Living attic vs larger footprint: where to build in extra space
Design considerations for Pretty Good House in zone 6a.
If one lurks on GBA for long enough, they might be led to think:
1) Basements are traditional but arguably not the most cost effective solution to build a PGH. They are a hole in the ground with the requisite water issues, use a lot of concrete, and cost a lot.
2) A traditional ‘cold attic’ (insulating at the ceiling place) is more cost effective—and arguably more robust and resilient—than insulating at the roofline. All other things equal.
All well and good. But when it comes time to talk someone into adopting both of these approaches, they might wonder where they are going to put all their ‘extra stuff’.
Sure, one can make the main floors larger in footprint, but then you’ve built a larger house just for ‘extra-space’ rather than utilized ‘bonus’ space inherent with attics and basements.
To be more specific: If the primary goal is to forgo a basement for the aforementioned reasons, might it be sensible to consider insulating the roof-line to gain the bonus 1/2 story under the roof? Consider that the structure is wide enough (28′-32′) and the roof pitch steep enough (10/12) that the ‘bonus’ attic space would be quite large and livable).
—————————————————————————————
Possible benefits to insulating the roofline for the added 1/2 story:
· The overall size of structure remains the same. This intuitively feels more resource efficient. You are taking advantage of space already existing—you’re just moving the location of the thermal control layer.
· You will never add that much square footage of living space as cost effectively (I imagine). With 2 main full-stories, you would be gaining roughly 25% additional living space, and a full 50% more space when storage is included (attic triangles).
Benefits to increasing footprint instead:
· Insulating the ceiling plane is more cost effective, apples to apples. (until what point, given an increasing footprint size?)
· If you’re OK with NOT increasing Sq. footage to the same extent as the bonus 1/2 story would, perhaps this is actually the more frugal approach?
What is a wise design mantra to approach this desire for ‘extra space.’?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
If I was building a house, I would design a storage room on a main floor. Attics and basements all have issues, moisture, cold/heat, etc. but he biggest issue is stairs and access. A dedicated storage room is the way to go. Think, walk down the hall and pull out the box of Christmas decoration not climb that stupid ladder and juggle the boxes down from the attic.
Coming from the the land of 2.5 story houses with basements, I'm a firm believer in both attic and basements. They make much more efficient use of land space and building volume.
Properly detailed it is also better for energy efficiency as it has a better surface area to volume ratio. There are some additional costs when it comes to building it but there are some benefits as well.
Around me, unfinished basements don't count into gross floor area of the house, so it is "free" square footage as far as the city is concerned. Same with conditioned attic with storage.
Stairs are the big drawback to any of these, I have to go down four stories from my office to get anything from the basement workshop. Doing that a couple of times makes for good cardio though.
Inefficient use of space space really bothers me, the sight of the silly tall empty roofs over McMansion land just feels wrong. If you have built the volume, that volume should be used.
Are you talking "extra stuff" like storage space for clothes, seasonal decorations etc?
Is that necessary to be in a conditioned space? I personally have a detached shop/garage for "extra space" (and we actually also have a 40ft shipping container for our outdoor/hiking/climbing/ski gear)
If you're talking "extra stuff" i.e. water heater, ERV, furnace/boiler/heating unit/Mini split it somewhat depends on what you're planning on putting there.
Personally, I insulated the roof, used the portion of the house with flat-horizontal ceiling above batrhoom, 2nd Bedroom, and entryway as a mechanical space. Running ERV ducting, dryer exhaust, and electrical in that space.
I'm a fan of bringing the "attic" space into the envelope. As long as the envelope is specified and detailed correctly it should be relatively straightforward to airseal and insulate (maybe recycled polyiso exterior foam?)
Benefits I see (open to discussion as always)
-Less worry about airsealing wire penetrations through drywall in ceiling.
-recessed can lights don't penetrate air barrier
-Space for ERV/ducting/HVAC in attic, without energy penalty of poor insulation
-Not trying to get attic door airsealed/insulated
-If used as storage space-not having to deal with insulation on floor of attic.
Tyler,
I think the answer is very site and climate specific. In urban areas where the footprint of the house is often limited, including a basement and cathedral ceilings as Akos advocates for often makes sense.
