Best Of

Ed,

Around me, ducting is always inside conditioned space. Even builders put it there, that means it is the lowest cost option. This makes sense as all the ducting can now be bare pipe, no need to insulate air handlers or deal with drainage in the attic. You are also not working on your hands and knees in 140F attic but like a human in a comfortable basement when installing.

It amazes me to see all this attic ducting south of the 49th parallel. I can see why it would be common in places where slab on grade construction is the norm, but it makes zero sense elsewhere.

The big energy issue with attic installs is not the lack of insulation but air leaks.

Any air that leaks out of the ducts/air handler/registers goes to heating the attic so it is 100% lost. This lost air now needs to be supplied up by de-pressurizing the house sucking in more outside air through all the leaks in the building envelope. Basically a double whammy of energy penalty. Typical number is about 25% penalty.

Like I said earlier, for major reno or new build, air handler or ducting in the attic is always the wrong answer. Somebody is trying to tell you otherwise, they are wasting your dollars both for the install and as well as operating cost down the road. Also ask them what they will charge to go and service that unit in that 140F attic down the road.

@Kaos mentioned that an attic is never a great place to put an air handler and ductwork . I agree. I even had the opportunity to change a customer's mind about doing just that.

It was a one-story home that had been flooded during Hurricane Sandy and was ultimately raised above the new FEMA flood-level standards. This created a 12-foot-high "crawl space" beneath the home, since the area below the flood level could not be counted as a basement. As a result, there was approximately 3 feet of usable space below the floor joists that remained above the flood level.

Relocating the air handler and ductwork from the attic to this space changed the cooling load calculation from 3½ tons to 3 tons. Simply moving the equipment and ductwork out of the attic reduced the required cooling capacity by 6,000 BTUs.

On a two story home, he challenge is often finding a suitable location for the equipment and determining whether the customer is willing to make the necessary sacrifices to have a better-performing system. Too often, the competition advocates for an attic installation because it avoids the need for soffits or a mechanical room and preserves valuable living space. After all, there is a perfectly good attic available to hide all that equipment. With high-velocity systems, that hot attic is less of a factor in equipment sizing. One reason is that the air does not remain in the ductwork as long as it does in a conventional duct system. It is moving much faster and therefore loses less energy to the hot attic.

Another benefit is that if the attic is insulated with R-50 blown-in cellulose, most, if not all, of the ductwork will be buried within the insulation. This further reduces heat gain from the attic.

Perhaps a small closet could be used to house the air handler so that service can be performed without having to enter the attic. I believe this is a better idea, even with the additional equipment and installation costs for this type of home.

However, with a renovation that is so comprehensive, I believe two zones and separate duct systems are easier to install and allow for larger trunk ducts and other options. Since the basement's low headroom was mentioned, I believe the basement headroom has to be included in the design, with perimeter trunk ducts used to keep the headroom from becoming a factor. I like the idea if an insulated room in the attic for the AHU by @Jon_blaney

I could write paragraphs on my experiences with buildings — tight or otherwise. Ranging from genuinely airtight houses (reliant on HRVs or ERVs for maintaining carbon dioxide and oxygen levels, never mind odours and VOCs and the like) to Cedric's home — built between 1780 and 1893, in stages. Along the way I've encountered efforts at tightening up pre World War II houses — which have invariably been dismal failures, either for succeeding too well with air quality, or rot and mould and structural failure from dampness to spending a lot of money with no real decrease in air exchange rates.

Obvious stuff — insulation where it's feasible, but always open cell or breathable. Good storm windows. On newer structures with poured concrete foundations, draught stopping at the sills. That sort of thing. o it, and worth the money (not on concrete block foundations, never mind stone!).

Blower door tests, where they are feasible — they aren't, always — are useful in determining infiltration rates. They are NOT useful to saying "too much draught".

How tight the home is or will be is ALL speculation until a blower door test is performed.

Not a myth. If — and only if — you can make the structure so tight, and locate the insulation and vapour barriers in such a way that you never get condensation in the walls or roof structure, you'll be fine, granted.

The problem isn't air movement — draughts — the problem is moisture. Or let me put it a little differently. Wood which is kept underwater — soaked — will rot very slowly, if at all. Wood which is kept with a moisture content below around 20% — that is never in contact with liquid water — will simply not rot at all. If wood is allowed to come into contact with liquid water, as will occur in a wall or roof structure which allows — for whatever reason — the temperature of the wood to drop below the local dewpoint (this may be a result of excess humidity or cold exterior temperature or some combination) it WILL rot and it WILL grow mould.

Not that this can't be managed; it can, with a genuinely tight interior vapour barrier, and this, of course, can be done in a gut rehab — although the level of craftsmanship required is often not there.

The worst combination is an exterior vapour barrier or "house wrap" and heavy, but not vapour tight, insulation.

I agree with @pecmsg & @Jon_blaney

That with two floors in a house that size it would be a big mistake not to put in two systems

@Jamie Hall That is a very common often reported myth. People need to breath, houses don't, they need to be able to dry (a lot of house heat leaking through the walls is defiantly a good way to dry them). Old houses sometimes do come with some ugly issues but those can be fixed during a gut job.

@EdTheHeaterMan High velocity is great for retrofit where it is easy to fish the smaller pipe through existing walls. They are pretty spendy to buy and not cheap to run (that high velocity takes a lot of blower power). With the place open, you can put in regular ducting and avoid all those costs. Ducting inside the conditioned envelope also does not need to be insulated.

Before even talking about any heating and cooling equipment, one needs a good man J.

From there, sometimes the answer is hydronic heat, sometimes regular ducts. Sometimes adding a couple of extra inches of insulation can save the cost of fancy equipment.

The answer is never an air hander in the attic though.