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Insulation for a Hot Climate

Insulation for a Hot Climate

Insulation is a very regional discussion. The best advice for a home in Boston can be disastrous for home in Phoenix and vice versa. In order to insulate a home properly, getting advice from a local contractor is essential. But since almost every bit of insulation advice I have ever seen on the interwebs is focused on cold climates, I thought it was time to give a little focus to those of you who live in the sunbelt instead of the rustbelt.

After all, my work is based mostly in Florida, and so insulation for a hot climate is comfortably in my wheelhouse. How do you know if you technically live in a hot climate? If you have more days that you use air conditioning than heat, then you live in a hot climate. If you heat more days than you air condition, then you live in a cold climate.

On a side note, the Nest Thermostat has a cool reports feature than shows you how many hours you heated and cooled in each month. We recently got one and the information has been super helpful! I posted a tutorial on how to install one here as well.

Obviously, this is a very black and white delineation, so for those of you in the middle, you’ll likely need a little bit of both types of insulation techniques.

Insulation For a Hot Climate

Insulation works much the same no matter where you are in that it slows down the transfer of heat. The difference is that in a southern climate the heat is outside trying to work its way in. There are three types of heat transfer: conductive, radiant, and convective and I written about the difference between them in an earlier post.

In hot climates, think about the temperature delta. For example, if you are living in Houston, Texas and it is 96°F outside and you want the indoor temperature to be 76°F indoors, that is a 20° difference you have to overcome. Now, let’s assume you live in Burlington, Vermont and the outside temperature is 14°F. and to get to a comfortable temperature of, say 68°F in the winter is a 54° difference. That’s huge!

It may seem miserable in the summer in the deep south, but the work to keep a building insulated properly is much easier than it is in the great white north. Despite what you may hear, you don’t need to worry about insulation for a hot climate nearly as much much as you do in a cold climate.

For those of you in the southwest states where temps regularly exceed 110°, the situation may be a bit different, but the ideas in this post still apply. You just need to take them further.

Where to Insulate

There are only two places you should be spending the bulk of your insulation dollars in a hot climate. Picking these two low hanging fruits first will yield big results with the least investment, so let’s start there.

The Attic

The brutal sun in the southern states will cook your attic and turn it into an oven (literally). If you want to bring your energy bills down, this is where you start. Focus a lot of energy (pun intended) on the underside of the roof to keep the heat out of the attic and not just the floor of the attic.

You can use products like radiant barrier on the underside of the roof, spray foam, or even good old fashioned batts of rock wool, denim or fiberglass. I discuss the different types of insulation you can choose and how to decide on the right one in a previous post.

Once you’ve insulated the underside of the roof to the max, then it’s time to focus on the floor of the attic if it is unfinished. If you have a finished attic, then the focus should be solely on the roof, but for those of us with open and unfinished attic spaces, every inch is fair game.

Use my tutorial on how to install blown-in insulation onto the floor of your attic. Blown-in insulation is dirt cheap, easy to install and does wonders to bring down energy bills. You can load up the attic floor with as much insulation as you can fit for very little money (think $600 for a 1300 SF house!).

There are a couple things to look out for when insulating an attic though:

  1. Knob & Tube Wiring – If you still have active knob & tube wiring, do not disturb or cover it in insulation- that’s how fires start. Call an electrician and have it removed immediately.
  2. Airflow – Make sure your insulation doesn’t block any airflow, whether it’s is to soffit vents, gable vents, or ridge and off-ridge vents. You attic needs to breathe to keep your house healthy.

How to weatherstrip wood windowsWeatherstrip Doors & Windows

The next biggest offender of heat getting in and AC getting out are your windows and doors. No, it’s not about them being single-paned, it’s about air sealing. The heat transfer that happens through doors and windows is because of the tiny gaps that allow air to seep through.

Learn to weatherstrip your doors and windows with my tutorials. It costs less than $20 a window to weatherstrip properly, compared to replacement windows, which can cost thousands of dollars and don’t really give the payback they promise.

