Proper HVAC registers and diffusers to reduce temperature stratification
I live in a house built in the 1950s heated by forced-air from a gas furnace. This is a single story house with basement. Basement is not heated, but is insulated along its perimeter with fiberglass. The main floor has 8 foot ceilings.
Cold floors have always been a problem. I realize the basement not getting heat probably contributes the most, but with some air temperature measurements, I measured high temperature stratification:
Floor (0″): 59F
Thermostat (60″): 68F
Ceiling (96″): 72F
A difference of 13 degrees seems huge & probably explains why everyone calls my house “cold” even though it’s getting lots of heat.
Started looking into things like a register’s “throw”, spread, drop, etc…but most stuff talks about overhead heating. I have baseboard registers where the ducts come out of the floor near perimeter walls. My baseboard registers simply blow the air up and most are right up under a window (makes me wonder if the heat is just going out the window instead of into the room).
I was wondering if there are any resources which describe what today’s guidelines are for maximizing forced air efficiency by register placement & diffuser selection, in case my home needs an update. For example, is there any reason for a home to really have 48″ baseboard diffusers? Is that hurting or helping air mixing?
Thank you!
In case it matters —–
I recently had an energy audit & my heat load was calculated to be about 30 kBtu. My furnace is 40kBtu, 90%. N.A.C./hr is 0.2. But yet it’s not comfortable when the heat isn’t blowing. For example my fingers and feet get pretty cold sitting typing on this computer even though the thermostat is happy.
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Replies
J. Pritzen,
The reason for the stratification you describe is almost certainly air leakage driven by the stack effect. The solution is to perform air sealing work in your basement and attic.
These two articles describe the necessary work:
Air Sealing an Attic
Air-Sealing a Basement
Hello Martin - did you come to that conclusion because my basement isn't heated & I don't have insulation between basement/main floor?
The energy auditor told me my house is pretty sealed up (it is 0.2 air changes per hour), so I'm not sure what more air sealing can be done (the biggest advice was my mail slot, poorly sealed/cheap windows, the tub hole, and improving insulation in the walls - which is not gonna happen).
Even if that is the issue, I am still interested in any resources on registers & diffusers that talk about their importance in maximizing system efficiency, just for my own knowledge.
Thanks!
J. Pritzen,
0.2 ach(nat) is about 4 ach50 -- which is OK, but not great. Tracking down the remaining air leaks -- the ones that are causing you to be uncomfortable -- is detective work.
With persistence and patience, you should be able to seal some of your leaks. Or you can hire a contractor familiar with blower-door-directed air sealing to help you.
There's nothing you can do with the duct registers/diffusers to fix this problem.
Registers blowing straight up under a window counteracts the natural convection of cold air cascading down the face of the cold glass, and was a common (and reasonable) thing to do when windows were all ~U-1 (or worse). Yes, it raises the heat load by putting 120F air next to the point of high-loss (the window), but it improves average comfort levels.
A 59F floor in a 68F room indicates a basement temp in the mid-50s. If the furnace & ducts are in the basment and the foundation walls are air-sealed and reasonably insulated it would stay in the low-60s, unless perhaps your subsoil is permafrost (?).
Short of a blower door & IR camera investigation, a $50 box store pistol-grip infrared thermometer and a pretty good window fan for pressurizing/depressurizing the house would be able to find most of the air leaks, or perhaps missing/failing insulation. Concentrate on the leaks in the basement, and at the ceiling plane of the first floor. Be sure to investigate the air-tightness of all plumbing stack, flue, & electrical chases too. Air could be convecting straight through to the attic from the basement, bypassing the first floor but chilling off the basement with the infiltration.
Sometimes cold first floors can be from band-joist leakage into the joist bays of a finished basement ceiling. If the basement is warmer than the measured 59F at the floor level that's a real possibility. But sections of first floor wall with missing insulation can also cascade cool air to pool down at floor level, with a similar sort of symptom. This is also pretty easy to spot with a in IR thermometer.
Are the ducts air-tight and insulated?
Poor insulation and air leakage both contribute to stratification. In many houses, the air leakage is the main problem, but in yours they might both be contributing.
Poor insulation also reduces comfort for a given air temperature, because a human body "sees" the temperature of the walls and floor through radiation, as well as feeling the air temperature.
I agree that an IR temperature gun could help you locate problems in insulation and air leaks. A strong range hood, if you happen to have one, could work similarly to a window fan as an alternative to Dana's good suggestion. In some areas, orange big box stores rent thermal cameras, which is a lot more fun than a IR gun.
Once you have the envelope weaknesses corrected, stratification will be minimal.
Ok thankyou. I guess I will try to rent the IR camera. I guess my confusion is I thought that was what the point of the ($600) energy audit was, but I guess it all comes down having to be your own detective. There is orange spray foam all over the basement & attic already at pipes & stuff. All of the obvious stuff mentioned in the articles looks to have already been hit.
