HVAC and/or attic ventilation

[big2bird]

Don't know if this has been covered in this thread but on a hot summer's night what would you normally like to have the attic temperature to get optimum results from your home A/C unit. I have a 2 story house and when I walk upstairs I can start feeling the air temperature increase. I believe the attic temperature has a lot to do with it so I installed a attic fan with a thermostat I just need to set the proper attic temperature which I figured around 100 degrees before the attic fan turned off.

This is normal thermal layering. I have seen two cures:
1)Use two smaller systems. Cool the top floor in summer, and heat the lower floor in winter. The return for the lower unit upstairs, and visa versa.
2)Have your HVAC system zoned. They install motorized dampers on each duct, with a stat in each room to rebalance the layering.

 

[big2bird]

My neighbor and I have been thinking of rolling several cans of Cool White roof coating/sealant on to the shingles, years ago I bleached them and got good results, this year it's marginal for some reason....they are normal shingles with the white/lt. gray gravel on them....just replaced the rolled roofing on my nearly flat roof room addition....white metal this time around....

I dunno how it would look from the street though...but on the backside, who cares?? it virtually can't be seen....:smash:

My roof is very light copper colored.

 

[big2bird]

My house is on a crawl space and its pretty cool in the crawl even on hot days. I thought I would try to take advantage of that cooler air and use that air to force the hot air out of the attic. The furnace room is in the center of the house so I ran an 8" diameter tube from the crawl space directly up in to the attic with an 8" diameter solar powered fan inside the tube. I found the 8" heating duct tube and the fan and solar panel at a garage sale but I'm sure new this couldnt cost more than $100. I reduced the attic temperature by 15 degrees. I would think tweaking this idea you could do much better,the cooler air in the crawl space is free.
The very little tweaking I did: Iniatally the 8" tube went just inside the attic and was at the ceiling in the crawl space. I extended the attic tube to go higher so the cooler air could fall and mix more with the hot attic air and force it out the vents more. I also extended the tube in the crawl space to go closer to the ground thinking that air is cooler. I did these 2 things at the same time but I think the attic extension was the real improver.
Initially the temp drop was 10 degrees but went to 15 after the extensions.
I monitored the temp drop with and old inside-outside thermometer that I buried in the insulation in the attic.

Roger, I find this idea brilliant. For me it would involve building a chase, and might entail a fire damper to be legal, but smart all the same.

 

[big2bird]

Minimum code (well maybe not CA code, but everywhere else) is with high (peak) & low (soffit) venting, one square foot of vent area for each 300 square feet of conditioned space attic area (if I remember right this late at night). That's minimum. And remember, "Built To Code" means built to the absolute minimum standards the law will allow...

The spinning turbine vents 'work' on the principle that the convection draft will get the turbine spinning which will then draw more air out. Think about your work/power engineering principles .(Where's Turtle when we need him? Where is Turtle, by the way??) You use passive airflow convection currents to spin a mechanical device to thus draw more airflow....Hey, buy two and I'll toss in a free magnetic ionizing fuel filter for your car guaranteed to improve milage & performance by 20%! All those fucking things are good for is to keep the neighbors awake all night when the bearings go bad and they start sqeaking.I understand that, but am trying to capture some of the afternoon sea breeze here.

No soffits, huh? bummer--where's you passive shading? Oh well, moot point. Install regular roof vents down low as possible without being blocked by insulation, and more up high. For HVAC generalities, you want twice the area of return than you have for supply--supply is higher pressure (forced), return is low pressure (passive). Same principle here---install as many low vents as you can (I don't have any numbers to give you, but I'll just toss out a combined total of 1 sq foot of vent area per 50 sq foot of attic area--more if you want--I know I have more than that in my little house) ) and then install at least 1 1/2 times MORE area of high vents as high up on the roof as you can than you have in low vents. The more airflow you have, the better. Put a few more low vents on the cool side, more high vents on the hot side of the roof. Enhance the convection effect.

Gable vents can help, especially if aligned with prevailing winds. Increase the size of your gable vents--make 'em as big as you can, get a breeze flowing through the attic! And/or an attic exhaust fan with thermostatic control can help a lot too. Again, exhaust (fan) on hot side of house, intake on cool side. And don't whine about service life---christ, any mechanical device has a service life (hence, the emphasis on passive ventilation). Do you bitch about how long the water heater, furnace, A/C last (Well, OK, yes we all do). How often do you service & repair those dam mechanical beasts we call 'cars'? How much do you bitch about that?My gables line up with the winds. I have doubled their size, and intend on adding more. When I cut the holes, the temps dropped drastically.

