Have many questions about HVAC, insulation, and ventilation
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I have many questions about HVAC, insulation and ventilation, but will just start with a few. If we decide on an exhaust-only system for a new build home, does also trying to button up the envelope tight work with that type of ventilation?
We will probably insulate with fiberglass blown in at attic floor because spray foaming the whole attic would be so much more expensive. If we don’t insulate at roof, do we need to install a radiant barrier at some location to try to bring the heat down in attic?
Thanks.
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C. Clark,
Q. "If we decide on an exhaust-only system for a new build home, does also trying to button up the envelope tight work with that type of ventilation?"
A. It's always important to try to make your envelope as airtight as possible. Striving for airtightness pays may dividends: lower energy bills, increased comfort, reduced chances of moisture problems in walls and ceilings, and better control of ventilation systems.
Even when builders strive to make a tight envelope, most exhaust-only ventilation systems operate well without any passive air inlets (because there are almost always enough cracks here and there to provide makeup air for a system that moves only 60 cfm to 120 cfm). However, if your blower door test shows that you have attained Passivhaus levels of airtightness (0.6 ach50 or less) -- most builders don't -- then you may need to install a passive air inlet for your exhaust-only ventilation system.
For more information, see Designing a Good Ventilation System.
C. Clark,
Q. "If we don't insulate at roof, do we need to install a radiant barrier at some location to try to bring the heat down in attic?"
A. If you are building a new house, you have an opportunity to get the details right -- so I hope you will make sure that there are no ducts or HVAC appliances in your attic. (That's standard GBA advice, and it's extremely important.)
Assuming that there are no ducts or HVAC appliances in your attic, you don't need a radiant barrier. Just install a generous layer of insulation on the attic floor. Take the money that you would have spent on a radiant barrier, and use the money to buy a deeper layer of insulation (if you want). You'll get more bang for your buck that way.
If you can find an insulation contractor who installs cellulose instead of blown-in fiberglass, you'll get better performance from the cellulose.
The benefits of radiant barrier are miniscule when the attic insulation is even at IRC 2006 code levels, let alone current code, and the benefit is primarily during the cooling season. During the shoulder seasons a radiant barrier limits the passive solar gain benefit that otherwise (very slightly) lowers the heat load on the house. For a heating dominated climate a radiant barrier would slightly increase energy use.
The only time radiant barrier has an economic rationale worth the cost is when there are uninsulated air conditioning ducts & air handlers above the attic floor insulation, and the attic floor insulation is half or less current code minimums. Spend the radiant barrier money on tighter air sealing and higher-R insulation.
It's almost impossible to retrofit air seal so tight that exhaust-only ventilation would not work. If you have atmospheric drafted combustion appliances it's important to test for backdrafting potential.
If the air leakage supplying the ventilation air is coming through the ceiling, blown fiberglass can become a potential indoor air quality problem due to airborne glass particulates. Cellulose is a generally better choice for open-blown attic insulation for several reasons, including that one. Fiberglass is somewhat translucent to infra-red radiation coming off a hot roof deck, absorbing the energy an inch or so below the top surface. This makes the temperature an inch into the fiberglass layer hotter than the attic air, so you're insulating against a slightly higher temperature with slightly less insulation. Cellulose is opaque to IR, and the warmest level will be at the surface, close to the attic air temperature. Cellulose is also more air retardent, limiting the volume of air moving through any leaks in the ceiling plane more effectively.
Thanks Martin and Dana. We are moving from a dry climate, and I am very concerned about building in humid climate, because we are very allergic to mold (bed-ridden allergic for me) so preventing condensation and the mold that comes with it are our primary concern, even over energy efficiency. I read the cellulose insulation can trap moisture, which steered me away from it and to fiberglass. If any condensation build up can happen in attic, I don't want cellulose to be a mold culture.
