house, all that nice warm moist inside air that you spend so much money on to create, simply leaks out.

This isn’t all bad. You need fresh air to replace old air filled with odors, excessive moisture and other yummy stuff from cooking, cleaning, washing and breathing. The key is build as tight a house as you can (limit infiltration), and use mechanical ventilation to provide fresh air and to vent odors. Mechanical ventilation can also be used to preheat the cold incoming outside air with the warm outgoing inside air, which is what makes it more energy efficient than just letting the wind blow through your house. We’re cover this a little later.

So its that simple you say? Build a tight house and ventilate mechanically.

Well this is where it starts to get complicated. What constitutes a tight house? How much fresh air is the right amount?

First, defining what makes a tight house means understanding a bit about how to measure air infiltration in a house.

Measuring Infiltration with a Blower Door Test

Air movement is often measured in cubic feet per minute (CFM) or air changes per hour (ACH). A value of 2 ACH means all the air in your home is replaced twice in 1 hour. CFM and ACH are mathematically related, so you can calculate one if you know the other. I find ACH an easier number to understand and use in my calculations, so I’ll use that.

A blower door test measures the air leakage of the house by depressurizing the house to see how much air seeps in to replace the air that gets sucked out. Air pressure in measured in Pascals, or P for short. It is sometimes tacked onto the CFM or ACH, e.g. 5 ACH50P (or simply 5 ACH50), means 5 air change per hour measured at 50 Pascals.

In my research, a tight house is defined anywhere from less than 5 ACH50, to less than 0.6 ACH50, and everywhere in between. The PassiveHouse standard for a tight house is 0.6 ACH50, which is the tightest standard I’ve seen. Based on my research, a 1 ACH50 target seems very doable for our house, considering my OCD tendencies.

But if your blower door test results show 5 ACH50, that doesn’t mean the house air is changing over 5 times per hour under normal usage, that’s only then you have a big fan trying to suck out all the air in the house. So we can’t use that number in calculating how much infiltration is adding to your ventilation needs.

We need what is called a normalized value to approximate what 5 ACH would be under normal conditions. Research on the web says to divide ACH50 by X to get ACHnat or ENIR (estimated natural infiltration rate). X depends on climate and location. I’ve seen ranges from 17 (Minnesota) to 30 (Florida). Of course this is an approximation.  When the wind blows outside, more air is forced into and sucked out of the house than when the wind isn’t blowing. But it helps in understanding how much of your ventilation is being provided by infiltration. I’m using an X factor of 20 figuring we’re closer to Minnesota weather than Florida.

So if I shoot for a target of 1 or less ACH50 in the blower door test, that means I can estimate a value of 0.03 ACH due to infiltration. For you energy nerds, that means I would loose roughly 5.5 BTUs/hr/1° due to infiltration, or over 11,000 BTU’s on a day when its -15° outside. Compare that to a leaky house (1 ACH) which might loose over 383,000 BTUs due to infiltration on the same day.

Now on to the next question.

How much fresh air do you need?

The code recommends that you need 3 cubic feet of air per minute (CFM) for every 100 square feet of conditioned space, plus 7.5 CFM per occupant. Our conditioned air square footage is 1200. You derive number of occupants from the number of bedrooms plus one (2 people as assumed for the master bedroom and 1 for each additional bedroom). We have a bedroom and an office, but by code standards, that office is another bedroom (which also effects the size of our septic system).

But you get the idea, we would need approximately 60 CFM. CFM can also be measured as air changed per hour (ACH), which is an easier number to think about. 60 CFM = .35 ACH. Which means that a little over a third of the air in the house changes over every hour.

I should note that lots of other folks recommend no less than 2/3 ACH, which is another third more than the code recommends. I haven’t figured this out yet. So when I do I’ll let you know.

If you remember from above, I’m shooting for less than 1 ACH50 or just 0.03 ACH due to infiltration. That doesn’t meet the minimum required value of .35 ACH. So I need to provide an additional .32 ACH in mechanical ventilation. Note that many older houses have a value of 1 or more air changes per hour just in infiltration. This means they don’t really need any mechanical ventilation because all their ventilation needs are being addressed by their leaky house.

Mechanical ventilation

Most folks get mechanical ventilation through a forced air system which can provide heating and cooling. In our case, we don’t need cooling. And I haven’t determined what type of heating or how much we need yet. So if we just look at pure ventilation (.35 ACH) and ignore heating and cooling (assuming we just suck in the -15° outside air and blow out the 72° inside air), we would loose roughly 134,000 BTU on a very very cold day, just to get some fresh air in the house.

If we used an air exchange system as mentioned in the early part of this post, we would be able to recover some of the heat that was being blown out of the house. If we assume an 80% efficient heat recovery ventilator (HRV) then we only loose 27,000 BTUs. That is a lot more efficient than the 383,000 BTU’s lost by just letting the wind blow through your house.

All this assumes my calculations are correct. Every time I go through them I find some horrible mistake. I’ll revise this post as I get more accurate numbers. But I’m getting closer. Hope you enjoyed, I certainly did.

Favorite Building Sites and Books

Just thought I’d share a few of the sites, links and books that I’ve been using a lot these days.