Where I build in rural BC, basements are unnecessary for frost protection and often difficult to achieve because of bedrock. Spreading out the house on the main levels is much easier and yields a better quality space.
One of the big problems I've found with including dedicated storage rooms in the house I design, is that they are immediately requisitioned for other uses - another bedroom, office, or hobby space. Storage is almost always relegated to marginal, and usually unsuitable areas instead.
I appreciate all these perspectives.
I purposefully kept 'extra stuff' vague because I am thinking beyond one situation, and partly because I think it remains conceptual and nebulous for those accustomed to having basements and attics (Mainers in my case) until the house is built.
For the purposes of discussion, assume it has to be conditioned space (though it's a good point that many things could and should be shed/garage bound).
And to Malcoms point, I do think there's a desire to have 'otherwise unusable' space to put those 'extras', rather than a nice room that could become a bedroom/playroom/office.
Tyler,
It may be different elsewhere, but under our building code the distinction between unfinished and finished spaces has almost disappeared.
A basement needs to be insulated, air-sealed, conditioned, be provided with electrical service, and the interior Vapour-barrier protected from mechanical damage up to the four foot mark by drywall. That doesn't leave a lot left to turn it into a nice room. The requirements for conditioned attics accessible by stairs are going that way too. .
You can always stash stuff in unconditioned spaces like in between the roof trusses, or the rafters over a garage, but if you are looking for anything more the opportunities for those neither nor spaces are quickly disappearing in modern code compliant houses.
Malcolm,
I see what you mean with basements, though one would hope it could still remain a bit utilitarian if one wished it to be (storage, workshop, mechanicals, etc). But I guess cost becomes the factor if code is requiring near-finish levels.
As far as the conditioned 'attic' (better term is probably .5 story, since it's not really reminiscent of an attic any longer) the triangles behind the 'knee-walls' (4'-6' in height?) are essentially useless for anything but mechanicals and storage, no? Or are you saying code would require those to be drywalled and such, and therefore it becomes cost ineffective?
It could be that 'extra-space' really is too vague here. To some degree, I'm even thinking of the livable space provided by the .5 story to be a bit of a bonus, with the storage provided by the triangle attics to be the real utility workhorse. A bit of a fallacy I think, since it costs money to get there. None of this is apples to apples.
The way I see it half story building are very difficult if not impossible to air seal.
A code approved stair to the attic will waste 300 or more square feet of prim floor space on the main floor.
When you move the insulation to the roof the surface area of the conditioned space just got at least 20% larger, making your building at least 20% less efficient.
The cost to build an R60 roof is astronomical and almost never gets built. When you move the insulation to the roof almost always ends up using spray foam on the inside or foam on the outside and most run out of budget well under R30.
In my opinion concrete is greener than foam.
I say build a 2 story or a one story and say no to the half stories.
Walta
Walta,
1/2 story is hard to air seal in older houses. It is relatively straight forward to design in a properly sealed and conditioned attic and it does not add a lot to the cost. This assumes the roof shape is simple, which should be for efficiency and durability.
There are many ways of building insulated roof, it does not have to be SPF. On a new build it is straight forward to design a vented roof with either blown in insulation or batts. Also keep in mind that In most climates the energy saved between an R30 and R60 roof is minimal.
Your correct in the roof insulation area. Conditioned attics do increase the insulation by 20% to 30%
P.S. Code approved staircase takes up around 50sqft. You can even reclaim most of that by building in storage under it.
I've been thinking about this problem a lot, and for the most part I think that "A Pretty Good House shouldn't exceed 2000 sqft" is advice tailored to people who have basements, garages, and attics to store things in, or to minimalist outliers in the American consumerist tradition.
I grew up in a house with 1300sqft living space, a 1100sqft basement, a 500sqft garage, a 300sqft behind-garage workshop, and a 400sqft shed (EDIT: and perhaps 500sqft of usable attic!). Then my family moved to a house with 2600sqft living space and an 800sqft basement. It was immediately clear that it wasn't as much of an upgrade as we'd hoped. The 'Den' immediately became a storage room and constant point of contention, we started storing a good number of things in an open carport (which did lead to theft), and in the small basement, we are certainly testing the floor loading capacity of the slab by piling containers & storage racks to the ceiling.