The duct will have to be in attic, because builder said constructing duct channels would be much more costly. So, back to my question of whether it would be just as effective to use wrapped insulated flex ducts as to spray foam them? Can you even spray foam flex duct? I will try to get contractor to use rigid duct for a short run in attic before branching off to rooms occurs, but designing and installing rigid throughout is not going to be possible for us costwise.
The fire retardents in cellulose kill mold and if your cellulose reaches moisture levels where mold actually grows on it you have much bigger problems than the mold- you have water leaks or MAJOR wintertime air leaks.
Trusses designed with for horizontal utility chases are not a huge cost adder, and putting the mechanicals in the attic is(almost) universally a mistake, making it harder to air seal, and adding to both the heating & cooling loads.
The mechanicals could also be ductless.
C. Clark,
It pains me to hear of a homeowner designing a new house with ducts in the attic. Now is the time when you can fix this problem inexpensively -- before it is too late! This detail really, really matters.
There are lots of options, including a crawl space foundation, a basement foundation, 9-foot ceilings and duct chases, open-web floor trusses designed to accommodate ducts -- or, as Dana pointed out, ductless minisplits. If your builder doesn't understand this basic issue, it may be time to look for a different builder.
Here is a link to an article that you may wish to read: Keeping Ducts Indoors.
By the way, what is the location of this new build? (By US climate zone or ZIP code)
Rafters & joists, or trusses?
Dana, ZIP is 29907. [Ladys Island, South Carolina.] Trusses. As I said, we will likely not be able to afford paying to have chases run for ducts, so will have to live with ducts in attic. I did create a 6 x 6 mechanical room when I modified our house plan to be on first floor, but duct will be in attic. Maybe the third time for this question will be the charm, but I would still like to know if there is any preference between spraying foam flex ducts or buying pre-insulated flex duct? Martin did answer a question that has been puzzling me when he said that even if we try to tighten house, there would still be enough infiltrated air through the envelope to suffice for make up air for a exhaust only system.That tells me that we can give the builder the go ahead to spray foam around all outlets, stacks, recessed lights, dryer vents,etc. and we will still get the make up air we will need. Since infiltration is not exactly the controlled method we would all like, I'm hoping for the best. We will have 2 bathroom fans, a laundry room fan and a range hood fan for exhaust.
https://buildingscience.com/documents/building-science-insights/bsi-012-balancing-act-exhaust-only-ventilation-does-not-work
http://www.energyvanguard.com/blog-building-science-hers-bpi/grading-an-attempt-to-get-ducts-inside-conditioned-space
http://www.energyvanguard.com/blog-building-science-HERS-BPI/bid/57709/How-to-Install-Flex-Duct-Properly
http://www.energyvanguard.com/blog-building-science-HERS-BPI/bid/38931/Case-Closed-Get-Those-Air-Conditioning-Ducts-out-of-the-Attic
Finally some info on foam over ducts.
https://basc.pnnl.gov/resource-guides/encapsulated-ducts
C. Clark,
Unlike Jon, I don't think that an exhaust-only ventilation system will cause problems, as long as your house has a well-designed heating and cooling system. The use of any ventilation system should be minimized during hot, humid weather, of course -- you want just enough ventilation to be comfortable and avoid odors.
I also disagree that it's essential to "Use a dehumidifier+humidistat at all times to keep the interior dry." In most cases, your air conditioner should do a good job at keeping your interior comfortable. During humid weather in the swing seasons (when the air conditioner isn't operating much), a dehumidifier may be necessary -- but not "at all times."
During the winter, when you are operating your heating system, it would be highly unusual to need a dehumidifier. Your heating system and your ventilation system should keep your indoor air dry during the winter.
Q. "Is any preference between spraying foam flex ducts or buying pre-insulated flex duct?"
A. The answer depends on the skill of the spray foam installer and the thickness of the spray foam that you intend to install. There are many types of insulated flex duct. If you choose insulated flex duct, it should be rated at R-8, not R-4 or R-6. Of course, all seams should be carefully sealed. The ducts should be carefully installed, well supported, and as straight and short as possible.
