Whether you want to build a new home or fix an old one, the way to ensure that you get the best performance is to do the building envelope right. That means installing the right amount of insulation and installing it well, and it means having an air barrier with minimal leakage. But how do you know when you've done enough air sealing? How tight is tight enough? I get asked this question a bit, and I love to talk about the measures for air leakage anyway, so let's dive in. First, of course, you have to be able to measure how much air leakage the house has. That's what a Blower Door is for. (If you're completely new to Blower Door testing, make sure you read our section about testing for air-tightness.) Then you have to choose how you want to specify the air leakage. The most common unit used by Blower Door operators is ACH50, which stands for Air Changes per Hour at 50 Pascals. I prefer cfm50 per square foot of building envelope, or better, cfm50 per hundred square feet of building envelope (sfbe).

(A cfm50 is a cubic foot per minute at 50 Pascals.) The two reasons for that choice are that (i) air leakage happens at the surface, not in the volume, and (ii) it's the best unit, in my opinion, to express what a Blower Door is really good at - measuring the amount of air moving across the building envelope at elevated pressure.flat panel ac unit Please don't talk to me about ACHnat ('Natural' Air Changes per Hour). carrier central ac units pricesI loathe that measure! ac and heat units for saleIf you're using a Blower Door, you can't measure it, and only researchers use tracer gas analysis. Now we're ready to discuss the actual question: How much air leakage should you aim for? OK, we're not really there yet. First, we have to know about your house.

Are you building new or fixing an existing home? If the latter, what's your budget and how complex is the building envelope? Let's start with the easy one: new construction. The rule here is that a house can never be too tight. The Passive House program takes houses about as far as you can go with air tightness, and their threshold is 0.6 ACH50. I tested a net zero house a couple of weeks ago that was at about 0.5 ACH50. A target that's more achievable for anyone - and which the 2012 International Energy Conservation Code (IECC) will require for most climate zones - is 3 ACH50. That's also the level that Joe Lstiburek identified as a good target in his great article on Blower Door testing new homes, Just Right and Airtight. That number — 3 ACH50 — translates to about 0.25 cfm50 per square foot of envelope, or 25 cfm50 per hundred square feet of envelope. Since roofers have already abbreviated 100 square feet as 1 square, I like the latter form the best. It gets it into a whole number form and is easy to remember.

Get your Blower Door number down to 25 cfm50 per square (or below), and you've got a tight house. The house I built ten years ago came in at 14 cfm50 per square (1.7 ACH50). The places to pay careful attention to in new homes are funky transitions in the building envelope, band joists, top plates, bottom plates, and myriad other details. The simpler the building envelope, the easier it is to find and seal the air leakage sites. We've published quite a few articles on this topic here, so click on the air sealing, air barrier, or building envelope tags to tags to the right to read about some of these details. This can of worms is really too big for the little article I'm writing here, but let me try to shed at least a little light on it. Do worms even like light?) The amount of air sealing you're going to be able to do in an existing home is limited - unless your budget isn't. Generally, with a good attack on the holes, you can get about a 20% to 30% reduction in your Blower Door number (whether you specify it in cfm50, ACH50, or cfm50/sfbe).

Sometimes you can get much more when you add surface area or volume by moving the building envelope. The first thing you want to know here, though, is: How leaky is your home to start with? If you're already at 25 cfm50 per square, it's going to be really hard to get a 20% reduction. If you're at 100 cfm50 per square, it should be a snap to reduce it to 75 cfm50 per square or even lower. The higher that number is, the more big holes you probably have in your building envelope. If you're starting at 100 cfm50 per square, however, don't count on getting down to 25 cfm50 per square unless you're doing a Deep Energy Retrofit. Sometimes Deep Energy Retrofits are called Deep Pockets Retrofits, for obvious reasons. The cost a fortune! Check out Martin Holladay's recent post called The High Cost of Deep Energy Retrofits at Green Building Advisor. I've done a number of air-sealing jobs when I was in the contracting business, and the results varied. Sometimes it was a frustrating 5-10% reduction, and other times it was an easy 20%.