I don't know how tall your attic is, but if you have R30 in it, well then add more---keep the heat away from the ceiling to reduce the broiler effect. Just don't let the insulation block the ventilation airflow.

You want to insulate the conditioned space, not the roof deck itself. Don't cook the shingles from below with reflective barrier directly under the roof deck. Let the heat absorbed by the shingles pass through and then be removed by being transferred to the medium of the attic air which is then removed by ventilation. But isolate that heat from the conditioned living space below.
See, I think that to be true also. Increasing sheathing temp sounds like a bad plan.
R6.5 duct insulation is better than nothing, but any A/C or heat equipment & ducting in an attic (whether California or Minnisota) is pretty fucked. Hells peckerwood, how do you think the efficiency of the A/C coil is affected living an environment like that? Again, moot point. You got what you got. Cover the ducts with more batts to increase the insulation. Flex ducts have a very poor airflow rate compared to hard tin ducts, therefor the air flow is restricted & slowed down, thus increasing its absorbtion rate of surrounding heat, and thus doubly reducing its efficiency in cooling the conditioned living space.I should have used R-8 ducts, but they are twice the price, and I installed 150 linear feet. Every duct is a home run to the plenum.

What color is your roof? Dark colors absorb heat, light colors reflect it thus reducing the heat absorption of the attic space. Moot point #3......
Light copper colored

So, to wrap up Johnny's presentation, the main points are:

Increase ventilation in the attic airspace (and then double it).
Increase insulation to the conditioned space & eqipment (and then double it).

Just get up really early when it still cool up there to service the attic fan, just as you periodically clean the A/C condenser, change the furnace filters & and check the fluids & shit on the cars.....


Schedule J and all that stuff I haven't gotten into.


And here's a website from aguy whom I have met in person and he's exactly the kind of man who would fit right in here on VetteMOD, or any construction sight---even theough he is an 'engineer'. He even makes fun of himself for that. I'm too tired right now to look for any specific articles so I'll have to leave it up to you look for them, but I know he has at least one about attic HVAC, and others about roof/attic/conditioned space relationships. (edit: If he contradicts any of my opinions above, listen to him, not me!)

http://www.buildingscience.com/documents/digests/bsd-102-understanding-attic-ventilation?full_view=1






Check out his other articles in the sidebars under "Digests", "Insights", & "Primers" as well--he's not only one smart motherfucker (and he'd be honored to called that) but he's a hell of a good & humorous writer as well.
Here's some more samples:

What do the American Revolutionary War, whiskey & NASCAR all have in common with building practices?
http://www.buildingscience.com/docu...on/?topic=/doctypes/building-science-insights

"Top Ten Dumb Things to do in the South" HVAC-wise
http://www.buildingscience.com/docu...th/?topic=/doctypes/building-science-insights

Let's make a model space shuttle from a brick, a cold beer & two cigars...
http://www.buildingscience.com/docu...ce/?topic=/doctypes/building-science-insights

I believe my weak point now is intakes. I need to cut in alot more. John, are the blocks between rafters necessary in my seismic zone? Can I knock out every other one? My intakes are between rafters about every 6th bay, and are 3.5x16". I am thinking every other bay would be very beneficial.

 

[big2bird]

It's pretty hard to make drastic improvements to an existing design and without gutting the house, only general guidelines will help. Only you can decide what will end up being cost effective.
Be aware that a SEER 13 rating for a couple of years has been the absolute minimum rating and that higher top rating seer often is not recoverable financially in the life of the unit. I know a lot of units work afte many years, but the efficiency of the condenser coils after 10 to 15 years are a small percentage of what they were when new and the unit will run a greatly reduced capacity. Keep in mind the rating was done under lab conditions. Installation results do vary.My new unit is SEARS 13, but the old one waas probaly SEARS 8 or less