In an attic with trusses, it's usually impossible to install the flex duct on top of the drywall. The flex duct ends up suspended from hangers, because the duct runs go 90 degrees to the trusses. That makes it difficult to insulate the ducts with spray foam.
If you can install some type of rigid foam support under the flex ducts that are 90 degrees to the trusses, that will make it easier to install the spray foam. If you can do that, and if you have a spray foam installer who is willing to add enough R-value to the ducts to approach the thickness of your attic floor insulation, you'll have an excellent installation.
At that point, however, the hassle and expense might be high enough to get you thinking, "Maybe it would have been easier to put my ducts indoors after all."
When the builder says "That won't work," it may mean one of two things:
a) I have looked into that and discovered a problem which I cannot figure out how to solve, or
b) That isn't how I do it and I don't want to change.
When the builder says "That would be really expensive," it may mean one of several things:
a) I have costed that out and the materials and labor impact are high,
b) I have never costed that out and really don't want to so I am not going to unless you are willing to pay me a lot, or
c) I tried costing that out, but my usual subcontractors don't want to do it so they gave me really high bids.
To figure out which one applies, you need to ask more probing follow-up questions. The more you learn, the better prepared you are to ask those follow-up questions and to evaluate the answers.
Thank you guys for sticking with me on these concerns. Today I plan to look into ductless splits, as I don't know what that is. If I can learn enough from you guys,so that the builder gets the idea that he can't manipulate me into whichever direction he wants to take HVAC, then maybe he and I can work out another option than ducts in the attic. But cost may be the deciding factor, as it is with many of us who don't exactly live the life of money-is-no-object. Without yet breaking out cost, it seems like spray foaming attic at roof would be best solution, so duct work would be in conditioned space, but cost may mean that blown in cellulose on attic floor and insulating duct work to R-9 or higher would be the method we can afford, unless the ductless split option might work.
Martin, you said that even if we try to tighten house, we will still have enough makeup air through infiltration for an exhaust only vent system., so if we try to seal with foam around every recess light, elec outlet, every pipe penetration of walls, every outside door and window, where does that leave leaks to come in? Do bath fan vents and range hood vents not have dampers to seal out air when fan is not running? I'm guessing that thermal bridging around studs in walls would be one place air can come in, maybe even in a fairly high volume.This is our first build, so my knowledge curve is steep and appreciate the guidance.
C. Clark. It is clear you are rightly concerned about budget. But I would note that Habitat for Humanity is building LEED Platinum and Net Zero homes for low-income families. (http://www.cleanenergyfinanceforum.com/2016/02/01/habitat-for-humanity-pioneers-affordable-net-zero-housing and https://www.youtube.com/watch?v=lI1q16S9aTc)
I think it is possible for you to build a "pretty good home" without blowing up your budget. A energy efficient home will pay many financial dividends over its useful life while being more comfortable to live in.
See this link for more on the pretty good house concept. Be sure to read the additional links in the sidebar as well. https://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/pretty-good-house
Forgot to add that we will be all electric with a heat pump, and we will not have a fireplace, which eliminates a lot of air infiltration. In addition to the Tyrek or whatever house wrap they use, do we also need a vapor barrier directly behind the drywall because of the hot humid climate?
C. Clark,
Q. "Martin, you said that even if we try to tighten house, we will still have enough makeup air through infiltration for an exhaust only vent system, so if we try to seal with foam around every recess light, elec outlet, every pipe penetration of walls, every outside door and window, where does that leave leaks to come in?"
A. Lots of places:
- Cracks under the mudsill that aren't sealed by the sill seal due to uneven concrete.
- Cracks between the subfloor the the bottom plates of walls, wherever the framers forgot to install a gasket, forgot to install sufficient caulk, or did a bad job of caulking.
- Between doubled framing members (top plates and studs), unless there is a bullet-proof air-sealing job at the sheathing layer.
- Between the window sash and the window frame (because weatherstripping is imperfect).
- Between exterior doors and the door frames (ditto).
- Between the attic access hatch and the hatch frame.