First, a book my rhyming upstate neighbor lent me, Builder’s Guide to Cold Climates by Joseph Lstiburek from the Building Science Press. My favorite part is the calculation to determine the proportion of exterior insulation (sometimes referred to as outsulation) to interior insulation. The idea it to ensure that the temperature at the interface of the two systems is above freezing so you don’t have condensation problems in the wall in the coldest months of the year. I included this calculation in the spreadsheet I attached to my earlier post on energy calculations.

On the same topic of insulation and how and where to put it, I enjoy following the the spirited debates on two sites, the “Energy Efficiency and Durability” category on the GreenBuildingAdvisor.com/community site, and the “Forums” on the Journal of Light Construction (forums.jlconline.com/forums) site. They often cross link to each other and tend to see some of the same people contributing to both. They cover lots of good questions about insulation, how to use the HERS index, foam vs cellulose, you name it.

GreenBuildingAdvisor charges for its research papers but also has a good free blog.

Now for my most favorite site, BuildingScience.com‘s research papers. They have a treasure trove of information, my favorite paper of late is the RR-0903 report on total wall R values which I references in my earlier post on Energy Calculations. But they have lots of good content on just about every building topic. And did I mention all their content is free?

I also monitor the 100k House blog. They are a great resource for energy efficient products. Check out this excellent post on Passive House Ventilation Design. They’ve done their homework and are building some great stuff in Philly for the right price range.

Another book I mentioned in the earlier post was The Passive Solar House by James Kachadorian. I’m not particularly interested in building a house in the way he pioneered (the solar slab), but its the best resource I’ve found to do the rough energy calculations for both heat loss and heat gain.

There’s also a new book coming out soon about net zero houses called  Toward a Zero Energy Home, from the same authors of Green From the Ground Up. I’m looking forward to it.

If you know other great building science related books or sites that I should check out, let me know.

Energy Calculations

Warning! This post contains lots of building science concepts and terms. I’ve tried to explain some stuff but this article is for the folks like me that enjoy the nitty gritty science of building.

As Jill pointed out a few weeks ago in her gardening post, I’ve been working on the basic energy calculations for the new house design. It took me a while to get the basics down, again. It’s been some time since I did this stuff in school and I’m pretty sure I’ve messed up some things, but it gives me a bit more understanding into how all the parts (walls, windows, floor and ceiling u-factors, infiltration rates, required ventilation, heat loss, solar heat gain, shading factors, heating degree days, vapor barriers, dew points in the wall and ceiling assemblies, etc.) contribute toward an energy efficient system.

I’m interested in the net zero house concept (make as little energy as you can to meet the power needs of the home, netting out over the year) as opposed to the off the grid house (which has the same concept except you have to net out every time the sun sets). The basic concept is to invest in energy conservation until its more cost effective to invest in energy generation. At a certain point the scales tip and its cheaper to invest in solar or wind power than walls that are three feet thick with quintuple-pane windows in a perfectly sealed box. There’s also opportunities to offset some costs, for example the extra cost of insulation in the walls and ceiling should balance out with a cheaper heating system.

I’ve put together a spreadsheet that captures one half of the net zero concept, the energy conservation part. I can fiddle with the r-values, glass area, window properties, shade factors, allowable infiltration rates, ventilation air changes, heating degree days (HDD), even square footages and see approximately how the system balances out in a year.

12" double wall section

Problem is that it all depends on what you put into the inputs. Do you use center of wall r-values, or averaged values based on thermal bridging and convective heat flows in the wall? This can change the outcome quite a bit. A typical 2×6 stud wall @ 16 inches on center is usually described as having an R value of 20. But studies show that it is closer to an actual R value of 13.7, or 32% less effective. But typically in all the examples of energy calculations that I’ve seen, they use the R 20 value, not the 13.7. Does this inflate the energy efficiency of the house, or does the calculation itself have built in factors to account for the inaccurate R values that are typically used?

I’ve been working with a 12″ wall detail. It’s 3 inches of high-density foam on the outside and 9″ of loose pack cellulose for a center of wall R value of 54. (This wall was used at a net zero house built in Townsend, MA. Link to article.) Although the double stud wall minimized thermal bridging at the center of the wall, it doesn’t address bridging at the top and bottom plate including the rim joist. I’m guessing the average R value across the whole wall is closer to 44. (For an excellent review of various wall systems and total wall R values, see Buildingscience.com research paper RR-0903. Buildingscience in my new favorite site.)

If you plug the 54 R value into the spreadsheet you would see that you need approximately 18.9 million BTUs per year to keep the house above 65 degrees. If you use the R 44 value, you would need approximately 19.7 million BTU’s  per year, or close to a 4% difference.

If you include solar gain in the calculations you see that the sun can potentially provide up to 86% of the total heating load of the house. Changing the window U value or more importantly changing the SHGC value (solar heat gain coefficient) can have a huge effect on the solar gain. Most new efficient windows limit solar gain, but you want that with a house that is positioned and designed to take advantage of the sun. (I used James Kachadorian’s excellent book, The Passive Solar House, to figure out the heat gain calculations.)

Anyway, if you’re interested in the actual calculations, I’ve included a PDF version. Add a comment below if you’re interested in the spreadsheet version.

I’ll add another post soon with links to more the books and online sources that I’ve found helpful. I have a series of posts planned for the next couple of months including window options, ventilation choices and calculations as well as more wall sections. I’m also planning a high-level cost estimate so I can approximate the trade-offs between energy conservation and generation. I hope all you building science geeks out there enjoy.