A package system is not a really efficient design, and I understand you are stuck with that. Hopefully it is a ground unit and not a rooftop. The biggest proglem is with those is the plenum ductwork both inside the unit and the run to the main duct system.
You can add thin foam insulation inside the unit (careful of waranty) and thick rigid foam (ie. 3") glued and sealed to the outside of the plenums.
On the plus side of a package unit, there is no reason for an installer to touch the refrigerant charge, so it should be correct. Residential units are often not charged properly and result in up to 30% in energy loss.
It's a down blow rooftop on the north side.
Attic temp makes a big difference.
Here we commonly use a full continuous 4 or 5" soffit vent around the complete soffit.
Without seeing pics, I hope I understand your soffit properly, but remove the stucco and blocking between the trusses (rafters), top plate and roof sheathing, and install a full soffit vent, such as those used with aluminum siding. PITA messy job, but a diamond blade on a 4 or 5" grinder works wonders. The vented soffit installs on the bottom of the tails, netween the fascia nd the wall, so any stucco damage above the top plate won't be seen.
Don't know what type of roof you have, but on our rentals, as well as ours, I put a white elastomeric rubber coating on the concrete tile and also on a shingled house. I did it to prevent reroofs (they will last forever with repainting), but on a shingled house it dropped the attic temp by 40° mid day by itself. Solar radiation is high in S Florida. I've been in attics as high as 145°.
If your ductwork is in the attic, it makes a big difference, especially when the system is idle and convection is heating up the ductwork.
Your looking for the canopy effect. Cool and breezy.
You must be carefull to not have your ductwork sweat in an attic, ideally all the ductwork should be in air conditioned space. That alone can save a bunch.
You can also get paint for your outside walls that has ceramic particles mixed in, (just like headers) and the reduces the radiant heat by a good percentage.

If you have a pool, you can add a water to refigerant condensing coil to the condensing side and use the superheated rerigerant to heat the pool and conversely the pool will drop the refrigerant pressures. Same principal as a pool heater, but it uses free electricity. On mine it dropped the condensing unit amperage draw by 3 amps, a direct electrical savings and also dropped the compressre head pressures translating into a longer lasting compressor.

Shade all your windows with awnings or at least reflective film etc.
For example, adding an 8 ft wide unshaded slider to a west facing wall here, required 1 more ton of a/c to cool the house. That is a lot and shows what the sun can do.
This was discovered by using a heat load program developed by Trane and accepted by Florida codes for design.
I still find this unbelievable, but wall insulation makes a bigger difference than attic insulation. Studies from Trane have proven that and is incorporated into the program.

Use a good programmable electronic tstat. Probably save the cost of it the first month.

Keep in mind, most of this code stuff, especially for structures, is just a minimum requirement, you can usually improve on it substantially.

Good luck figuiring the cost effectiveness.

I have dual glazed windows on the west side, and the two south facing kitchen windows are next. The are in the shade of the eaves though. I DO have a 24" overhang, it's just open and not boxed in like a soffit.
I like the peanut grinder idea. I have one, and a diamond blade will pay off in savings. Just need a retrofit vent design that I can have made up. I have one in my head.

 

[JPhil]

" I believe my weak point now is intakes. I need to cut in alot more. John, are the blocks between rafters necessary in my seismic zone? Can I knock out every other one? My intakes are between rafters about every 6th bay, and are 3.5x16". I am thinking every other bay would be very beneficial

How about a hole saw & 'Norval' them?

 

[big2bird]

" I believe my weak point now is intakes. I need to cut in alot more. John, are the blocks between rafters necessary in my seismic zone? Can I knock out every other one? My intakes are between rafters about every 6th bay, and are 3.5x16". I am thinking every other bay would be very beneficial

How about a hole saw & 'Norval' them?

I could on 1/2 of them. The rest are stucco, and that would require a carbide 2" hole saw, or a diamond core bit. I think Noonies idea is better for that.

 

[DWncchs]

My house is on a crawl space and its pretty cool in the crawl even on hot days. I thought I would try to take advantage of that cooler air and use that air to force the hot air out of the attic. The furnace room is in the center of the house so I ran an 8" diameter tube from the crawl space directly up in to the attic with an 8" diameter solar powered fan inside the tube. I found the 8" heating duct tube and the fan and solar panel at a garage sale but I'm sure new this couldnt cost more than $100. I reduced the attic temperature by 15 degrees. I would think tweaking this idea you could do much better,the cooler air in the crawl space is free.
The very little tweaking I did: Iniatally the 8" tube went just inside the attic and was at the ceiling in the crawl space. I extended the attic tube to go higher so the cooler air could fall and mix more with the hot attic air and force it out the vents more. I also extended the tube in the crawl space to go closer to the ground thinking that air is cooler. I did these 2 things at the same time but I think the attic extension was the real improver.
Initially the temp drop was 10 degrees but went to 15 after the extensions.
I monitored the temp drop with and old inside-outside thermometer that I buried in the insulation in the attic.

