Attic Cooling, pretty much a math question

[chiang mai]

I'm trying to get my attic ventilation right and have run into problems, here's the picture:

The footprint of the house measures 1,600 square feet and has four sets of gable end vents for hot/exhaust air and these have been enlarged to provide a total of 6.6 square feet of ventilation.

Input of cooler air is provided in two locations: there's a series of 20 air intake vents at the eves (spread evenly around the house) which provide a total of 2.2 square feet of air flow, and an in-house attic hatch way that provides an additional 6.75 square feet of cool air flow.

I understand that, as a guideline, a house of this size that does not use continuous ventilation at the ridge line, requires 1 square foot of ventilation for every 150 square feet of floor space, By that calculation my house requires a total of 10.7 square feet of ventilation which should be split, 40% at the gable vents and 60% at the soffits for intake air. From the above I calculate I currently have 8.95 square feet at the soffits and 6.6 square feet at the gables so the overall numbers and the balance seem correct.

BUT, the roof void remains very hot and there is no real sense of airflow in the roof void. The ceiling is insulated with up to 12 inches of bat type insulation plus I'm in the process of fixing radiant barrier foil against the rafters.

My question is, does the existence of the roof hatch to provide upward air flow from the centre of the house have a negative affect on the balance or should that air intake all be dispersed to the eves/soffits?

Thanks in advance for any useful inputs.

[CharlieH]

MOVED to DIY Housing

[IMHO]

I have mentioned several times on these forums that managing thermal expansion is way more effective than managing thermal buoyancy, but every time it just ends in bunch of armchair experts trying to debunk me. So I'll leave this thread to them I think

[krisb]

[chiang mai]

I have mentioned several times on these forums that managing thermal expansion is way more effective than managing thermal buoyancy, but every time it just ends in bunch of armchair experts trying to debunk me. So I'll leave this thread to them I think

I'd appreciate your input, especially since I don't have any idea what thermal buoyancy means!

[IMHO]

I have mentioned several times on these forums that managing thermal expansion is way more effective than managing thermal buoyancy, but every time it just ends in bunch of armchair experts trying to debunk me. So I'll leave this thread to them I think

I'd appreciate your input, especially since I don't have any idea what thermal buoyancy means!

Thermal expansion: hot air expands

Thermal buoyancy: hot air rises

[chiang mai]

Ok look, I'm not about beg for answers on this subject nor to cater to prima donna's, does anyone else have useful input they feel they can share on the OP?

Edited June 15, 2015 by chiang mai

[IMHO]

Ok look, I'm not about beg for answers on this subject nor to cater to prima donna's, does anyone else have useful input they feel they can share on the OP?

Sorry, didn't mean to come across that way. I also don't have much time now to get in depth, so I have to keep it brief.

In short, what you should be focussing in on is soffit ventilation - as much of it as possible. Ridge/gable ventilation and things like "whirlybirds" (i.e. techniques focussing on 'hot air rises') are nowhere near as effective as everyone imagines. Heavy use of soffit ventilation is able to take advantage of prevailing breeze to create airlfow in the roof, which is simply way more effective.

Turn your house into a big sail, and direct the air flow in & out of the soffits (through the crawl space) and you'll then get that 'sense or airflow' you're looking for - and lower temps.

It's also the most effective way to extract expanding hot air.

[arjunadawn]

I have less knowledge on how to do this then all of the posters above. Does anyone know who might be contacted in BKK to do such work on my home?

[chiang mai]

Ok look, I'm not about beg for answers on this subject nor to cater to prima donna's, does anyone else have useful input they feel they can share on the OP?

Sorry, didn't mean to come across that way. I also don't have much time now to get in depth, so I have to keep it brief.

In short, what you should be focussing in on is soffit ventilation - as much of it as possible. Ridge/gable ventilation and things like "whirlybirds" (i.e. techniques focussing on 'hot air rises') are nowhere near as effective as everyone imagines. Heavy use of soffit ventilation is able to take advantage of prevailing breeze to create airlfow in the roof, which is simply way more effective.

Turn your house into a big sail, and direct the air flow in & out of the soffits (through the crawl space) and you'll then get that 'sense or airflow' you're looking for - and lower temps.

It's also the most effective way to extract expanding hot air.

No problem and thanks for your input.

[T_Dog]

Google "heat transfer methods" and learn about radiation. You can move all the air you want up there, but that roof tile will get hot and radiate into your living space long after the sun goes down. Insulation is a key factor of keeping the heat out the house in Thailand. It does not take as much as you might think.

[Dakling]

A simple way to reduce the problem is to install an exhaust fan with a thermal switch. Put it in front of one of the gables at the back of the house, run some ducting to the highest/hotest part of the roof and you should notice a big difference. Increasing soffit vent area is definatly a great idea but can be bothersome if they are already installed and are difficult to access. The foil will help a lot with the radiant heat if that is the problem. I would also consider insulating the ceilings if you want to run aircon.