How significant could a bathroom fan (exhausting to the roof) with a poor air damper be? I always hear it tapping & wonder if that's a leaking hot air.
Dana - don't know if it matters, but the air coming out of my vents is only about 95-115 degrees (depending on distance from furnace) & this is on the slowest blower fan speed.
I should have measured the basement temp when I took those other readings, but the main floor temperature has never gone under the basement temperature. In other words, if I warm the house to 68 and and it's 59 in the basement, if I killed the heat, after some hours, the whole house will be 59.
I'm not sure about insulation gaps on the main floor, but the rear of the basement is not insulated. We do have drywall over the basement's ceiling (energy auditor could not see the rim joist).
Only a small section of ductwork is exposed, and those are air tight (with mastic). The ductwork that isn't exposed is between the basement ceiling drywall & subfloor for the mainfloor, which I guess is all in that rim joist area that we also can't scan. Ductwork is not insulated.
Charlie - what defines poor insulation? In my energy audit, all I see is that it says "double brick, [length x width], U value 0.130" and I think it said I had 1" fiberglass on the walls somewhere, but are you saying poor insulation due to the type of insulation, or poor insulation due to not holding up to what it used to?
If one of the basement walls is completely un-insulated that's a guaranteed contributor to the cool floor temp problem!
When it's 20F outside and 55F in the basement, every square foot of above grade foundation is losing 35 BTU/hr, and the below grade portion is losing something like half that. If there's 2' of above-grade foundation x 30' wide that's 60 square feet, 2100 BTU/hr of heat loss, and you're probably losing another 2500-3000BTU/hr out the below-grade section. Even one uninsulated basement wall in a 55F is the heat-loss equivalent of a decent sized insulated first floor room.
If there was no access to air-seal and insulate the band joist & foundation sill that's a highly likely contributor to the problem.
A wall U-factor of 0.13 = R5.6 whole-wall, which is about half what you get out of a 2x4 studwall w/ R15 batts. Is that the above-grade first floor, or is that the basement?
I have lower exit air temps than you on the air handler zone in my sub-code 2x4 framed1920s antique, but the floors never drop below 65F in a 68F room (except for patches of floor directly below larger windows when it's under 25F outside). The insulated basement (R15-R18 continuous foam) and the conducted heat loss off the ductwork, hot water heater etc. keeps the basement about 65-66F even when it's in positive single digits F (like it was last night.) My whole basement's heat loss at 65F (including slab losses to the ~52F sub-soil) is only a fraction of the loss of a single side of un-insulated foundation wall in a 55F basement.
Where is this house located? (ZIP code, &/or climate zone.)
"I was wondering if there are any resources which describe what today's guidelines are for maximizing forced air efficiency by register placement & diffuser selection, in case my home needs an update. For example, is there any reason for a home to really have 48" baseboard diffusers? Is that hurting or helping air mixing?"
There is something that does exactly that, and it's the ACCA Manual T. Most people are familiar with Manual J and maybe Manual D, but there is also Manual S and Manual T. Manual T is all about air distribution. According to Allison Bailes ( http://www.energyvanguard.com/blog-building-science-HERS-BPI/bid/29237/HVAC-Design-Done-Right-Manual-J-S-T-D ), "The questions you answer here are: Where will the supply registers, diffusers, or grills be located? Where will the return grills be located? What type of register, diffuser, or grill will you use? How big does it need to be?
Good choices here will eliminate problems of feeling drafts from the moving air or having inadequate mixing of the air. It's possible to get enough conditioned air into a room but still have it uncomfortable because all the air just sits at the register."
According to manual T, "During heating an isothermal throw of from 4 to 6 feet is required when the outlet delivers the design air flow rate (CFM)." For cooling it's 6 to 8 feet, so 6 feet is a good throw if you have both heat and AC. I doubt manual T is followed in the vast majority of residential HVAC installs.
Dana - sorry should have mentioned that prior - this is Illinois, so I guess that is Zone 5A. The design temperature in this audit says -4F.
Looking at this report closer, the 0.130 U-factor is of all my above-grade first floor walls. It says I only have 1 inch of fiberglass.
The basement walls (with insulation) read a U-factor of 0.065. The basement walls without insulation reads a U-factor of 0.138.
So it sounds like my basement walls are better insulated than my first floor walls. Adding up all the rooms, the first floor is 53% of the total losses, basement is the rest.
I'm unsure how to interpret this - is more air sealing still in order or insulation needs to be addressed?
Michael - thanks for that pointer. I will look more into the Manual T. Not sure how to scientifically measure throw, but out of my baseboard registers, the back of my hand hardly feels anything after a couple feet.
