Archive for the 'Air sealing' Category



More sheetrock: gaskets and taping

Thought I would share a few more sheetrock progress photos. The entire second floor ceiling is now sheetrocked and taped. This means we’re ready to foam the intersection of the walls with the ceiling, from the attic side. Then we just have to install the east door before we’re ready for the blower door test, sometime in the next week or two.

In the meantime I’ve been stapling gaskets to the exterior walls on the 2nd floor. Our primary air seal is at the exterior of the shell, but we’re also trying to seal the interior shell to prevent any warm moist air from circulating near the cellulose. The gaskets are important in 3 areas, ceiling, floor and at the intersections with interior walls.

Ceiling: The ceiling gasket is redundant with the foam that will be applied from the attic space. Basically, I don’t trust the foam or the gasket on their own, but I’m hoping both together will catch any spots we miss.

Bottom: The gasket at the bottom of the wall, together with the gasket under the wall that we installed during framing, keeps interior air from entering the exterior wall space at the intersection with the floor.

Intersection with interior walls: The vertical gasket at the intersection with interior walls keeps interior air from slipping into the exterior wall through the corner stud.

Problem area

The stairwell is adjacent to the north exterior wall and penetrates the floor planes to pass from the basement to the first and second floors. Following the approach described above, I should run gaskets around the exterior wall and at the ceiling and floor, but what about the truss area between the 1st and second floors?

The ceiling over the 1st floor is open all the way to the exterior sheathing, and to the exterior wall behind the stair. I think we’re going to need to block the area at the trusses, so air from the 1st floor ceiling can’t pass into the exterior wall behind the stair.

It’s little details like this that are rarely documented in most air sealing guides (because they are less typical) and easy to miss. I’ve been going back over all our details to make doubly sure we haven’t missed something silly.

Let the sheetrocking begin!

Well, at least for the 2nd floor ceiling.

Normally, you wouldn’t start the sheetrock till the walls are insulated. But in order to test how well (or poorly) we’ve sealed the house with a blower door test, we need to complete the air barrier, and that means the 2nd floor ceiling. This is the area where the air barrier transitions from the exterior of the house to the interior. It will be one of the trickiest areas to seal properly.

We installed gaskets around the top edges of the walls, but because we’re using strapping, it made it difficult to close all the gaps. So we’ve decided to use 2-part foam only around the edges where the ceiling meets the walls. This should ensure that we have all possible gaps sealed.

Lessons Learned

We used poly boxes at all exterior wall electrical box penetrations, but we totally forgot to do this at the 2nd floor ceiling. We’ll have to use a combination of tape and foam to seal light fixture electric boxes in the ceiling.

I think I’ve mentioned in an earlier post that a smarter way to air seal the top of the enclosure would have been to extend the exterior sheathing and air barrier over the ceiling joists. Then pile the roof insulation on top of the sheathing. This method has been used by Marc Rosenbaum, although the air barrier in this case was the roof.* This approach would necessitate raising the roof up a bit to ensure proper insulation levels at the eves. The extra expense of sheathing the top of the ceiling joists would have likely been offset by the extra labor and material costs spent sealing all the gaps with foam.

* http://www.southmountain.com/smclibrary/articles/2011_01_JLC.pdf

Attic access detail, the ‘cork’

I started working on the details for our attic hatchway a few months ago. The internet is fairly void of super-insulated air-tight attic hatchways. I found this curious considering all the net zero and Passivhaus work going on to date.

The problem is how to insulate and air seal the attic access to the same standards as the rest of the ceiling. Our ceiling will be insulated with 24 inches of loose cellulose (R-75), so I wanted to make sure the hatchway was at least equal in R value and air tight. The trick was to find a way to achieve this and still be able to open and close the hatchway with relative ease.

Our solution is nicknamed the ‘cork’. Essentially it’s a 24″ deep hatchway filled with 2 insulated components. The first component is air sealed to the interior drywall. It is a piece of plywood screwed to the ceiling within a gasket and 8″ of left over rigid insulation (R-40) glued to the top of it. This is a fairly standard approach to sealing the attic hatch. The second component (the cork) is 14″ of rigid insulation (approx. R-70) held in place by a hinged top plate.

To gain access to the attic we will first remove the lower panel from inside the house, then we pop the top barrier out into the attic. To close the access we reverse the process.

The total insulated value is R-110, but that is misleading. Due to inefficiencies of sealing the insulation to the hatchway, air can circulate inside the hatchway in tiny gaps at the edges and between the two insulated components, so I just tried to cram as much insulation as possible into the hatchway as tightly as possible but still be able to pop the cork to gain access to the attic. Only an inferred camera will tell if my efforts have been successful.