The most reduction I ever achieved was about 40% in a house that started with about 120 cfm50 per square. We encapsulated the crawl space and did a lot of work on the kneewalls and can lights on the upper floor. If you're looking to make your home tight and you really want some numbers to go after, here are my recommendations: New Homes:  25 cfm50 per hundred square feet of building envelope (or 3 ACH50) Existing Homes:  Get a Blower Door test, and see if you can reduce the number by 20% or more. Infiltration Occurs at the Surface, Not in the Volume It's the Hole - Understanding What a Blower Door Is for Mind the Gap - Air Leakage at the Top PlatesIn this Friday, Oct. 16, 2015 photo, the charred remains of the Doctors Without Borders hospital is seen after it was hit by a U.S. airstrike in Kunduz, Afghanistan. Military gunships are loaded with expensive sensors to hit targets. But that’s not the same as knowing what to hit. A lesson is emerging from last month’s tragic U.S. airstrike in Afghanistan that killed at least 30 people at the Kunduz Trauma Center: having human eyes on the target when calling in air support can prevent innocent people from dying.

Despite this, the military does not always require “eyes on” the target before an commander can call in a strike with an AC-130, one of the most dangerous weapons in the U.S. arsenal. The U.S. Special Forces commander who phoned in the strike on the Kunduz Trauma Center was a half-mile away from the hospital at the time he made the call, the AP yesterday reported, according to one former intelligence official who had seen documents related to the incident. The story follows previous reports from the New York Times stating that U.S. special operators in fact did not have “eyes on” the target when ordering the airstrike, suggesting that a joint terminal attack controller, or JTAC, was not physically in the vicinity. Defense One asked Air Force Special Operations Command, or AFSOC, for clarification as to whether JTAC “eyes on” were required for an AC-130 airstrike. AFSOC public affairs specialist Erica Vega responded, essentially, no: “Military doctrine defines and requires several types of control measures for Close Air Support.

Visual acquisition of a target is one of many possible control measures to ensure safe and effective employment of Close Air Support. JTAC’s are not required to visually acquire a target prior to an engagement if other control measures are in place,” she wrote in an email. The military’s manual for calling in close air support points out that a JTAC with direct line of sight to a target is always preferable. “[Close Air Support] in support of stability operations often depends heavily on detailed and timely intelligence, detailed coordination with the maneuver commander’s plan, appropriate munitions, and JTAC with ‘eyes on’ the intended target.” Modern AC-130s are outfitted with GPS and an inertial navigation system, as well as infrared and synthetic aperture strike radar that can acquire targets at long distances. The plane can hit multiple moving targets at once via a variety of systems radar, laser, and coordinates uplinked to the inertial navigation system, or thermal cameras.

The presence of so much sensing and targeting tech is one possible reason why the Pentagon may have substituted “eyes half a mile away” for “eyes on.” “Ideally, a JTAC would have eyes on the target, but that is not necessary with the advanced sensors and communications between the AC-130 and the JTAC on the ground,” said Mike Haytack, a retired Air Force JTAC, last month. Yet, acquiring a target with radar or thermal imaging is very different from determining whether or not the target is appropriate to kill. As The Daily Beast originally reported, the military “actually requires relatively little scrutiny of the target area before  [AC-130] gunship crews opens fire, compared to the much greater restrictions the Defense Department imposes on the pilots of other aircraft types.” A series of documents from the Joint Staff obtained by the ACLU under a Freedom of Information Act Request carves out a number of exceptions for rules regarding collateral damage estimation and targeting, depending on factors including the type of weapon used (105mm or Howitzer grade and smaller) and conditions on the ground.

So what happened in this case? The Special Forces unit whose commander called in the strike, “was under fire in the Kunduz provincial governor’s compound,” at the time, according to the AP. Gen. John Campbell, top Afghanistan War commander, said in October at the Pentagon, “the Afghans asked for air support from a Special Forces team that we have on the ground providing train, advise and assist in Kunduz. The initial statement that went out was that U.S. forces were under direct fire contact. What I’m doing is correcting that statement here.” This could happen again. President Obama announced last month the U.S. would keep 9,800 military personnel in Afghanistan through 2016, in part to support Afghan forces “in extremis”, essentially in extreme danger. That means more potential for U.S. close air support for Afghan forces, Campbell testified. The stage is set for a smaller United States military presence rushing to protect Afghan forces with airpower in situations where those forces are less likely to have familiarity with facts on the ground.