- Through a sump in the basement that doesn't have an airtight sump lid.
- Etc.
Q. "Do bath fan vents and range hood vents not have dampers to seal out air when fan is not running?"
A. They do, but the dampers aren't airtight.
Q. "I'm guessing that thermal bridging around studs in walls would be one place air can come in, maybe even in a fairly high volume."
A. Either I don't understand your question, or you are wrong. "Thermal bridging" refers to heat flow via conduction. It has nothing to do with air leakage.
C. Clark,
Q. "Do we also need a vapor barrier directly behind the drywall because of the hot humid climate?"
A. No. That would be a very bad idea, especially in your climate, and especially if you plan to install an air conditioner. For more information, see Do I Need a Vapor Retarder?
Thanks. I will revisit thermal bridging to better understand how it works. Also, is there an article or book on the physics of air/heat/vapor flow/condensation in regards to how these pass in and out of, or drop water in, buildings, and what are the methods and materials used for controlling each of these conditions. I get confused about things like what outside or inside conditions create neg or pos pressure to push or pull air in or out, where certain sheathing or barriers belong in the envelope to stop air and moisture, allow permability, etc. If I can understand the basics of moisture, air, pressure, temp, etc, maybe I can more easily grasp the logic of what barriers do and where they belong in the layers of a wall, crawl space, roof, etc.
Keep in mind that "bed-ridden allergic" puts you well outside the norms.
Note that humidistats only turn on when mold inducing conditions occur - when other mechanisms are working it has no effect.
C. Clark,
Q. "Is there an article or book on the physics of air/heat/vapor flow/condensation in regards to how these pass in and out of, or drop water in, buildings, and what are the methods and materials used for controlling each of these conditions?"
A. There are lots of books out there. You might consider reading:
Water in Buildings by William Rose
Residential Energy by John Krigger
There are lots of articles on GBA as well, including these:
How Heat Moves Through Homes
Air Barrier or Vapor Barrier?
Do I Need a Vapor Retarder?
Questions and Answers About Air Barriers
How to Use the Psychrometric Chart
If you like to read, and you believe in educating yourself, you might want to check out the links on this page:
How To Do Everything
ZIP 29907 is fairly heating/cooling balanced location with roughly as many base 65F heating degree days as cooling degree days most years, but it's been trending toward cooling-dominated. Base 65F is the presumed heating/cooling balance termperature for 2x4 framed house that aren't super-tight. Tighter and better insulated houses will have a lower balance point.
In a 2x6 /R20 type house (the current IRC code minmum with a heating/cooling balance point base is closer to 60F, and it then becomes a heavily cooling dominated location.
Either way, if you installed a true vapor barrier anywhere in the walls stackup it would be on the "wrong" side of the stackup large fraction of the year. Using plywood or OSB sheathing as the exterior vapor RETARDER (typically 1-5 perms, depending on moisture content) and standard latex paint on air-tight wallboard (3-5 perms, typ), the house would be able to dry at reasonable rates seasonally in either direction, without letting too much moisture in via vapor diffusion. The only time it needs more than that is if it has masonry, brick, or stucco on the exterior, which is a special case.
Mini-split heat pumps solutions can work well for those with allergies since it does not pressurize or depressurize the rooms, causing outdoor infiltration via potentially allergen-laden paths. If airborne allergens are the problem, going with a heat recovery ventilation system with a good filter is probably going to be healthier than exhaust only ventilation, along with with ductless heating & cooling. (aka "mini-split heat pumps")
With any heating & cooling system, it's important to size the equipment correctly. In new construction calculating the loads with a "Manual-J" method is the industry gold-standard. That is best performed by a professional engineer or RESNET rater who are being paid for the accuracy of their numbers, and (sadly) NOT an HVAC pro or general contractor, who regularly oversize the equipment several fold, yielding higher up front costs and lower comfort. And they need to be aggressive in their air leakage and insulation quality assumptions, otherwise the size creeps up, and you're forced in to oversizing.