In the suburban working-middle class, we pick up a set of skis when we see them at a yardsale for $6 *regardless of the fact that we've never been and never intend to go skiing*, and then we store them in our garage/basement indefinitely. To keep the option open, to keep it economical. This hoarder-adjacent tendency is colored by material scarcity - a person with money can afford to keep the money in their pocket and keep their apartment empty and simply rent the skis ($54/night/person) if they ever decide they need to, a person without money has to snap up what opportunities are offered. There are a lot of ways that this cheap-stuff-maximalism is a pathological tendency, but damned if it wasn't occasionally useful in my family.
Some notes:
There is a world of difference between the cost of an attic that holds up cellulose, and an attic that holds up storage.
The cheapest attics these days are 2x4 trusses that can't be loaded much on the bottom chord and present 2x4 web obstructions every few feet inside the putative "room". They have no subfloor surface - you're straight-up attaching drywall to the underside of the trusses (or to strapping you apply to the bottom chord from underneath), and then blowing insulation onto the drywall. This is useless for storage, and you couldn't even walk on it.
To make usable storage space, you either need an attic-style truss that uses a floor joist as a bottom chord and changes around the support schema, and add floor sheathing inside the cavity, or you need to frame a full platform floor (just like your second floor) and then put rafters or trusses on top of that. An attic-style truss is going to be awkward to use with cavity insulation and airseal - too much of "the interior is preattached to the exterior". Rafters were a lot cheaper back when houses were smaller and good lumber was slightly larger; Nowadays you're probably looking at something like TJIs or parallel-chord trusses; Engineered lumber starts at something like 5x the cost of kiln-dried sawed lumber.
Geometrically, in a small home with a modestly pitched roof, you just don't get much of a "bonus" in that room, particularly with how much stairwell you have to put in. A 30x40 2-story shoebox gable with a 6:12 pitch, modeled with infinitely thin rafters, has a ridge 7.5 feet above the floor of the attic; There's only a 6ft wide aisle in which you can move around with 6ft of headroom. Raise the heel 2ft and your 'bonus room' is much more realistic, but still only 14ft wide at the 6ft tall level. At 12:12 pitch with a 2ft rise, things look a lot more promising, with a 22ft wide space at the 6ft tall level... but at 12:12 pitch your home is a sort of tower, with these infinitely thin rafters extending to 17ft above the attic floor at the ridge (add actual thickness of the rafters * 1.4 plus the sheathing/cladding thickness for actual peak).
A typical attic truss is also not especially great structurally. The codes are generally designed for L/360 deflection at 40lb/sf loading. I assure you, my basement slab exceeds this loading. In the commercial world, without knowledge of use they tend to design for 100lb/sf loading, which is about the densest you can pack a crowd before they start to trample each other. When they do have knowledge of the loads - there is special care taken to reinforce around things like compact filing systems, which can run all the way up to 250lb/sf. In the residential world, people start to talk about special reinforcement when the words "hot tub" are bought up, but a storage system gets genuinely heavy, and any kind of structure built above an "open-plan" first floor is implicitly challenged.
If I use 27-gallon totes (4.3 sqft) and fill a stack of 5 totes with 50lbs each, I'm at 62.5 lb/sf before considering any kind of shelving system.
On top of all this, L/360 is a *span-relative* deflection criteria. If your span is fully 30ft (you have no intermediate load-bearing walls in a 30x40 house, you're strictly supporting the attic using truss bracing from exterior walls), L/360 considers a full 1" of absolute deflection to be acceptable; I don't know what this does to your ceiling drywall.
So how do we solve all this?
I think we bring the storage needs of the 80th percentile of the population inside the conditioned envelope, and give them extra space besides. I think we make houses simple and low embodied carbon but *huge*. Most of the cost per square foot of a house isn't in framing materials; Extra floor area is cheap if you're not tacking cabinets and quartz countertops on top of all of it. I think that 64x48x2 at $90/sf with cheap finishes, simple design, and a lot of empty space starts to look a lot more reasonable than 40x30x2 at $190/sf loaded with architectural features and nice-to-haves and densely packed.
