Insulating for A/C
 

I know when you insulate for heat, you want to start from the top of the house and go down, and ways to do that include properly insulating the attic, caulking and sealing around recessed lights, vents, etc, and working down from there.

My question is, is insulating for A/C simply the reverse?

Be sure your attic is well vented in summer. Otherwise it gets up to 150 degrees or so and your drywall ceilings will act like radiant heaters. Some folks have the self spinning stack type vents in their roofs and then cover them up for the winter, others have thermostatically activated electric fans through the roof or at the gable ends. Make sure there are vents under your eaves also so cool air can enter the attic.

Watch out for the sun's rays also- close the east window shades in the morning and close the west shades in the afternoon.

Not necessarily. You're concerned with heat getting in instead of heat getting out, but one of the main sources of heat intake is the roof, just as it is of heat loss in the winter.

One main difference is that AC would greatly benefit from cool roofing materials, whereas regular asphalt shingles are fine for keeping the house warm. Cool roofing materials are a bit more expensive, but they may pay off in the long run depending on the local climate. Of course, that's mainly an option if you're putting on a new roof anyway.

Insulating your boiler and hot water lines should help with the AC as well.

Hrm, OK, My roof has no vents at all, and no sophets. So I'll plan to install a thermostatic roof fan in the spring. Thanks for the advice :)

Thanks for the advice, I googled "cool roofing materials" and found a lot of great stuff!

If you put in a roof fan for cooling purposes, you will also want to put in some kind of vents to allow air to flow into the attic space, so you have good circulation.

Light colored roof shingles make a big difference, too. Black roofs should be illegal on an energy saving basis.

You can buy solar powered fan roof vents as well, they do cost a bit more but are self contaned so they will save you the cost of wiring, and save you on your electric bill.
The ground is pretty close to 55F all year round once you get below the frost line, the main reason for insulating the foundation of a house is to keep the cold from the frozen ground out, so in the summer nearly all that heat is coming from above, like was said, white, silver, grey, or any other light/reflective color will help keep your house cool in the summer, and in the winter those light colors will help keep the snow on your roof, and that snow will act, to a point, as insulation, keeping your roof 30F insted of the -10 that it might be outside.
while you are adding insulation to your atic, make sure that you don't block off any vents to the outside that might be in your eves, you can buy chanals that keep that space from filling with insulation so that you keep air in that unheated space moving, and most roof vents are labled with how many cubic feet of space they can vent so do your best to make sure you have enough.

While you are in the attic, think about installing a radiant barrier. You can find rolls for sale on the web if you google it.

I read an interesting article in a Fine Homebuilding publication this week that talks about the inefficiencies of A/C and heat pump systems in general as a result of improper installation and sizing. What really caught my attention in the article however was the fact that the majority of systems are installed with an incorrect level of refrigerant, causing a 20% or greater impact on efficiency from the moment they are turned on.

They also mention that the greatest energy load for most homes in the summer is a result of A/C use, comprising a majority of kWh used. So put another way, anybody faced with $200 electricity bills during the summer in a warmer climate could potentially save at a minimum, $20/month just by assuring that their system is operating with the proper amount of refrigerant. (Likely more.)

I just did a major renovation, and while the walls were open I decided to go big and did spray foam.

https://farm3.static.flickr.com/2282/...46ad114095.jpg

It's very expensive, but the benefits are that its totally airtight and waterproof. Very efficient. I don't have actual numbers, since I haven't moved in yet (soon I hope). And then, I can't A-B-A it :/

One thing that I've heard about AC systems is that you should run your ductwork in the conditioned space. Less heat gain from hot attics and if there's a leak, it'll just blow cold air into the room.

Yup, the ductwork and air handler should ideally be in the conditioned space for best efficiency. Another article in the same magazine suggests that it is far better to have conditioned crawl spaces than unconditioned vented ones, both from an efficiency standpoint, and from the standpoint of keeping moisture accumulation low. As for attics, piling it deep never hurts and is easy to do with loose fill.

In fact, after reading through that issue, I have firmly decided that whomever installed our HVAC system didn't really have or clue or didn't care enough to bother with doing things right. So tommorrow we are having it serviced, admittedly for the first time since we moved in here 5 years ago. (And we suspect is hasn't been touched since it was installed 11 years ago aside from filter changes and when I cleaned the air handler myself and replaced a faulty motor start capacitor.)

