Two walls Vs Q-con blocks

Air conditioning, fans, and anything related to keeping it cool, such as insulation. This would include any posts generally discussing how to keep it cool, such as which types of blocks are better insulators.... ideal wall thickness for keeping an A/C house cool, etc.

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Postby Attila » Fri Jun 13, 2008 8:52 pm

tung wrote:
cruzing wrote:...

Even if we were doing a double wall of thin q-con, I think we'd want the external wall thicker than 7.5cm. As I said though just my opinion.

Attila used the some of the bigger q-con blocks. You could pick his brain for some answers.

cruzing
Attila

I believe that you used quite a thick AAC block on your build; what thickness block did you use, qcon or superblock. How do you rate the coolness of your house and lastly do you reckon your leccy bills have been reduced.

Cheers

Tung


Hi Tung,

I used 20 cm Q-Con Blocks, for all outer and all loadbearing walls.

About the coolness of my house, well, yes, it looks cool, indeed, but that was my wife picking the cool colors 8)

Oh yeah, it feels cool too, even with sunshine on the walls in the afternoon the inner side of the walls is not getting hot, not even warm. Before the heat traverses the 20 cm Q-Con it is already evening and that's it.

However I have a big eve, keeping the wall in the shadow as long as possible, and good windows, PVC, with heat reflecting glass. It all belongs together.

Can't say much about the electricity bill, as you most probably think about cost of the power to run a/c, and I don't have any, don't want them, and don't need them, just ceiling fans and open windows, letting the cool breeze in.

If I would have hot and heat radiating walls I probably would depend on a/c, but without such heat radiators it's absolutely fine the way I have it.

Yes, I would choose again the 20 cm Q-Con blocks, or equivalent.
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Postby Jack » Fri Jun 13, 2008 9:45 pm

Re the thermal mass discussion, a ground water cooling system would be a nice way to go if the temperature differential between the air and ground in Thailand is large enough to be practical. And maybe leaving the windows open at night would cool the mass enough to be useful during the day although for a lot of Thailand the day/night temperature differential isn't that big.

But if using an aircon, even with a small highly efficient one, it still seems like it'd be more energy intensive than just direct cooling with good insulation. For one thing the bedroom you cooled at night probably isn't the room you'll be using during the day. So you'd lose that coolness anyway. And even if it is, energy transfer between the aircon, the walls, and you is a lossy process that's likely to lose more energy than straight aircon cooling. But I bet it would feel better to walk into a cool room without having the fans blowing and the aircon noise.
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Re: Two walls Vs Q-con blocks

Postby tung » Thu Jul 03, 2008 8:27 pm

Just seen a big house being built here in Hua Hin using Qcon blocks. Strange that they are using 75mm blocks and have two walls of them, no cavity, therefor making a 150mm wall. When I asked why not just buy 150mm block I was told by top notch malaysian contractor that the heat will not be transferred to the inner block as quickly if it was a 150mm single wall. You learn something everyday :o
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Re: Two walls Vs Q-con blocks

Postby Attila » Fri Jul 04, 2008 6:59 am

tung wrote:When I asked why not just buy 150mm block I was told by top notch malaysian contractor that the heat will not be transferred to the inner block as quickly if it was a 150mm single wall. You learn something everyday :o


He's assuming that if there is no direct contact between the 2 walls then you interrupt the heat transfer, forcing it to go through the air. This is theoretically correct, as long as the walls really do not touch at all. However due to the lack of distance between the walls my guess is that the effect is very very small.

A 20 cm block wall would certainly have a better effect, and would be faster to build, saving time.

Post a picture showing how that top notch contractor is building, to see if it is really so top notch.
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Re: Two walls Vs Q-con blocks

Postby jazzman » Fri Jul 04, 2008 1:39 pm

There are different requirements for insulation between Hua Hin and Udon Thani province 800 km from the littoral. In HH right now i is hot and sticky. In northern Isaan it is beautifully cool, just occasional rain, and there will never be a need for a house to be equipped with air con or any sophisticated passive solutions - both of which incur considerable expense. Attila's house is in a very similar climate and location to that of HH, but it is beautifully cool inside, day and night, all year round, but the budget to get there was rather different from the budget for the house in Kud Chap.
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Re: Two walls Vs Q-con blocks

Postby grant » Fri Jul 04, 2008 3:14 pm

jazzman wrote: sophisticated passive solutions - both of which incur considerable expense.