Roger, I find this idea brilliant. For me it would involve building a chase, and might entail a fire damper to be legal, but smart all the same.

Thanks, I have toyed with the idea of moving more air with an AC fan instead of solar but it works good enough for my area. I would wonder at what point moving too much air would exhaust the cold air in the crawl space . My crawl space has about 3 inches of gravel which I've been told rock will hold the cold from the night before. I guess depending on where you live it would be a balancing act. The rock holding cold or heat I got from reading an article on an "envelope home" (and my accountant had one).The bottom of the envelope house stores cold or heat in the heavy stones and uses it to supplement hot or cold.

 

[mrvette]

My house has just a typical old 8' ceiling, and the l/r-hallway has been capped with another layer of shitrock to make it smooth and uniform, that starburst pattern done near 40 years ago was all butchered up from previous wall removal....I just wish I did that in the kitchen too, but didn't think to just cap it with another layer....by time l/r came around, that was suggested, and I jumped ON that one....it's pretty good insulation, beleive it or not, as there is roughly 1/4 inch of airgap between the layers....

I know that one way or another that attic air has to go......

I should get the HVAC pipes wrapped again, for the insulation....just be a 8f drop from one end to another....I have a HVAC tech button in the bonnet made of duct board...and stuck another in the end vent....

at least the stucco is white, so it's cool as can be on the walls....

I going to do the shingles....but one project at a time....

 

[big2bird]

My house has just a typical old 8' ceiling, and the l/r-hallway has been capped with another layer of shitrock to make it smooth and uniform, that starburst pattern done near 40 years ago was all butchered up from previous wall removal....I just wish I did that in the kitchen too, but didn't think to just cap it with another layer....by time l/r came around, that was suggested, and I jumped ON that one....it's pretty good insulation, beleive it or not, as there is roughly 1/4 inch of airgap between the layers....

I know that one way or another that attic air has to go......

I should get the HVAC pipes wrapped again, for the insulation....just be a 8f drop from one end to another....I have a HVAC tech button in the bonnet made of duct board...and stuck another in the end vent....

at least the stucco is white, so it's cool as can be on the walls....

I going to do the shingles....but one project at a time....

I'll clarify. My concern is not the ceiling. It is COOL to the touch. My concern is heat gain into the ductwork, and moving them to a conditioned space is not an option. Adding duct insulation, or lowering attic temps is an option. Power venting is not. I'm trying to reduce power consumption, not raise it.
Solar fans are still in infancy, and CFM claims by their manufacturers are unrealistic.

 

[73 Mike]

I'm not sure you caught John's idea of using roof vents mounted as low as practical on the roof. This is fairly common practice in instances where the soffit isn't accessible for whatever reason. The idea is to promote convection and this works just fine. Just looks kind of dorkey but so will a Bird on a ladder with a carbide saw cutting his stucco.

Also, I don't know if your nights get cool enough in your area, but we have had great results adding a whole house fan. It got up to 100 yesterday but by 10 o'clock it was cool enough to open the windows and turn that on. 15 minutes later, the house was at 65 degrees.

 

[big2bird]

I'm not sure you caught John's idea of using roof vents mounted as low as practical on the roof. This is fairly common practice in instances where the soffit isn't accessible for whatever reason. The idea is to promote convection and this works just fine. Just looks kind of dorkey but so will a Bird on a ladder with a carbide saw cutting his stucco.

Also, I don't know if your nights get cool enough in your area, but we have had great results adding a whole house fan. It got up to 100 yesterday but by 10 o'clock it was cool enough to open the windows and turn that on. 15 minutes later, the house was at 65 degrees.

You know Mike, I would not mind on the backyard side, but on the street side, I think it would look dorky.

 

[mrvette]

I know you just replaced your roof, but that metal shit I finally broke down and put 400' of it on my room addition....it's FAR better than any shingles.....the guy up around the corner put that on....spent about ten grand on it, just a ~1800' rambler too....but HE chose a dorkey BLUE, and it looks like shit, does have some insulation, styrofoam, I think under it to the old shingles/roof paper....IF I had the MONEY, I"d to that in white.....get hot NOW, bitch....

:crutches::flash:

 

[DC3]

I also need to know the truith about radiant heat barriers. They look promising, but the roofers say it decreases shingle lifespan. My roof is new, and that is not desirable. They also require ridge vents, but some say they don't work as well as they claim.