Don't forget the vapor barrier for the attic. Keep the attic vents open in the winter. Vapors from the house rise and make the insulation wet. Wet insulation is inefficient.
One of the main reasons for insulating the foundation is thermal storage. Keeps the temps much more even.

Wow Boxchain, how much did that run you?

$1.25-$1.50/sq ft of outside wall area (not house sq ft). I know you can buy fiberglas for $0.50/sf, but you still have to install it (they come out and spray it for you) which is never any fun.

It's an old house (1880s) with no vapor barrier, in order to have one I would have to tear out all of the siding (much of which is still in good shape, old cypress) and replace it. SO the foam also acts as a vapor barrier. And living in New Orleans, I've see what happens to fiberglas when it gets wet :O

Lastly, the foam actually adds structural integrity, something you can't have enough of in a hurricane prone area :)

They do make foam that you can apply with minimal sheetrock damage, but it's more expensive. The other route you can go is blown cellulose, which my old landlord did in my last apt. It works pretty well, but it tends to settle, so you lose insulation at the top, and rodents love to nest in it.

It gets better, most homes are severely under-insulated.

Current green standards recommend R-49 ceiling (house-to-attic) and R-30 floor (basement-to-house), and more can be done at the roof level and then of course windows and walls.

I found my ceiling was R-11 and I just this last November DIY project took it to R-20. It was a PITA but the difference I felt immediately, the heat pump ran less and for shorter periods of time, also it takes longer for the house to react to an outdoor temperature change and when heated it stays warm longer.

My floor is also R-11...
As you can see I have work to do, but I have to be in the mood for it because it's royal dealing with these fiberglass rolls.

Other things I noticed:
I keep my thermostat for inside set to 64f, this helps considerably but...

My basement is sealed in the winter months, and tends to stay in the low 50's (52-56f). On a warm day, opening the door to this basement for a period of time of 4-6 hours raises the temperature by about 8 degrees up to just over 60, like 62 or so (we do get some 70 degree days in winter here in VA).
Doing that, several hours later raises the interior temperature of my house by 4-6 degrees.
I found it interesting how it can be 62 in the basement yet raise my temps to 68 inside the house merely via this apparently silly method.
Not opening the basement door in contrast, on a 70 degree day still helps, but not as much.

Well the good news is that everything is ok with the system except for a motor start capacitor that needs replacing at a cost of about $10 the DIY route. The unofficial opinion from the tech was that it wasn't worth considering any kind of system upgrade unless something major failed.

So I'm just doing the regular stuff myself like placing R-5 foam around the air handler and considering an HRV.

get online (google) and type in radiant barrier insulation. It's called insulation but it actually reflects 90+ % of heat so in a well ventalated attic it works great. Power fans are over rated, phyics can do the job for free

power vents are ideal if you can't fit enough normal roof vents, like on a hip roof or if you have another weird shape, I've installed them and you can feel them pulling the hot air out of the attic space.

You insulate the house as if you are insulating to save energy costs. There is no pre-defined "order of operation" I know of that cannot be hotly contested. Insulate the attic first for heat? Ok, fine, but then you don't seal the basement or insulate it, and that nice cool AC air you paid good money for quickly flows right down into the unsealed and uninsulated basement and seeps into the earth lickety split. Contrary to popular belief, you cannot ignore a basement in your insulating scheme because "earth is a good insulator". It may well be, or not, but once the energy is outside your cinder blocks all you're doing is keeping/retraining the heat/cool outside your house.

Insulating and sealing, ideally, would be attic/walls/basement, at once (or as close to once as you can get). R38 (for temperate zones) and then top it off with a good radiant barrier in the attic. Remember to seal every wire/pipe entry you find in all areas first (there are exceptions of course, chimneys you have to block around and bathroom vent fans you don't cover either), and don't forget to seal the sill plate area where the basement and house pad meet with a good 20 year caulk (highly important). Also ensure you have very good circulation in the attic, that your soffit vents are clear and that your gable/sill vents are open and clear and moving air through convection (or fan if need be). The next best thing to do is to add some kind of shade (artificial or natural) to all windows that are exposed to the sun for any length of time during the day. Exterior shade is far better than interior shade (curtains) since it stops the heat/radiant heat from ever getting in to dissipate.