In your location the 99th percentile temperature bin is about +30F (Beaufort's is 31F: http://articles.extension.org/sites/default/files/7.%20Outdoor_Design_Conditions_508.pdf ) and the first percentile temperature bin is 92-93F. and those should be the outdoor temperatures at which the loads are calculated, not anything significantly higher or lower than that. Equipment sized for with sufficient capacity at those temperatures will usually have more than sufficient capacity to cover the loads when temps stray above and below those temperatures. The number of hours a year is 8760, and 1% of that is 87.6 hours. The 99% and 1% numbers mean that a typical year the total amount of time when the outdoor temperatures are above the 1% bin adds up to 87.6 hours, and below the 99% temperature, another 87.6 hours. Equipment step sizes won't align exactly on your loads, and the next size up will have sufficient capacity to cover the load for most or all of those 87.6 hours.
An independent competent Manual-J performed by an engineer reviewing the house plan details costs about $500-800 in my area (YMMV), and right sizing the equipment will save you far more than the cost of the calculation. Only rarely have I seen Manual-Js from HVAC contractors done competently, and I've never seen one (competent or otherwise) from a general contractor.
A common mistake with ductless systems contractors is to put a ductless head in every room whether it needs it or not. With room by room load numbers and the function of the room you can make reasonable assessments as to which rooms don't really need it, and which do. (A bedroom will usually want a ductless head, or the shared output of a mini-duct cassette, whereas you probably don't care if the laundry room runs a bit hot or cold.) The head-per room can end up being ridiculously expensive and the compressor way oversized for the loads, but in some markets even that is cheaper than running ducts.
If ducted, a trussed roof a "Plenum Truss" is a reasonable way to go for a single story house operated as a single zone with a ducted heat pump. It's a cost adder, but a LOT cheaper than site-built soffits and chases, with easier and more reliable air sealing too:
https://www.greenbuildingadvisor.com/blogs/dept/building-science/use-plenum-trusses-keep-ducts-out-your-attic
With a plenum truss the duct chase is effectively pre-framed, reducing the on-site labor, with higher dimensional precision.
With simple roof lines it's possible to design for Net Zero Energy in that climate without super-insulating the house, and even a IRC 2015 code-min house can get there if it's sufficiently air tight, with the proper roof orientation if you don't go nuts on the size of the window area (which raises the cooling energy use), particularly west facing windows (which raise the peak load numbers). See Table 2, page 10 of this document:
https://buildingscience.com/sites/default/files/migrate/pdf/BA-1005_High%20R-Value_Walls_Case_Study.pdf
You are in climate zone 3A, so look at the values found in the row for zone 3.
Those R-value numbers are "whole-assembly" with the thermal bridging of the framing factored in, not center cavity. But that table was done back when heat pumps had HSPF 10 /SEER 15 type performance, whereas pretty good ductless has SEER over 20 (some over 30) , and HSPFs in the 12 range (some as high as 14). Typicall rooftop solar was 15% efficiency panels, whereas this year they are 16. What the higher efficiencies mean is that you can pretty much drop a climate zone.
The R15 wall performance recommended for zone 2 would can be had from 2x6/R20 construction without modification. To bump it up to the ~R20 for the row 3 would require about 3/4" of exterior EPS foam (at about 30 cents per square foot.) But seriously, meticulously air tight 2x6 R20 will be fine, if you preserve the south facing pitch of the roof for solar with no plumbing stack or flue penetrations, etc. An R50 attic with trusses means adding an other 2" of fluff over an R49 code-min, which is barely a cost adder.
But that all goes to the dogs if you punch holes into the attic and need to support the parasitic loads of air handler driving infilrtation and direct gains/losses on the ducts & air handler.
The rate at which PV solar pricing is falling, actually installing it will become a no-brainer investment WELL before the shingle warranty is up. Depending on local rates, subsidies and net metering considerations, it might be a no brainer right now, if it can be rolled into the mortgage (assuming you're not building out of pocket, not always a good assumption). A house without heating & cooling bills can support a larger loan and still have cash left over.