Passive cooling techniques are by design very simple and inexpensive, rarely add additional cost and often result in cost savings. Traditional houses in the tropics have used them for generations. The problem with building western houses as most foreigners and foreign developers do in the tropics is they are not designed for passive cooling (with the possible exception of Australia) and most westerners try to adapt the same approach as they do with houses in the west i.e. insulate the hell out of the place to keep the heat out. With the exception of the roof, no special materials for insulation are required if you incorporate passive cooling techniques from the start. Aerated concrete blocks such as Qcon and Superblock are certainly more efficient at minimising heat transfer between spaces but are really only necessary for walls that are exposed to full sun. For shaded walls the heat transfer between a Qcon wall and conventional concrete block wall is minimal in Thailand because the temperature difference between inside and the outside is not that great (25 deg in and 32 deg out for example) when compared to the US or Europe where the difference can be 25-30 degrees in winter. If you use air conditioning, you will save a lot more in cooling costs by ensuring the room that is cooled does not have gaps in the doors and windows everywhere. My belief is that Qcon and Superblock (from a heat transfer point of view) are great for people and developers who do not have the time or inclination to incorporate passive cooling techniques from the start or are limited by their site or location. This current topic although very interesting is really about splitting hairs; The difference between the two will be negligible.
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Re: Two walls Vs Q-con blocks

Postby thaifly » Fri Jul 04, 2008 3:40 pm

GIDDAY GRANT ....ITS THE THAIFLY FROM MAE RIM ...hey cobber....a very intellegient post...if i may say so ......... inother words take your pick......hows the louver situation?????........its a intellegient gidday to all...its the thaifly from mae rim
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Re: Two walls Vs Q-con blocks

Postby jazzman » Fri Jul 04, 2008 3:43 pm

I agree entirely Grant. Please be so good as to post a comparison of the prices of Qcon blocks, and breezeblock, and some scientific side-by-side comparisons of their properties of thermal insulation / conductivity in a language we can all understand.. The readers of CTH who are not in positions to be magnanimously disposed to the environment will be very interested.
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Re: Two walls Vs Q-con blocks

Postby grant » Fri Jul 04, 2008 5:51 pm

Thaifly, thanks to you and Jazzman my worries about sourcing quality louvres are over. I've found a local supplier here for the steel louvres like you have and I'm also waiting on samples of the fancier louvres from Malaysia.

Jazzman, I'll see what I can do about putting something together for a comparison.
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Re: Two walls Vs Q-con blocks

Postby jazzman » Fri Jul 04, 2008 7:05 pm

Good stuff Grant? Any chance of knowing the price of the louvre mechanisms?
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Re: Two walls Vs Q-con blocks

Postby Jack » Sat Jul 05, 2008 10:37 am

Grant makes some valid points. Those are the sort of ideas I worked with when I was designing and siting my house. I put the narrow side on the east-west axis and on the west side I have a bathroom, bedroom, and utility room. I have one meter eaves and several meters of porch on the north and south side along with a covered carport to minimize the solar heating. I used foil under the roof and good roof ventilation.

Although I lived in Florida for 50+ years I guess I'm not as heat tolerant as Grant or Jazzman. Udon doesn't have cool ocean breezes and the average highs for February through October are 31 to 35 degrees with some days getting into the 40s. So I am putting in air conditioning. But as Grant points out the temperature differential isn't like in the colder parts of the west, I usually set the thermostat to 27, so I'm not going overboard with insulation. I'm putting 4" of fiberglass batts over the ceilings that the package rates at R-14. I've calculated my double block walls at about a little over R-9. I got this number adding up render at 0.2/in, 3" block at 0.6, and 2" of fiberglass at 7.7. Most of these numbers came from http://www.coloradoenergy.org/procorner/stuff/r-values.htm and I had to make guesstimates for some but they should be in the a ballpark. The R value for a 7.5 cm AAC block from that site is 3.75 which sounds low but may be right.

I decided to go with the double block instead of single AAC block because it was cheaper, insulation was at least as good and probably better, and I like the thicker walls hiding the columns. 20 cm AAC block weren't in the budget. Something that was hard to factor into the decision was that AAC blocks have a lower thermal mass so that if the sun does hit them they don't stay hot as long. Of course the flip side of that is that if they cool down at night they won't stay cool as long during the day. I don't think thermal mass is factored into the R-value.

So far I'm pleased with how my walls are doing and when the house is complete and we have some hot sunny days I'll try to get some actual performance data.
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Re: Two walls Vs Q-con blocks

Postby grant » Sun Jul 06, 2008 9:33 am

I've made an attempt to make a comparison of the two options questioned by the original poster. To calculate heat transfer through a wall, the following formula can be used (http://utwired.engr.utexas.edu/conserva ... g/cool.cfm):

Q = U * A * deltaT
Q = heat transfer through a wall in BTU per hour.
U = 1/R where the R value (also known as thermal resistivity) of the wall materials in the wall will each have an R value and they are combined to arrive at a final R value.
A = area of the wall in square feet.
deltaT = temperature difference between inside and outside in degrees F.

To make the comparison, I've calculated the heat transfer in a room through 3 walls. This assumes the 4th wall is comprised of windows and the ceiling is insulated so they will be constant in all comparisons. For a room that has 3 walls that are 8.2' (2.5m) high and 16.5' (5m) long this gives a total area of 406 square feet. For the temperature difference, I've used an inside temp of 77 deg F (25 Deg C) and an outside temp of 95 deg F (35 deg C) giving a deltaT of 18 deg F.

Qcon block 20 cm:
U = 1/R (where R = 8.4 for a 20cm block) = 0.12
A = 406
deltaT = 18
Q = 0.12 * 406 * 18 = 877 btu/hr = 257 watt/hr = 1.16 baht per hour at 4.5 baht per kilowatt-hr.