I hope this is helpful to others out there building super insulated houses with attic access from the inside. If you have a simpler detail, please don’t tell me. I feel silly enough spending 2 days building the cork. But please do post your solution to make it easier for the next person.

One last word, the code requires attics be accessible, but I don’t believe it specifies whether it has to be from the inside or outside of the house. Assuming you have an attic that needs access and your local code allows outside access, I would recommend exterior access based on my experience building a super-insulated air-tight attic access hatchway. Interior access can be done, but it’s a lot of tedious work.

Foam continued…

While the solar installers have been on the roof, Warren and I have been in the basement foaming the rim joist area. We are foaming this area for two reasons. First, we need to seal the water proofing barrier to the top of the concrete wall. The water proofing is also our air barrier and keeps any radon gas from seeping out at the top of the wall. Second, this would be a difficult and time consuming area to staple up a net and blow cellulose.

Here’s the rundown on our approach.

  • We’re using a two part foam made by Touch’n Seal. They make standard and fire retardant type foams. We’re using the fire retardant type because we’re not planning to finish off the basement ceiling any time soon.
  • We are foaming this area to a thickness of 7 to 9 inches to ensure we meet our stated R40 minimum. The foam is rated at 6.23 R per inch for the fire retardant type. The R value is a bit higher for the standard foam.
  • We’re using the 600 board feet kit. It includes two tanks, A (white) and B (red). We bought the longer hose set, 30 feet. There are 2 hoses. The liquid from each tank is combined at the nozzle where it combines with air to create high expansion foam. The nozzles are replaceable. We’ve used quite a few as the foam clogs them easily.
  • The temperatures have been in the 60’s and 70’s, but the foam really works best at higher temperatures. The tanks should be at 80 to 90 degrees before use and the surface temperature where you’re spraying should be above 70 degrees. Warren built a foam box that we use to warm up the tanks before use. We dropped in a small electric space heater to get them up to the required temperature range.
  • There’s a bit of conflicting information as to the surface temperature. We asked because we were experiencing some shrinkage away from the wall. The foam would expand initially, then pull away from the wall slightly as it cured. It only takes few minutes to cure. But the second and third sprayings seem to be doing a better job. The first application pulls away a bit, then provides support for the next application, holding it in place better. Our last spray day was quite warm out, but the higher surface temperatures didn’t seem to make a difference.

We’ve used 3 kits so far and have covered the attic rim area and half the basement rim area. We’re guessing we’ll need a little over 2 more kits to finish the basement and foam the north and south rim joists at the second floor. The east and west rim areas at the second floor are easy enough to net and blow cellulose.

Lessons learned?

If we had hired a professional crew to come in and spray all the areas and I wasn’t there myself watching the process, I might think this was the best stuff in the world. But having gone through the process of doing it ourselves, I would absolutely not use this much foam again. I’m so glad we decided against the ‘flash and batt’ approach. This stuff is nasty, and we can’t even recycle the containers. The instructions that come with the containers didn’t even mention using air breathing masks. We had to call the company to get a recommendation on the right type of mask to use. Get one that blocks organic compounds and make sure you have plenty of fresh air while working.

I knew the foam was not a green product, but I was willing to trade the hazards for the long term benefits of a more air-tight, super insulated envelope. Now I’ve changed my mind. A little foam in strategic spots is fine. But if I design another home like this I would go out of my way to find other solutions.

Just as I was putting the finishing touches on this post, the always timely GBA posted this article, “Waiting for EPA Action on Spray Foam Insulation“.

The cost of infiltration

There are several ways to approximate the cost of a leaky house. Before I proceed, however, I just want to mention there are other much larger longterm monetary benefits to building a tight house, like a longer lifecycle and less maintenance due to keeping critters and moisture out of the building enclosure. I’m sure there’s a way to calculate the value of these additional benefits, but it’s beyond my abilities (and you’d have to use a lot of assumptions to get there).

Having gone through the numbers, there is some value to understanding the factors that are used to estimate the cost of infiltration, as well as the proportion of infiltration cost to the overall energy costs required to heat a house.

The primary method to estimate infiltration is to calculate the energy required to condition the air replaced by infiltrated air. First we must convert the ACH50 value derived from the blower door test to a ‘natural’ value (ACHnat) that might be experienced when the house is not being pressurized by artificial means. Then calculate the energy required to heat the naturally infiltrating air on a yearly basis. Then calculate the cost of the energy used.