Ideally you insulate where you can afford to do so, regardless of order of operation. Most folks pick the attic because it's the easiest to get to, then neglect the other areas thinking they've done their job. The energy consumption improves, sure, but not nearly as much as if they'd taken on the whole house holistically as they should have. A well sealed and insulated house with windows shaded to keep out maximum radiant heat can obtain 50% to 70% less average energy consumption than a normal mass production home like most people live in.

See here: https://www.builditsolar.com/Projects...nservation.htm

For more insulation details

Power vents are great, but you have to have everything sealed below them for them to be effective. Most folks put one in without Great Stuffing/caulking all air leaks up to the attic, then are baffled as to why their energy bills aren't going down as much as they thought they would. :)

Create a good airflow with a fan and unsealed holes will suck up AC air into the attic as well as from the soffits.

And also, sealing the ductwork then insulating it, to save on both heat and AC, is a must. That's probably the biggest payback per dollar thing you can do, outside of a full blanket insulation project on the home.

The air might flow down to the basement IF you have an open door (the small amount of draft beneath a closed door becomes inconsequential at the temperature delta in question), but then that air doesn't flow into the earth. The air remains contained in the basement, and IF the earth is warmer than that air, it will conduct some heat into that air, depending on the R-value of your concrete/finished basement walls.

However, in reality, the basement is bound to be naturally much cooler. The earth and the thick concrete walls both act as high-capacity long-term ballast, and all through the summer my basement is nice and cool even if I haven't been using A/C -- and the front half of my basement is half-exposed with uninsulated wood walls instead of full-height concrete.

The situation in a car is far different. My guess would be that it really isn't strongly affected by temperature-related air density differences because the volume is so small, the height is so small, and the air is constantly moving, so insulation should be evenly distributed.

It will go down through any openings that allow air through, not just open doors. Why cool your basement with AC (assuming it's not finished) that you don't intend to use for basement cooling? Seems quite a waste, but easily and cheaply accounted for with a can or two of good-stuff and/or caulk and an hour or two on a Saturday morning. All leaks become consequential if there a lot of them spread across an area. That's why folks seal attics after all, and seal baseboards.

Basements are much cooler, without a doubt. That kind of points to how bad an insulator cinder block/concrete is, though it is true that you'll never be able to fully fight the entire planet earth when it comes to temp in a basement. Summer time it's fine since it requires zero to cool it, but in wintertime it's a bit of a PITA when it comes to saving energy especially if you have uninsulated floors (or rather, uninsulated basement "ceiling") and don't seal the sill/concrete joint. Big heat sink and energy waster.

I was talking about heat transfer, btw, not air movement, but wasn't very good at expressing that initially. :)

When you say the front half is exposed with uninsulated wood instead of cinder/concrete, you do mean that the wood is overlayed on the concrete/block right? Can't say I've ever heard of a wood-only basement (though my not hearing of them does not mean they don't exist, lol).

Assignment to all homeowners for this weekend: ^^^^

We've been slowly accomplishing our shading plan - but parts of it can only work as fast the folliage will grow!

This last year, we put up an arbor over our back porch and west facing sliding doors. It has cottage (climbing) roses planted at all 4 corners of it to fill in a lattice on the west side of it and over the top. It should do allot to reduce solar heating in the summer without sacrificing much of the minimal heating we get during the winter.

We have also placed an inexpensive bamboo exterior shade off the eave to block out heat into our bedroom window on the same side. We had two successive 90+F days recently that made for a great test of it's effectiveness, and whereas our upstairs bedroom had routinely been 2-3 degrees warmer than the rest of the house with the AC running, the new shade cut that differential down to about 1 degree. And that makes a significant difference when you are trying not to overcool the rest of the house just to get a good night of sleep. For $24, it should payback on the investment before the end of summer.

Good point, I failed to include other small openings. I think my point still stands though -- cooled air isn't going to displace significant basement air back up to the main floor unless you have big openings and a big temperature delta.

Sorry, I knew that was going to be unclear and meant to describe it better but I forgot. My house is on a hill and the grade is probably five feet lower in the front, so the concrete wall is only half-height in the front, with a wood wall (complete with single-pane uninsulated windows) going the rest of the way. Despite any ineffciencies from that (and the door to my basement is always open), my ~1000 sq foot raised ranch uses less energy than most similar houses.