Sealing these openings not only gets rid of hot drafts, but will beat those energy bills into submission as well.

Don’t Waste Your Money Insulating Here

I have a 1920s bungalow in Orando, Florida. Our energy bills are under $175 a month in the summer consistently and that is with a family of four. I have insulated very sparingly and strategically to keep our bills low and I want to show you where you should absolutely NOT insulate because the payback is so minimal that it’s not worth it.

The Crawlspace

The crawlspace is possibly the most comfortable place around your whole house year round. It is shaded from the sun and is cool in the summer and relatively warm in the winter. Why would you need to insulate an exterior area that is usually within 10° of the inside temperature? I’m not mad at you if you insulate the crawlspace, but usually it’s not worth the time, expense and considerable difficulty it requires.

Replacement Windows

Don’t waste your money replacing your windows. Replacement window companies have often been dragged into court by the FCC for lies and deceptive advertising. No window will cut your energy bills by 30-50%. Weatherstripping your original windows will let you keep your money and your windows.

Exterior Walls

Pulling down plaster or siding to stuff the walls full of insulation causes a lot of expensive work to put things back together and often causes problems. I’m not against insulating the walls of a house, but you must have proper water management in place before you pump a wall full of any kind of insulation.

Old houses often have no way to ensure water isn’t getting into the wall cavity, and once you fill it with insulation (aka a sponge) you get mold, rot, and health issues. A deep energy retrofit is a possibility, but think about the payback. If you send $25,000 to completely revamp the shell of your house and get a monthly savings of $150 on your utility bill, that is going to take you about 14 years to see a return on your investment. Always look at the return on your investment when it comes to insulation.

Final Thoughts

I hope this has given you some guidance on how work with insulation for a hot climate. It’s not as complicated as it may seem. Always, always start with the low hanging fruit and work your way up the tree, as money and time permit. You’ll be better off in the end if you do it that way and so will your wallet.

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40 thoughts on “Insulation for a Hot Climate

  1. I live in a home that is near 30 years old. I live in southern Utah and the temperature gets easily into the 100’s. during the summer it just seems to run non stop, which I expect, but around 1 in the afternoon it will just start climbing on the thermometer almost one degree and hour until around 6 or 7. I have had the air conditioner checked out and they say it’s fine. I had someone come last year and they told me that I was at 9 inches of insulation and he said It was half of what it should be. once the sun goes down it starts cooling pretty easily. any ideas? the guy before told me about all these things like a pressure test and attic fan and this and that.

  2. Hi Scott,
    Recently moved into a wood-frame, aluminum-sided house in Largo, Florida. Garage unfinished with no insulation on walls. Cannot pull off siding to install vapor barrier, but I want to insulate and install blueboard.

    Any advice or opinion on how to minimize humidity or possibility of mold growth? It’s dang hot in there!

    1. Jim,
      I’m about to do about the same thing to a pre-fab “tough-shed” so I can use it as an office. I’m installing 30# roofing felt in each bay of studs, leaving a 1/2″ air gap between it & the exterior siding & stapling it along both sides (roofing felt comes in 36″ wide rolls – so cut in 1/2, one roll will do about 18 bays at 16″o.c. for just $15+/-).
      Inside the felt; I’m using 3/4″ closed cell, Polyisocyanurate thermal insulation Board with Radiant Foil, facing the exterior. I’ll seal all four sides of this board in place with a canned closed-cell Insulating Foam (the kind that is used for gaps around doors or windows). This will provide a Moisture/Vapor/Thermal Barrier along the exterior of the wall studs & carries about an R-6 insulating value & can be done for just a few hundred dollars. My 12×32 shed will be about $500.00…by comparison, just 1″ of closed-cell insulating foam sprayed as a DIY was estimated at about $1000.00 & would only achieve an R-7 insulation value.
      On the interior of the wall, between & flush to the face of the studs, I’m going to use 1″ Extruded Polystyrene (XPS) Rigid Foam Insulation. This provides R-10 insulation value; but, and more importantly, leaves a 1.25″ space (assuming 2×4 walls) between the two rigid insulation boards for any electrical or plumbing in the wall cavity & allows for the material to be removed if I need to get to any of those components. I will also seal along the interior of the wall with caulking to prevent air leakage over/under top & bottom plates. Then additional rigid insulation can be added to the interior space for higher R-value, or just finish the wall with the interior surface material of your choice.