Converting ACH50 to ACHnat is based on many assumptions including the climate zone, height of the house and the degree to which the house is shielded from the wind by surrounding vegetation and other structures. Energy Star recommended a value of 17.8 for ‘well shielded’, 14.8 for ‘normal’ and 13.3 for ‘exposed’ locations in Zone 2. Let’s use the normal value for now.

Dividing 1 ACH50 by 14.8 equals 0.067 ACHnat. This means approximately 6-7% of the air volume in the house (992 ft3) will be replaced by infiltration each hour. [This is not entirely accurate since the volume of the basement is included as conditioned space, yet it is not heated directly.] Let’s assume the inside temperature is 65°F and the outside temperature is 10°F. It takes 0.018 BTUs per hour to heat 1 cubic foot of air 1°F. 992 * 0.018 * 55°F = 982 BTU/hr to heat the infiltrated air.

1 BTU equals 0.293 Watts, so 992 * 0.293 = 288 Watts or 0.288 kW. At $0.14/kWh it would cost $0.04/hr while the temperature outside is 10°F. (We’ll take into account that an air source heat pump (ASHP) can be 2 to 3 times more efficient at converting a kW to a BTU later.)

We can use heating degree days (HDD) to estimate the cost over a typical heating season. In our climate area we have 7100 HDD in a heating season. Substituting 7100 for 55 in the above calculations and multiplying times 24 to get a daily value means you would spend roughly $125 for one heating season just to heat infiltrated air.

But as I mentioned earlier, an air-source heat pump can be 2 to 3 times more efficient at converting electricity to heat than simply using electrical resistance. There are other factors that also temper that number. I found the following formula in several blower door manuals (see here, section 6.3.d). The formula introduces two correction factors, one for the efficiency of the heat source and one for everything else.

Annual Heating Cost = (26 x HDD x Fuel Price x CFM50 / N x Seasonal Efficiency) x 0.6

  • 26 is the result of multiplying the heat capacity of air (0.018) x 60 minutes x 24 hours.
  • HDD is the Heating Degree Days (7100).
  • Fuel Price is $/BTU. 1 BTU = 0.000293071 kW. Multiplying times the price of electricity in our area $0.14 kWh gives us 0.0000410299.
  • CFM50 is 245 for 1 ACH50.
  • N is the Energy Climate Factor (14.8).
  • Seasonal Efficiency is the efficiency of the heat source. ASHP range in values from 1-3. Let’s use 2.5. This is one of the correction factors I described earlier.
  • 0.6 is the second correction factor. I found a brief description of what this value represents here.

Using these inputs, we will spend about $30 per heating season just to heat infiltrated air.

If you set Seasonal Efficiency and 0.6 each to 1, then you get roughly the same value I described earlier, $125.

$30 represents the annual cost for air infiltration and it’s less than 6% of the estimated $510 we will spend annually to heat the entire house (space heating only). We can now compare the cost of heating the infiltrated air per heating season for a number of different ACH50 and CFM50 values.

ACH50 / CFM50 0.6 / 137 1 / 245 2 / 469 3 / 704 4 / 939 5 / 1174
Annual cost ($) to heat infiltrated air 17 30 60 90 120 150

In fact, these numbers should be even less because I’ve included the basement volume in these calculations. The basement is not a heated space but it tempers the temperature in the house.

If we can hit a target of roughly 1 ACH50 we will pay 20% of the cost of an EnergyStar house with 5 ACH50.

You can also compare the different values based on Energy Climate Factor.

Another way to think about it, our cost is roughly proportional to the amount of air leaked in a blower door test, 6% leakage equals 6% cost of our projected annual heating bill. Saving 6% per year is a lot better than the interest I’m earning on my other investments, and those savings increase as the price of electricity goes up.

Just remember, 6% in this example is based on the expected performance of our house (insulation values, % of window area, orientation, climate, exposure, etc.). A house with lower insulation values, and a warmer climate will find a different % of savings by building a tighter house.

How tight is tight enough?

As the snow piles up outside, I have turned my thoughts to Spring. Not only because it’s warmer and nature is waking from it’s long slumber, but also because I hope we will be conducting our first blower door test by then. Specifically I’ve been considering what type of results I should be expecting from the blower door tests, and how to interpret them.

We made the decision early on to build an air tight house. This guided our choice of building materials, Zip system and tape for the exterior sheathing, gaskets and acoustical sealant to seal all other connections, and foam to seal gaps at the rims, windows and doors. The blower door test will help us seal any gaps we missed. But what target should we aim for? How do we know when the house it tight enough?

There are a few standards that define acceptable infiltration levels in a way that can be accurately verified by a blower door test. The PassivHaus standard requires <= 0.6 ACH50. Energy Star 2.0 requires <= 5 ACH50, and Energy Star 2.5 and 3 require <= 4 ACH50. The building code defines prescriptive methods to control infiltration rather than specify testable targets.