Sure it will. Unequal air pressure will equalize in pressure over time, as all gases do, flowing from point of higher pressure down to point of lower pressure. It may well depend however on how you have your venting/ducting setup and if you have any, or not, in the basement. If you have an intake only if unfinished, like most unfinished homes do (modern), it would suck air from the basement into the system, causing pressure differences that would have to be made up from somewhere. That somewhere is the closest area of higher pressure presummably, which would be the floor above it. If you have an AC'd (or otherwise "blown into the basement as well") basement it would be a different case of course.

On the other hand the unequal pressure would also leak into the attic through unsealed electrical wire holes and fans/canister lights/chimney openings as well as through any other openings you have as well with the outside, eventually. Whole home sealing is what I prefer, for just this reason.

It's one of the reasons you seal up an attic, so that you're not blowing air conditioned air (or heated air in winter) from lower rooms into the attic, either through gas equalization or temperature difference.


Neat setup. I wouldn't sweat the uninsulated windows (if by uninsulated you means it's not some kind of double glass with gas setup), the seal around the window and blocking it off from the sun in summer is more economical than buying a neato expensive window.

Home efficiency comes from many different factors. The thread was just asking about "which would you do first". My answer, seal and insulate everything. If push came to shove, and I was given a choice of only being allowed to insulate and fully seal one area, and was forbidden from doing any other areas forever after, I'd probably go with the attic. Real life though, I'd canvas the house sealing and insulating because leaks happen and they matter no matter where they come from.

I thought we were talking about convection caused by differences in temperature, not pressure equalization. Either way I just don't think the total effect is significant, because the temperature and pressure differences are just not very big; the basement is bound to be nearly the right temperature already, in the summer.

Come to think of it, I think that side doesn't get much sun exposure during hot parts of the day. It's funny, I have a half dozen really nice vinyl gas-filled double-pane low-e windows sitting in my basement (I got them for free) waiting for me to install them somewhere, but my wife hates the idea. We have NO windows on the ends of the house, only the front and back. She can't imagine what it would be like and is deathly afraid of modifying things like houses and cars.

Oops, I came in late and didn't read the previous posts.

Sealing the house so thoroughly is good if you're also ready to vent properly too. However, I think if you overdo it it ends up not paying for itself. There's a certain amount of inefficiency that has to be accepted, either in the form of unsealed drafts or on-purpose vents. You can pay to have tighter control on that inefficiency but the point of diminishing returns is closer than you think.

The implications of insufficient venting are more far-reaching than one might intuitively think, and sometimes can take longer to hit than you'll know or own your house. Having been in construction for years (and glad I'm out of that racket!) I know enough to know that my understanding of ventilation needs is rather more vague than I used to believe.

There are, of course, the simple and common concerns to address. There's indoor air quality, which tends to be far worse than most people think; though if you think it's good, isn't that good enough? There's direct construction consequences; most people know that an improperly ventilated roof will rot its sheathing before the shingles die naturally, but most do not know that the shingles may die early even if the sheathing holds up. Those are simple and easy examples.

Here's another one: Getting a good draft for wood burning requires intake somewhere, and my house is too well sealed for it, so I often have draft problems despite a well-designed internal chimney. Hello smoke! I burn waste wood that was landfill-bound, which costs me nothing, saves others money, saves landfill space, and probably doesn't contribute any more pollution than if the wood rots in the landfill and the landfill gas is either vented or burned (not to mention all the forest around my house to absorb it anyway, and the closed-loop format of wood as fuel).

See, part of why my house is so efficient is that it's well sealed because it's a modern modular home, meaning that it's built almost 100% traditionally except that it's built in a factory and shipped to the site to be placed on the foundation. Our indoor air quality really suffers from being so well sealed, though, and all winter I suffer with it. A little draft here or there would be much better than opening windows!

I was just talking about sealing and insulating. The "how" of how the air would get down there, at least in my view, is pretty much inconsequential. Sorry I wasn't more clear.


I've said it before, I'll say it again. Women. :)


No worries.

I completely agree. Home energy efficiency is a multifaceted prospect. It has to be completely thought through. Good observations.

Can't say I have a wood burning stove (or wood burning anything at this point).

Why not just poke a really small hole or two here or there and cover it with screen and a door? One way up high, one way down low, and let nature do its work (just thinking out loud, not sure if this would be truly a good idea, lol).

Well, I was considering a cold air intake for the fireplace, but I've read about them and was convinced not to do it -- though I no longer remember why.