Tell us more about the house (size, shape, etc), or post images of relevant parts of the plans.
Thanks Dana and Martin. It looks like I will be reading for about a year. Dana, the layman got lost several times. This is what I got from your appreciated and detailed explanation.
1. In our climate zone, do not wrap house with Tyvek or similar, use OSB or plywood instead.
2. Unless I talk to some people who know for a fact that the HVAC contractor knows how to do Man J, look for a building engineer who specializes in the calc.
3. The plenum truss might be an option to run ducts, but the ducts would still be in uncond space, just above attic floor insulation. So, the link you gave me is about having contractor build a drywall enclosure just over attic floor and ducts, yet all within the plenum space within truss, then condition that small space. I have a feeling this guy will look at me like I had just downed a snifter of squid ink, but who knows, maybe he has done this type of installation before.
4. The ductless system, as I understand it is individual units in every room. If so, will have to check if allowed in covenants.
Even though you guys lose me sometimes, I appreciate your taking time to try to help.
Sorry, Dana, got sidetracked reading about crawlspaces and forgot to answer you question. The house will be right at 2400 sq feet and will be a rectangle. Crawlspace of about three feet high. The way the homes in this subdivision are built, I haven't seen but one enclosed crawl. Almost all are covered all around with1x6 boards run from pier to pier and spaced apart about 4 inches. So would you call that a vented space to the max? Haven't asked anyone more probing questions yet as to possible condensation and mold problems. Mentioned before that we will have heat pump with air handler in conditioned space on first floor in a mechanical room of about 6x6. Does code call for a mech room to be ventilated with louvers or grills? Does it require its own supply register?
Three bedroom, 2 bath, open floor plan kit/great room/dining room. Don't have the lot yet so don't know which direction house will be oriented. We are looking at a north facing lot and a south facing lot. Plenty of large trees to provide shade. Not sure what else you might want to know.
C. Clark,
Q. "In our climate zone, do not wrap house with Tyvek or similar, use OSB or plywood instead."
A. Tyvek is not a vapor barrier or a vapor retarder. It is a water-resistive barrier, and it is vapor-permeable. It is perfectly normal to wrap your house with Tyvek -- in fact, installing a water-resistive barrier is required by the building code. Tyvek is not a substitute for OSB or plywood. When OSB or plywood is installed on the exterior side of a wall, it's called sheathing. Sheathing serves many purposes, the most important of which is structural.
Q. "The plenum truss might be an option to run ducts, but the ducts would still be in unconditioned space, just above attic floor insulation."
A. No. You are wrong. The purpose of the plenum truss is to provide a space for ducts that is on the interior side of the attic insulation. See the drawing below. For more information on this issue, see Keeping Ducts Indoors.
Q. "The ductless system, as I understand it is individual units in every room."
A. No. Wrong again. For more information, see Rules of Thumb for Ductless Minisplits.
Q. "Crawlspace of about three feet high."
A. Is this a one-story house or a two-story house? If it is a one-story house, why don't you just build a sealed crawl space and put the ducts in the crawl space?
.
Tyvek has a permeance greater than 25 perms. Polyethylene and foil has a vapor permeance of about 0.05 perms or less. Don't use foil or polyethelene sheeting in the stackup.
Unless you use a structural sheathing product with an integrated weather resistant barrier (eg Huber ZIP or ZIP-R) you will still need a weather barrier, such as Tyvek or #15 felt, etc.