  3. We just tore down all the drywall in our house in Pensacola Florida. The drywall was damaged and had a few too many generations of textures. The house is brick and we notice the outside walls really hold the heat and are warm to touch, we want to insulate the walls before putting up new drywall but are now concerned after reading your post about moisture and mold. Your thoughts?

  4. We have a 70 year old house in Houston. When we had insulation blown in many years ago we were told not to insulate over the attached double garage. We were told insulating over the garage would cause moisture and mold problems. We have one small air conditioner vent that blows a small amount of air into the garage. It is time to add more blown insulation. Should we insulate over the garage?

  5. Thanks for posting about the outside wall insulation that help to protect the walls from cavity. its really helpful for me.

  6. I have a 1974 town home in las vegas nevada and the units are separated by block walls. I have notice my bathroom gets direct sun on the neighbors side of the wall because i get it in the same area but i have a covered patio in the area and he doesnt. The bathroom is very hot in the evening in summer and freezing cold in winter. I plan to remodel that bathroom and was thinking i should reinsulate while dry wall is open but sure what product to use

  7. Is it worth getting an insulated garage door instead of just a steel garage door for a home in Florida? The goal is to keep the garage cooler. Thank you.

    1. So I am moving to a tropical Island and building a B&B. I have been told by some island ex pats, that they say let the house breath and no exterior wall insulation. That it is totally different from when I grew up in our construction family. Tyvek and insulate was always the standard, now I am looking for the the best solution, Fiberglass or foam?

  8. Dry hot Southwest climate. Vapor barrier in crawl space? It is vented and no ducts run through, through,only one short run of Plumbing. Small simple home. Seems like a vapor barrier would trap in heat while earth would absorb it.

    1. Vapor Barrier is for vapor. Yes use it. Heat rises for the most part. Does not sink into the earth.

  9. I have a garage in central Texas with walls that only have felt paper. The outside walls are wood siding with no sheathing.
    I want to put up plywood and then possibly drywall on top. The felt paper has a few missing areas on the wall. It appears good under the studs. The garage is 70 years old. I was thinking of putting in some fiberglass for sound proofing and insulation to use the area as a shop. Do you think I should put up the fiberglass. I was planning on putting the paper side of the fiberglass against the felt paper.

    Thank you.

  10. Hi we are in southern North Carolina about 50 miles from the coast. Our winters are not as mild as Florida but no where near as cold as Virginia north. We are getting a vapor barrier in the crawl space with open venting to the outside. If we don’t necessarily need insulation in the crawl space do we need to update our ducting (currently rigid) to the flexible insulated. Nothing is wrong with the current ducting

  11. What about a metal ‘barn roof’ guest house with lofts in South Texas? Everyone seems to recommend spray foam, but the cost! The concern is moisture build up.

  12. Building is 130 year old 3 bay, 2 story townhouse in Galveston, TX. . No insulation, lath/plaster walls, balloon framing, vented exterior walls from crawlspace to attic, About to start major renovation.
    My builder thinks that adding a central AC system will cool lead to water condensing from humidity in air inside wall cavity onto back of plaster and wants to remove the plaster, fully insulate, seal top and bottom and go back with sheetrock. I get that we’ll get some better R value but us the condensation concern valid? Then there’s the cost. My thought to focus on sealing on exterior walls, Windows, doors and restricting air movement in walls by stuffing insulation at top and bottom vent openings and skip the wall insulation. Plus roof /attic radiant barrier and maximize attic floor insulation. What say you?