There are lots of amenities you can provide cheaply that simply take up too much space to be sane in a renovation:
* Want HEPA-rated air filters that you only have to change once a year? You can do that, as long as you're willing to devote a closet-sized duct space.
* Want solid sound insulation from outside noises? Double stud walls and triple glazed windows.
* Want to be able to have loud sex in the MBR without the kids hearing? A double interior wall.
* Want to be able to run wires and plumbing simply and not screw around with the air barrier? Service cavities a few inch thick along every wall make that possible.
* Want perfectly reliable appliances? Don't spend 10x as much for dubious "commercial grade" quality; Don't spend more than your appliance costs to get it installed same-day.
Buy cheap ones, but buy two of everything. They won't fail at the same time.
* Want less painful stairs? Shallower pitch is within reach, it just costs floor area
* Want the option for a future elevator? Stacking closets or leaving empty space in the middle of a U-shaped stairwell gives you the option.
* Want a first-floor bedroom? In a small house with traditional layout and normal family size this tends to make the second floor massively oversized. But one man's "massively oversized" is another man's "extra storage space".
Lastly: A 30x40 house's attic is pretty constrained as far as usable space, and the benefit isn't enormous considering the costs, since you only have 30ft horizontal to work with. But in a much bigger house, even with shallower roof pitch, there's enough headroom to use much of it in the event you want to screw around with stairwells and airsealing. Some of the costs are the same, but you earn several times as much storage space payoff.
Historically, we have always been obsessed with natural light from windows. As a result, every "large house design" ends up being an "expensive house design", because there is no square to cube ratio - every room has to be exterior-wall-adjacent, creating exotic fractal shapes and tumorous rooflines.
I think the advancements in LED lighting and the shift to non-draft-based ventilation have chipped away at that. We build plenty of office space far from an exterior wall - why not residential storage space, or theater/game rooms where natural light is actively counterproductive?
In actual practice large houses scale in cost even more, because glazing ratio goes up with market level and finishes go up in cost with market level. That doesn't need to be the case. You can build a huge house cheaper per square foot than a small house, if you try to value engineer it.
https://www.youtube.com/watch?v=jdne_aCjLuU
There is a lot to unpack there. Houses need storage and a flexible layout. What the most cost efficient way of providing that depends on the site as Malcolm points out.
In urban areas, a house with a finished basement is worth ~100k more, a 1/2 story ~150k. Both are no brainier.
Even in cottage land, the value of a cottage built with a basement is worth the building costs, thus most have one.
There are many ways of building a house, most cost adders of conditioned attic or full basement are noise compared to the overall cost of a new house build.
Energy efficiency wise, a code min insulated basement adds very little to building energy use same with a properly detailed conditioned attic space.
P.S. Engineered lumber is about the same cost as dimensional lumber here.
A lot to unpack indeed.
I should note that, in these parts (Maine), it's fairly common to have steeply pitched roofs. It's part of why, almost intuitively, it feels like those spaces should be used (because they can be fairly voluminous). That, and perhaps because there are so many existing .5 story structures around (mainly 1.5)
Burninate,
You've got farther along the road thinking about this than I've been able to.
The minimalists around me seem to be mainly involuntary ones who have had the lifestyle forced on them by the circumstances of their homes.
I think the garage is ripe for a re-think. Almost no one here can get a vehicle into it because of all the stuff. Developers have begun to acknowledge that by providing a space and door too small for all but the tiniest car. The door is almost a vestige of the space's old function. I'd like to see that process go farther and create a space more directly designed for storage, that could also host a home business or workshop. Getting rid of the garage's dominance of the front facade of North American houses would be architecturally transformative.
I think the OP's original premise was made to be an invitation to respond. There are a great number of queries in the Q&A section that are about creating a conditioned attic space. Second place is about routing ducts for HVAC and ERVs. The reason there are so many questions is because there can be so many problems there and is so difficult to execute correctly.
If one is building from scratch in an area where the land footprint is not exorbitant then, to me, it seems preferable to build single story with an unconditioned attic space. Use raised heel trusses with a high enough "heel" to allow both good insulation on the perimeter and to also allow a 9 foot ceiling that is air tight. Then put in a false ceiling a foot below that to allow a 12" high area to run ducts and wiring in conditioned space. Load up the attic with cellulose and don't ever expect to need to go in there again. Don't forget to provide a large enough closet in conditioned space for both a HVAC unit and ERV.