I don't know if it's something that is typically measured. It is (or should be) part of the design with the register selection. The register manufacturer will publish information on the throw, but you need information like CFM and velocity that would be part of the manual D design. http://www.hartandcooley.com/assets/files/7s/420_421_rz421_531.pdf
J,
I think your primary source of cold air is your barely-insulated walls. The air near the walls is cooled by the cold walls. That cool air is more dense than the rest of the air in the room and it falls to the floor. Hence, you have cool air on the floor. You could get some benefit by insulating the not-yet-insulated basement wall, or from finding and sealing additonal air leaks, but the stratification and comfort issues would be improved the most if you upgraded your wall insulation.
Charlie - yes that was one of the recommendations from the audit, but the expense, payback, time, and effort just don't add up. I would have to remove all the drywall/tile/windows/etc.. in the house, install insulation & then put the finish back on. Apparently my walls just have thin metal studs nailed to the brick with drywall on top and a thin fiberglass batt stuffed between (about a 1 inch space).
I was also reading the following which kind of sounds like insulating a brick wall shouldn't be undertaken unless the brick is in good condition & water drainage has been taken care of.
Insulating Old Brick Buildings
How often is EIFS cladding used? I read that it's a way to insulate the outside of the brick, and I honestly don't care about preserving the outside brick appearance. Wondering if this would be a more economical approach & it sounds like it's safer for bricks.
J. Pritzen,
Q. "How often is EIFS cladding used?"
A. EIFS is a tried-and-true approach that has been used on thousands of older masonry buildings in Europe and North America. Call up an EIFS contractor for more information.
One of the discussion points that sometimes comes up here is that the benefits of insulation in terms of fuel use are relatively easy to calculate, but the benefits in terms of comfort are harder to quantify. This situation is a great example of that. You might not find the fuel savings worth the investment, but you might find the comfort worth the investment.
"So it sounds like my basement walls are better insulated than my first floor walls. Adding up all the rooms, the first floor is 53% of the total losses, basement is the rest.
I'm unsure how to interpret this - is more air sealing still in order or insulation needs to be addressed?"
It means that 47% of your heat is going out your uninsulated air-leaky basement, which is why the basement & floors are so frickin' cold. The unisulated basement wall is probably at least 25-30% of the basement losses, the rest are the slab and the windows/doors. If you insulate and air seal the stuff that's easy to get to it WILL make a difference.
Basement walls insulated with an inch of rigid foam trapped against the foundation by a 2x4 batt insulated studwall or a (mold-riskier) 2x6 batt insulated studwall come in at about U0.65, which is a whole-wall R15, and getting close to code-min. (U0.050 is current IRC code minimum for basement walls in zone 5. ) And yes, that's twice the performance of your above grade walls that have only an inch of fiberglass.
It's also true as others have noted that the low-performance first floor walls are a likely contributor to the cold floor, but that's a bigger project to fix. Start by fixing the cheap & easy stuff, like the uninsulated part of the basement and chasing down the remaining the air leaks, both of which are very good bang/buck. You DON'T need IR imaging and a blower door to find the bigger holes- a window fan, a wet finger, and a $50 infra-red thermometer can do a lot. Insulating the remainder of the basement and getting serious about chasing down air leaks will probably cut the whole-house heat load by a solid double-digit percentage, and raise the temperature of the basement (and first floor floors) by 3-5F, possibly more.
Thank you Dana. Don't know why I didn't just add up the numbers myself, but I guess I didn't realize how much the uninsulated basement wall was draining energy -
The uninsulated basement walls only comprise 30% of total basement wall surface area, but yet account for 60% of the total energy loss out of the basement walls!
Further, if I did insulate those walls to R11, I'm seeing a 15% reduction in the total house heat loss which would put the whole house in the 20k BTU/hr range!
All of this neglects air leaks though. Lights are all caulked to the ceiling, the conduit in the attic looks all sealed with foam, plumbing & electrical is all sealed with foam (except the big hole under the tub), the duct boots are sealed to subfloor - it's gonna be some hard work for me to find any more air leaks, especially if an energy auditor with a blower door & IR camera didn't identify anything else aside from my windows being cheap & the mail slot having air come in.
ps: As far as EIFS cladding on the brick...at a cost of at least $15,000 my comfort will just have to suffer!
Other than all of the above, you could get a somewhat warmer floor by adding topside R value - for example a thick rug and high R value pad in the areas where you spend the most time.
Registers should send high velocity air horizontally across the floor (the opposite of a diffuser).
It sounds like even if I significantly reduce heat loss (disregarding wall insulation), I'll still have a problem (with comfort) because of the convective loops that will always be occurring near the walls.
This is a small house with small rooms, so it's impossible to not be near a wall. Every room has at least one exterior wall bounding it, with bedrooms & living room all having at least two. So given the cost of a retrofit, I'll have to live with the fact that we're uncomfortable & everyone thinks my house is always "cold" even though it's gotten tons of "energy efficient" assessing.