So why the big range from 0.6 to 5? And what is ACH50?

The range is due to the different goals of the organizations behind these standards. PassivHaus is a non-governmental organization started in Germany. They set the infiltration standard at the lowest level they thought was easily attainable in residential construction as part of a strategy to reduce overall energy use of the home to very low levels and protect the enclosure from moisture that is transported by the infiltrated air. EnergyStar is a governmental organization that works with the construction industry to set standards. They set targets they think can be achieved by builders over a period of time without upsetting a large population of their constituency. The code is the least common denominator.

ACH50 is Air Changes per Hour at 50 pascals. Basically air is blown into (or out of) the house up to a specific pressure, 50 pascals (very low pressure, 0.00725 PSI), and then see how hard the fan has to work to keep it at the same pressure. From this they can calculate the cubic feet per minute at 50 pascals, or CFM50, of air that is replacing the air that was sucked out at this standard pressure.

To find ACH50 which is in hours, multiply CFM50 * 60 then divide by the House Volume. ‘House volume’ is rarely defined and sounds rather vague and open to creative interpretation. Is it the exterior dimensions and the building height? Or is it the interior dimensions? Do you include the basement?

Because house volume is the divisor, the bigger the value the better your resulting ACH. Which leads me to believe that people often fudge their volume by using the outside dimensions instead of the interior volume which can be quite a difference when your walls are 12 inches thick. PassivHaus even subtracts the interior floor and wall volumes (gotta love German precision).

As for the basement, if the air barrier separates the house from the basement then don’t count it. In our case, the air barrier continues down the foundation wall and under the slab, so I have including the basement volume. This decision has several consequences. Our ACH value will be lower than if we just use the house volume area, and it will cause inaccuracies later when calculating the cost of infiltration since the basement is not heated directly. I think this is why PassivHaus generally recommends building on slabs, not basements.

Let’s look at an example. If our blower door test in the Spring reveals a value of 240 CFM50, then we could end up with the following ACH values depending on how you define ‘house volume’.

240 CFM50 Exterior Interior Interior minus
interior walls
and floors
Dimensions 32’ x 22’ x 23.5’ 30’ x 20’ x 17’ +
28.5’ x 18.5’ x 8.5
trust me
Volume (ft3) 16,544 14,682 13,697
ACH50 0.87 0.98 1.05

Listing the different standards we can see the range of allowable CFM50 values.

Standard Max ACH50 allowable Max CFM50 allowable
PassivHaus* 0.6 137
Uphill House Target** 1-2 245-489
Energy Star 2.5 and 3** 4 979
Energy Star 2.0** 5 1,224

* 13,697 ft3 = Interior volume (including basement), minus interior wall and floor volumes.
** 14,682 ft3 = Interior volume (including basement).

Not only is volume vague, why use volume at all? What does the volume of air in your house have to do with the air tightness of your envelope? Another option is to use surface area of the envelope or building shell instead of volume. See EarthCraft House Guidelines to calculate surface area. EarthCraft recommends a target of <= 0.5.

Marc Rosenbaum also likes specifying air leakage using the surface area because it is not biased towards larger volume homes. He refers to this measurement as CFM50/ssf or shell square footage, and sets a minimum target of <= 0.05 CFM50/ssf for super-insulated homes. (See his presentation on the Efficiency Vermont site.)

Using Rosenbaum’s recommendations the resulting CFM50/ssf are:

4,189 ssf* Super-insulated Conventional
CFM50 200 400
CFM50/ssf 0.05 0.10

* Our shell square footage is 4,189. Slab and ceiling (32 x 22 x 2) + N/S walls (32 x 25.75 x 2) + E/W walls (22 x 25.75 x 2).

Considering the vagaries of ACH, the air leakage ratio (CFM50 / ssf) should be open to less creative interpretation. But there is a similar standard that only counts surface area above ground. See the MLR – Minneapolis Leakage Ratio definition. They also state that a value < 0.5 is good and > 1 is bad, same as EarthCraft, yet they count substantially less surface area, pushing up the allowable air infiltration.

Comparing the PassivHaus and Energy Star standards with Rosenbaum’s recommendations, I’m inclined to shoot for a target range of 200 to 300 CFM50.

So far we have only considered the various standards for air tightness, not the resulting costs of infiltration at various rates nor the additional costs of constructing a tight house.

Next week I’ll post some of the numbers I have been working on to quantify the cost of infiltration. Toward the end of the project I will try to quantify our cost of building a tight house.


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