I looked into this one time before and got the same answers you did about shingle life. But I found the decreased life is due to installations that prevent the movement of air between the roof and the radiant barrier. I was told to make sure there is a gap for air to enter the space between the roof and the barrier at the lowest point and another gap at the peak point.

I never followed through with this on my house so I can't tell you how it worked but I did something similar to a storage building I have. I didn't install a radiant barrier on the storage building but I did completely insulate and seal the attic space so as to have a dry dust free somewhat conditioned storage area. I was told I had to leave a gap between the roof and insulation for airflow to avoid shingle problems. I found some plastic trays at the hardware store just for that purpose that stapled to the rafters. I installed continuous ridge and soffit vents and made sure the air flow could get to the space between the roof and insulation. It's been 10 years or so since I built the building and the shingles seem perfectly fine.

Hope this helps.

Forgot to look for the attic ventilation brochure I think I have. Will try again Monday.

DC

 

[big2bird]

I know you just replaced your roof, but that metal shit I finally broke down and put 400' of it on my room addition....it's FAR better than any shingles.....the guy up around the corner put that on....spent about ten grand on it, just a ~1800' rambler too....but HE chose a dorkey BLUE, and it looks like shit, does have some insulation, styrofoam, I think under it to the old shingles/roof paper....IF I had the MONEY, I"d to that in white.....get hot NOW, bitch....

:crutches::flash:

NO. CASE CLOSED

 

[big2bird]

I also need to know the truith about radiant heat barriers. They look promising, but the roofers say it decreases shingle lifespan. My roof is new, and that is not desirable. They also require ridge vents, but some say they don't work as well as they claim.

I looked into this one time before and got the same answers you did about shingle life. But I found the decreased life is due to installations that prevent the movement of air between the roof and the radiant barrier. I was told to make sure there is a gap for air to enter the space between the roof and the barrier at the lowest point and another gap at the peak point.

I never followed through with this on my house so I can't tell you how it worked but I did something similar to a storage building I have. I didn't install a the radiant barrier on the storage building but I did completely insulate and seal the attic space so as to have a dry dust free somewhat conditioned storage area. I was told I had to leave a gap between the roof and insulation for airflow to avoid shingle problems. I found some plastic trays at the hardware store just for that purpose that stapled to the rafters. I installed continuous ridge and soffit vents and made sure the air flow could get to the space between the roof and insulation. It's been 10 years or so since I built the building and the shingles seem perfectly fine.

Hope this helps.

Forgot to look for the attic ventilation brochure I think I have. Will try again Monday.

DC

Owens Corning "Raft-R -Mates" ring a bell?

 

[big2bird]

I'll start with more intake vents, and take the old insulation off the old ducts, and drape it over the new ones. That will double the blanket thickness to R13.Why not recycle the old stuff? It will just cost me my time.

 

[mrvette]

I'll start with more intake vents, and take the old insulation off the old ducts, and drape it over the new ones. That will double the blanket thickness to R13.Why not recycle the old stuff? It will just cost me my time.

This joint built in '72 same as my car, funny as that's when I met/married my first ex wife....anyway, the metal ducts are wrapped in paper backed insulation, I suspect if yours is too, it's too dry to work with by now....I been talking to my wife's son in the HVAC business to wrap another layer on these, and so he ducking the summer heat....maybe soon....he thought the delta T over the run was a bit much, I think for that air speed, it's horrendous....



:nuts:

 

[big2bird]

I'll start with more intake vents, and take the old insulation off the old ducts, and drape it over the new ones. That will double the blanket thickness to R13.Why not recycle the old stuff? It will just cost me my time.

This joint built in '72 same as my car, funny as that's when I met/married my first ex wife....anyway, the metal ducts are wrapped in paper backed insulation, I suspect if yours is too, it's too dry to work with by now....I been talking to my wife's son in the HVAC business to wrap another layer on these, and so he ducking the summer heat....maybe soon....he thought the delta T over the run was a bit much, I think for that air speed, it's horrendous....



:nuts:

The ducts are all new Gene.

 

[73 Mike]

Bird,

So you're planning on draping pieces of insulation over the ducts sandwiching them between that and the ceiling insulation?

It amazes me that there isn't anything better out there. I've got the same problem except most of my ducting is hung from straps. I think it's covered in R 0.5 if you don't count the dust.