Questions:
1) On air sealing, would it make sense to apply Drylok to the bare basement wall? Some document I was reading indicated masonry/brick is air permeable and I'm wondering if that's a big source of air leakage from the basement. Since the other walls are drywalled, they have an air barrier (drywall) but the uninsulated walls do not.
2) On insulating the basement walls, I read that spray foam is the best. Is this something I can just buy a spray foam kit & spray it on the walls? I don't plan on covering it up since it's where the furnace/water heater are (both are now sealed combustion). Or do I _have_ to hire an insulation company because they have better stuff?
J. Pritzen,
Q. "On air sealing, would it make sense to apply Drylok to the bare basement wall?"
A. Drylok is sold as a waterproof coating. The manufacturer of Drylok makes no claims that the product provides an air barrier.
Q. "Some document I was reading indicated masonry/brick is air-permeable, and I'm wondering if that's a big source of air leakage from the basement."
A. The standard method of air sealing leaky CMU walls or cracked concrete walls is to coat the walls with cement-based plaster (parging). This is the method used by radon mitigation contractors. If you want to know how leaky your wall is, you could hire a blower-door contractor to test your house.
Q. "Since the other walls are drywalled, they have an air barrier (drywall) but the uninsulated walls do not."
A. Maybe you should insulate the uninsulated walls.
Q. "On insulating the basement walls, I read that spray foam is the best. Is this something I can just buy a spray foam kit & spray it on the walls?"
A. Yes. The type of kit you need is called a two-component spray foam kit. These kits cost hundreds of dollars, and it doesn't take many kits before the cost of a DIY project becomes higher than the cost of using a spray foam contractor. For more information, see Marc Rosenbaum's article on the topic: Basement Insulation — Part 2.
Q. "I don't plan on covering it [the spray foam] up since it's where the furnace/water heater are (both are now sealed combustion)."
A. When spray foam insulation is installed in a basement, it has to be protected on the interior side of the foam for fire safety reasons. For more information on this topic, see Thermal Barriers and Ignition Barriers for Spray Foam.
Martin, how does the blower door number tell me if the leakage is from the foundation wall?
For the spray foam - this is only two walls that are uninsulated, or 30% of the basement wall surface area - seems uneconomical to hire a big company for such a small space that would take them 5 minutes.
The article linked to - those polyiso boards - are those safe to use around a furnace? If I am not sure I can get the boards airtight, that would also be not-recommended, correct? This is why I was going for spray foam.
J.,
Q. "How does the blower door number tell me if the leakage is from the foundation wall?"
A. When a house is being depressurized by a blower door, the home performance contractor (or the homeowner) can locate air leaks by feeling with their bare hands. At 50 pascals of depressurization, air leaks become quite noticeable.
If you decide to get a blower door test, you'll know whether your basement walls are leaky or not -- and where the leaks are largest.
We found during our blower door test that cranking up the blower higher than 50 pascals made it even easier to locate the few leaks we had.
Fire rated Dow Thermax polyiso is more fire-safe than anybody's 2-part closed cell spray foam, and it's a heluva lot greener too, since it's blown with pentane instead of HFC245fa (a powerful greenhouse gas, 1000x CO2 for 100 year global warming potential. In some juristictions the fire rating of Thermax is good enough to use in a boiler-room, others would still require the thermal barrier. In some jurisdictions intumescent paint would be allowed for fire risk mitigation on spray foam, but in most a rated thermal barrier against ignition would be required if there is an ignition source (like a furnace or boiler or hot water heater) in the same room.
Never put rigid or spray directly against a flue pipe- it would need a minimum clearance of at least an inch (sometimes 2"- local codes vary) even with a rated thermal barrier.
To meet IRC code min for foundation insulation in zone 5A would take 3" of closed cell foam, or 3" of polyiso. In a dry basement with no history of flooding it's safe to go with 1" of polyiso trapped to the wall with a 2x4 studwall insulated with unfaced rock wool or fiberglass (and not denim or cellulose) batts to achieve comparable performance. Though that assembly is not explicitly called out in code, it would still meet code on a U-factor basis. The half-inch gypsum on the interior side need not be painted, though if painted, make it a standard latex paint- it neither needs nor wants an interior side vapor barrier, which would create a moisture trap.
Thank you again, Dana.
The foil on Thermax acts as a vapor barrier, correct? Is my thinking correct that as long as there's no (air) gap between the Thermax & cement, that's why it's permissible to use a vapor barrier on the wall?
What if your walls are not perfectly flat, like mine? The basement is cement up to 5 feet, then a couple feet above grade is brick. If I had a rim joist, it would probably be considered to be 3 feet above grade. The interior above-grade brick surface is uneven & not sure it's too uneven to safely put a vapor barrier on the wall.