Even if the HVAC contractor has Manual-J software and knows how to use it, there is ample evidence that they are more likely to skew the input data to fit some of their own inherent assumptions and biases. That bias is always to the high side, since they have a natural fear of undersizing, and push the load numbers higher to avoid getting having to get the call from a sweaty irate customer when it's 98F outside. It doesn't take many thumbs on the scale to end up with a calculated load sizing 30-50% higher than reality, but since the step sizing of the equipment is in large increments the equipment chosen ends up closer to 2x oversized, sometimes more. This happens more often than not:
eg: Reality is a cooling load of 1.6 tons, the contractors number skew that to 2.2 tons, the next size up is 3 tons,so that's what gets installed. When a third party correctly calculates the 1.6 tons, the equipment gets sized at 2 tons, which is still 25% of margin, with enough capacity to cover the load at temps well above the 1% outside design temp. Oversizing by 2x often has the consequence of less effective latent cooling, higher indoor humidity, and lower comfort. It will cool the place faster, but can leave you clammy when the the sensible load (temperature) isn't very high.
I assume the pier foundations are to manage storm surge flooding? That can affect whether the crawlspace is really suitable for housing the mechanical systems & ducts, but it's still a strong possibility, especially if using mini-split mini-duct cassettes rather than large air handlers.
The ductless heads do NOT belong in every room. I know it's long, but in the middle of response #21, read the paragraph that starts with :
"A common mistake with ductless systems contractors is to put a ductless head in every room whether it needs it or not. With room by room load numbers and the function of the room you can make reasonable assessments as to which rooms don't really need it, and which do. "
These are split systems with outdoor condenser/compressor unit, and between 1 and 8 interior "heads" or sometimes mini-duct cassettes to deliver the cooling/heat to where it's needed. A ductless head is a small coil with a blower and cooling/heating coils inside, that recycles the air within a room rather than moving it via ducts through a large central air handler. This allows you to run zones efficiently, but individual room micro-zones is excessive, and never really needed. The capacity of the head needs to be reasonably matched to the zone/room loads to run efficiently, which is why the Manual-J needs to be on a room-by-room basis. These systems modulate the blower speed and compressor speed over a wide (but not infinite) range, and if sized so that it can modulate tracking the load they can be extremely efficeint, beating their SEER numbers. Putting a 1 ton head in a room with a load of 3000 BTU/hr usually doesn't work, because the minimum speed of the head isn't much lower than 3000 BTU/hr, so it just cycles on/off, which is less efficent and less comfortable. A half ton head can more than cover that load, and will modulate much much lower. Similarly, the compressor in the condenser unit can only go so low, and will be less efficient if cycling rather than modulating. I've seen proposals for 8-head multi-splits systems (with a head in every room) where the minimum modulated output of the compressor was barely below the design load for the whole house, guaranteeing that it never (EVER) modulate with load. Don't let them make that mistake!
For a typical code-min 2400' rectangle house the cooling load at 93F will be roughly 2-2.5 tons, but it could be less than 1.5 tons,
depending on the particulars. One of the guest bloggers here is in GA, and has compiled a database of his Manual-J calculated loads of real homes for the total load and square feet of conditioned space, and created this info-graphic, which plots the square feet of house per ton of cooling against the total house size:
https://www.greenbuildingadvisor.com/sites/default/files/images/Bailes%20graph%20for%20Manual%20J%20blog.preview.png
You'll see that in the neighborhood of 2400 square feet there is at least one that comes in at about 1900 square feet per ton, which means the total load for that house was about 1.25 tons (for the whole house!). But the typical was clustered around a ton per 1000', which for a 2400' house would be 2.4 tons.
His whole blog article lives here:
https://www.greenbuildingadvisor.com/articles/dept/building-science/manual-j-load-calculations-vs-rules-thumb
C Clark,
Are you buying a tract house (i.e. Production Builder) where you choose one from a handful of plans and the builder allows some modifications to the interior configuration?
Saying that I thought Dana said Tyvek or other wrap would be replaced by OSB or plywood was a big brain cramp. Got it now. I also misunderstood the drawing, Martin. I thought the bottom of the overall truss triangle was where the attic floor would be, which would make the plenum and ducts above the floor and insulation. Are you saying the attic floor would be just above the ducts shown in the drawing, which would make the lower portion of truss actually in a cavity between ceiling of first floor and the floor of attic? I'm having a hard time picturing what the design is. Sorry if I got it wrong.
again.