To me its important to have one's priorities straight about where one will need future flexibility. If one looks at the vast number of question about remedying existing problems then having a false ceiling below an unconditioned attic would seem to solve many of them. The problem is you have to design for it at the beginning, preferably in new construction. The OP has that opportunity.
I like Eric Habegger's plan, which allows you to have single story, slab on grade, unconditioned attic, and ceiling service cavity inside the conditioned enclosure, all at the same time.
If you had a mechanical room within conditioned space for HVAC and ERV (instead of just a closet), is it reasonable to omit the false ceiling in said mechanical room and route ducts, etc., through the top 12 inches of the wall into the ceiling service cavity? It would all still be beneath the 9 foot air-tight ceiling. The mechanical room could be sized to also include water heater and laundry. Not as much room as a full basement, but it seems like it could save you some money, even with a bigger footprint.
I would want a false ceiling that's reasonably nice to look at, not one of those acoustical tile on aluminum frame and hanger wire contraptions you see in office buildings. Maybe some type of lightweight truss - just enough to hold up drywall, light fixtures, and air registers.
Maybe "false ceiling" is the wrong terminology as its too reminiscent of the old acoustical tiles. It should be made of sheetrock I think. I'd like to take credit for the idea but I read about it in the Journal of Light Construction. I'm sure the idea has been around even longer than than where I read about it but I'm glad you like the idea.
I think it could also be adapted to mini-splits instead of a single full size HVAC unit. Dana mentioned a ceiling mounted indoor unit that has been introduced by one of the manufacturers. You could mount it in the cavity and avoid the big bulge (TM pending) many of us dislike. I like the ceiling mount idea but would not want to go into the attic to service it that comes with a normal installation.
I have been thinking about this subject a lot too. Around my area (Colorado, CZ 5B), most houses, at least those built before quite recently, have basements. Few of these are truly conditioned (no slab or foundation wall insulation), but most are somewhat sealed and have some windows, and stay reasonably temperate. Any 'finished' basement space is usually retrofit later by the homeowner. Some newer houses have vented crawlspaces (frozen pipes anyone?) or slab on grade.
So my vote, for Colorado climate anyway, is for a conditioned (sealed, insulated) but not necessarily finished basement with a vented, unconditioned attic. Build a simple, moderately pitched (say, 6/12) roof on raised-heel trusses. Loose-fill cellulose insulation above the ceiling, no ducts, storage, or anything else in the attic.
Once you decide on a vented attic, a basement offers a lot of advantages: It provides an accessible space for an ERV/HRV + ducts, water heater, furnace/ducts or boiler, if used (my choice would be ductless mini-splits with electric resistance backup), water tank (if you're on a well), laundry facilities + utility sink, electrical and plumbing distribution, chest freezer. You could have some open area for a home office, workbench, some storage. Install a suitable ignition barrier over any interior rigid foam; use the slab as the floor, sealed, if you want.
If you have a slab on grade foundation, either by choice or necessity, you can locate the equipment in a main floor mechanical room, but ducting and wiring need to be figured out if you want to keep it all inside the air barrier. Plenum trusses could be useful here, or maybe furring below the trusses and ceiling air barrier. In any case, I think omitting the basement would usually increase the building footprint somewhat, but which design is most cost-effective? I guess it depends on the details.
Finally, I agree with Malcomb that the garage dominating the front facade needs to change. Drive down most streets, and you're mainly looking at a bunch of garage doors. I can appreciate that people have a finite lot width and need to park somewhere, but I think parking could be moved to the back or side with a little planning. The house I grew up in (northern California, 1950's) was typical - small house on a big lot. The one-car garage, more of a wood shack really, was in the back yard, accessible via a narrow driveway alongside the house. Even back then, the car got displaced during the summer by a ping pong table (watch your ribs!). My choice, lot size permitting, would be to have a separate storage/utility building instead of a conventional garage, with an adjacent covered carport. The carport roof could be a good location for PV panels.
"What is a wise design mantra to approach this desire for ‘extra space.’?"
Hire a good architect.