Double 10 cm Block Cement w/ 1" air gap:
U = 1/R (where R = 2.6 for the two blocks and a 1" air gap) = 0.38
A = 406
deltaT = 18
Q = 0.38 * 406 * 18 = 2777 btu/hr = 814 watt/hr = 3.67 baht per hour

I also made a comparison with a single 8" cement block with air holes:
U = 1/R (where R= 1.11) = 0.90
A = 406
deltaT = 18
Q= 0.9 * 406 * 18 = 6577 btu/hr = 1927 watt/hr = 8.7 baht per hour

So it's obvious from this comparison that a Qcon wall is more efficient. This is all fine and reasonably straightforward, however to make a cost comparison between the two you have to take into account your planned air con usage i.e. how many rooms and for how many hours per day, and the temperature variations where you live. This is far more difficult to calculate as it varies tremendously for each person/family. For myself and my wife, it's rather simple, we only use air con in one room of the house (the master bedroom) for about 8 hours in a 24 hour period. In Phuket at 10 PM the outside air temp averages about 29 degrees and we set our air con at 25 degrees. By morning the average outside temp is about 25 degrees so the average during the night is let's say 27.5 This results in the following cooling cost comparison over one month:

Qcon: 77 baht per month
Double Cement Block: 225 baht per month

The difference of 148 baht per month between the Qcon and double cement block would not be enough for me to select one over the other on heat transfer basis alone. I also did this calculation for an average European winter climate where I used a deltaT of 45 deg F (25 deg C) and the cost difference then becomes 1562 baht per month for only 8 hours per day. In reality, the average would probably be closer to 14-16 hours per day or a cost difference of 2,928 baht per month, certainly significant enough for most people to consider. And that's just for a single room not the entire house. Other factors such as ease of use come into play as well and ask any electrical or mechanical contractor in Thailand what they prefer and they will always tell you Qcon or Superblock; they are far easier to cut through and work with.

Obviously cooling systems have to overcome many other factors such as gaps between windows and doors, heat given off by surrounding concrete terraces that have absorbed heat during the day and then release it at night, air con units that give off heat right outside the room they are trying to cool (why not capture this heat to heat your water?). These can be significant contributors to cooling costs and will likely account for the majority of night time cooling costs along with heat transfer through the ceiling and slab. Pay attention to the type of roof and insulation you use. Money spent here is money well spent. I currently rent a house that has no insulation in the roof or walls and the roof has dark brown tiles with no ventilation. The inside temperature of the house is always 3-4 degrees C higher than the outside temperature even at 6 AM. The only time it approaches the outside temperature is when it's been overcast and raining for several days in a row. Imagine the additional load this places on the A/C system when it's being used. This is a prime example of how not to build a house (and it was built by a British guy in his mid 30s who claims he was a builder in England before moving here).

I hope this is useful. I've gone through the calculations more than once so hopefully they are free of errors but no guarantees. :D
Last edited by grant on Mon Jul 07, 2008 7:06 am, edited 1 time in total.
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Re: Two walls Vs Q-con blocks

Postby Jack » Sun Jul 06, 2008 10:30 pm

I think you've run into the same problem that I did when I first started comparing block types. The Q-con site uses the metric R-value of °K·m²/W while the US sites use the English R-value of ft²·°F·h/Btu, The conversion factor is 1 °K·m²/W = 5.67446 ft²·°F·h/Btu. http://en.wikipedia.org/wiki/R-value_(insulation). But your argument still holds. With shaded walls if you don't use your AC much you're not going to save a lot of money insulating the walls.

One of the reasons I went with good wall insulation was for walls I couldn't shade. It's hard to quantify but the radiated heat from walls with sun exposure makes even an air conditioned room a lot less comfortable.

I used the same sort of electric usage analysis you did when I decided not to buy the inverter style ACs. For the amount that I use my ACs it would have taken longer than their expected operating life to offset the increased cost of the units.

I've suffered similar problems with the lack of insulation in rental houses that you have. In the one I'm living in now I just decided to put batts in the ceilings of some of the rooms myself. Any time after noon on a sunny day I can tell a real difference walking from a non-insulated room into an insulated one. It's been worth the cost of the insulation in terms of increased comfort level and decreased AC usage.
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Re: Two walls Vs Q-con blocks

Postby grant » Mon Jul 07, 2008 7:09 am

Jack, you are correct and thanks for pointing it out. I've changed the numbers in the above post so they are now correct. Qcon is far more efficient but it still depends on your overall cooling needs.
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Re: Two walls Vs Q-con blocks

Postby cruzing » Tue Jul 29, 2008 2:29 pm

If you use a single wall of the six inch concrete block you will still have some radiant heat unless your walls have adequate overhangs....with two stories it gets more difficult.

If you do use the single wall concrete blocks.... every other three holes, or is it three blocks??? (need to ask the MR to refresh my memory) you need to fill a hole with concrete....doing this in the same places on each course all the way to the top creating extra support to the wall.(like poles within the wall)

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