Dana, I will have to do a lot more reading to catch up with some of what you are saying, but have some time, so maybe I should just stop asking questions for now so I might better understand the detailed answers you provide after I have some more basic knowledge digested.
Chris, custom home modified from a few plans. I don't know what else I can accomplish toward energy efficiency given limitations of budget, but at least I got the mechanical room on the first floor of one story house. And it is located almost dead center, so duct runs shouldn't be more than thirty feet to any one room.
C. Clark,
Q. "Are you saying the attic floor would be just above the ducts shown in the drawing, which would make the lower portion of truss actually in a cavity between ceiling of first floor and the floor of attic?"
A. No, I'm not saying that. This attic has no floor. It is not intended to be used for storage. Above the chase that holds the duct is an air barrier (for example, taped drywall). Above that layer of drywall is insulation. As I said, no floor.
Under the chase is drywall (with no insulation). The ducts are on the interior side of the insulation.
Martin, So under the chase the drywall is fastened to the truss, and above the chase the drywall is also fastened to the truss(and the side pieces of drywall too)? And since it sounds like the drywall enclosure for ducts will be just down the center of the plenum space, you can only place insulation on top of that drywall (need something to place it on). I am lost as to where you put rest of insulation that is not over drywall enclosure, since you have no attic floor. What does it rest on to insulate the entire area above the ceiling below? I guess it would have to be batts run over the lowest members of the truss, since there is no floor to blow cellulose on top of? .
C. Clark,
Cellulose is usually blown over the drywall ceiling. It isn't installed on top of a floor, in the vast majority of cases. If you want your attic to have a floor, the floor is usually installed above the insulation. (In most cases, this requires planning ahead -- for structural reasons, as well as to ensure that the framing members are deep enough to hold the required thickness of insulation.)
When it comes to insulating the vertical sides of the plenum truss, it's necessary to include stud bays and InsulWeb to hold blown cellulose, or to insulate the vertical sides with batts or spray foam.
Martin,
Got it. thanks. The only thing it leaves me wondering about is, if you ever need to service ducts or reseal them, or to inspect the roof components, etc. how do you do that if you have no attic floor to walk on? That will be the last of questions about this topic. Very much thank you for the info.
So much sturm und drang about attic ducts.
Since you have a two-story house with a centrally-located first-floor mechanical room, it seems like there's a very simple, cheap, and eminently buildable solution to this quandary: use open-web trusses for the second story subfloor, and run all the ducts inside of them, with ceiling registers on the first floor and floor registers on the second floor. This will inherently keep ducts within the building envelope, minimize duct runs, de-couple the HVAC distribution design from the crawlspace and attic design, and simplify the attic insulation. Use a standard and familiar ducted heat pump, probably 2 tons but maybe 3 depending on the size of the house.
In an area that's wet, floody, and full of termites, I would build as much as possible out of masonry instead of wood. The simplest approach would be ICFs instead of complicated wood framing, which, as you're discovering, everyone has a different opinion on and involves many many steps and layers and opportunities to get it wrong and introduce leaks that can attract mold wood-destroying insects.
C. Clark,
Most attics don't have a floor. If you need to enter an attic, you have to to place your feet carefully above framing members so that you don't put your foot through the ceiling. If you want, you can plan ahead and include a catwalk down the middle of your attic -- but most attics don't have catwalks.
Thanks to all of you. I feel I have climbed the learning curve some and am a little better prepared to talk HVAC with the builder.
Clark,
We routinely provide access in our trussed roofs by fastening 16" lengths of 2"x10" to the bottom cords and running a 2"x10" catwalk on top.
You want to avoid mold but are proposing to pull warm, humid air into your walls and cool it down (when the AC is on)?
When the AC is on, maintain the house at a slight positive pressure. All incoming ventilation air should be run through a filter (not the walls). Use a dehumidifier+humidistat at all times to keep the interior dry.
See here for "It is important that homes in hot, humid climates be pressurized slightly with respect to outdoors."