    1. Don’t spend the money for what the GC is recommending. In a perfect world he is right but the execution and reality is that it will likely cost way too much and yield minimal results. Insulate the attic like crazy and weatherstrip the doors and windows as well as potentially adding interior or exterior storm windows and you’ll have a pretty efficient house. The other stuff will cost thousands and will likely never give you a return on your investment.

      1. Scott – we had to elevate our 1848 home post Katrina. We are on Northshore of Lake Pontchartrain in Louisiana, about 100 yards from lake edge. What was a 3′ crawlspace under house is now 7-1/2′ to lowest horizontal member on brick & CMU piers and steel frame. We poured a slab over all the footings when done. Visqueen was placed between footings before slab pour. All our floors are old pine, mostly two layers (original subfloor or two floors?). The floors are so cold now we are utterly miserable in winter. Central heat wants to run continuously. While we know we want to improve attic and window/doors, the cold blowing up from under must be addressed. What is our best course of action. Please help!

        1. Hi! I’m researching the same question for our home in Old Mandeville! Did you ever get an answer?

  13. Is there any concern in South Florida of adhering foil faced poly-iso to the inside face of an exterior CMU wall, then leaving a 1″ air space and then installing a framed wall? Will the grains of moisture in the exterior air work their way through the CMU wall and then condense on the exterior side of the foil? If the joints of the poly iso are not taped, is that enough to ensure that the wall dries out? I cannot find any literature via an internet search… so any coaching on technical info would be greatly appreciated!

  14. Thanks for recommending not insulating the crawlspace. My husband and I decided to renovate the 1930s Craftsman home we bought last month, and we need to redo all of the wall insulation. I’ll let him know that we don’t need to do the crawlspace, and we’ll be able to use the money we would have spent on that area on a more important part of the house. http://www.tracysinsulation.net/services.html

  15. I spent a lot of time last year researching this topic and came to the exact same conclusions. The predominant amount of online information applies to cold-weather homes, so I really appreciate your post. It was particularly interesting to read that you too have determined that a priority for homes in hot climates requires addressing the underside of the roof. Cheers!

  16. I”m ready to do some work to get my 1916 four square up in Jacksonville as energy efficient as possible for the long term. I have an open attic- just lattice on front and back of house between the attic outside. A couple items I’d love your opinion on: it has blown in insulation now, but what if I wanted to put some plywood down for storage? Spray foam under plywood? Could you enclose it ie close up open lattice with some vent or would that make matters worse? Just want a bit of storage and max effiency

    1. We have a 3 car detached garage. We are putting up sheetrock on the ceilings and walls. Should we insulate the walls? We are not going to insulate the attic space. We live in hot and humid Houston, Texas.

  17. Thanks for the info Scott. I am surprised you recommend against insulating crawlspaces. I thought it was a moisture issue? For hot and humid climates like New Orleans, what do you think about insulating directly under the floor in a crawlspace, either with rigid foam or batting?

    1. Joel insulation and moisture control are two different things. I would recommend installing 6 mil poly sheeting over the dirt to keep the moisture from entering the crawl space. That will make a big difference and is cheap.

  18. Hey Scott I purchased my house with cellulose blown in through the siding (small holes were drilled and plugged into the siding). Is there a concern with no vapor barrier or the “sponge” effect like your article states? If so, should I remove it?

    Thanks for your articles!

    1. If water is getting to the insulation then yes there is concern. It is a sponge and will hold water and cause rot, but it depends on whether things are getting wet or not.

  19. What is the solution for weather stripped two craftsman casement windows that meet in the middle? I have two sets and there is no frame in the middle, the one window has a brass latch into the frame and then they have a brass latch that locks them together in the middle. The prior owner bunched upholstery fabric and tacks in it to keep the draft out. hahaha. I want something that looks nice and preferably some what blends in or not really easy to notice. Last winter we had heavy rains that beat against that window and leaked water all over our mantle.

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