Soffit venting and gables alone are much less effective if you have no breeze. Cost of running the fan is minimal.

Edited June 15, 2015 by Dakling

[Dakling]

Also keep the hatch closed, you are just letting heat into the living area.

[facthailand]

I have less knowledge on how to do this then all of the posters above. Does anyone know who might be contacted in BKK to do such work on my home?

Yes, please send him to my house, too! Seriously, I have not been able to find anybody to work on gable vents and soffits in Bangna.

In fact, I haven't been able to find louvred gable vents for purchase in BKK. Any leads, please.

I'm thinking I'll use 1/4-inch heavy screen for soffit venting. For both, the main thing is to keep the rain & varmints out. Don't think mosquitos will like the attic. I'm hoping this will keep the attic cooler; if need be, I'll add insulation above the ceiling. The underside of the roof is already sprayed with foam insulation; didn't do a dang thing: no vents for circulation and no air gap!

Has anyone had experience looking for vents and/or installers at Hardware House? I'm told these are the most complete building suppliers in the Kingdom.

[chiang mai]

Google "heat transfer methods" and learn about radiation. You can move all the air you want up there, but that roof tile will get hot and radiate into your living space long after the sun goes down. Insulation is a key factor of keeping the heat out the house in Thailand. It does not take as much as you might think.

I do have a radiant barrier (in the process of being) installed plus the ceiling is covered with 12 inches of fiberglass, aluminum encased foil. Whilst there is a problem with conducted heat being transfered from the steel in the roof, to the tops of the concrete walls, the major problem is I;m certain, the balance of air flow between the eves and the gable vents.

[chiang mai]

Also keep the hatch closed, you are just letting heat into the living area.

Heat rises, the the attic hatch serves as as source of inbound cooler air to replace the hot air that vents through the gable vents to the outside..

[MaeJoMTB]

Put a spinner on the roof, fit an electric fan on one of the outlets and suck it out.

One or both should do the job, not much wind in Thailand to move it around on it's own.

[chiang mai]

A simple way to reduce the problem is to install an exhaust fan with a thermal switch. Put it in front of one of the gables at the back of the house, run some ducting to the highest/hotest part of the roof and you should notice a big difference. Increasing soffit vent area is definatly a great idea but can be bothersome if they are already installed and are difficult to access. The foil will help a lot with the radiant heat if that is the problem. I would also consider insulating the ceilings if you want to run aircon.

Soffit venting and gables alone are much less effective if you have no breeze. Cost of running the fan is minimal.

The ceilings are covered with 12 inches of foil encased fibreglass, installing a gable vent fan is a last resort, my next step is almost certainly to get the roof repainted from its present dark brown, to a light grey.

EDIT to add: we have three aircon units but prefer not to use them, this exercise is about trying to make the climate controls of the house self sustaining, if that doesn't sound too geekish.

Edited June 15, 2015 by chiang mai

[chiang mai]

I have less knowledge on how to do this then all of the posters above. Does anyone know who might be contacted in BKK to do such work on my home?

Yes, please send him to my house, too! Seriously, I have not been able to find anybody to work on gable vents and soffits in Bangna.

In fact, I haven't been able to find louvred gable vents for purchase in BKK. Any leads, please.

I'm thinking I'll use 1/4-inch heavy screen for soffit venting. For both, the main thing is to keep the rain & varmints out. Don't think mosquitos will like the attic. I'm hoping this will keep the attic cooler; if need be, I'll add insulation above the ceiling. The underside of the roof is already sprayed with foam insulation; didn't do a dang thing: no vents for circulation and no air gap!

Has anyone had experience looking for vents and/or installers at Hardware House? I'm told these are the most complete building suppliers in the Kingdom.

I used a company in Samkampheng (Chiang Mai) to make custom sized enlarged louvered vents, they cost THB 2K each made out of hardwood. If interested PM me and I'll send you contact details.

[chiang mai]

Put a spinner on the roof, fit an electric fan on one of the outlets and suck it out.

One or both should do the job, not much wind in Thailand to move it around on it's own.

The gable fan is an option but for me, the spinner is not. It fails on two accounts for me personally. The first is that it is hugely ugly and I'd rather throw big bucks at aircon before planting one on my roof, if I lived in a factory I might feel differently.

The second fail point is that friends who have installed them claim there is very little improvement in the air temperature in their homes.

[Crossy]

A post in all caps has been removed.

[bankruatsteve]

Per degree of heat added to "air" will make it expand. It will rise only if there is something cooler beneath it. Let's assume your attic has no ventilation and the radiation from the sun heats it up. The pressure will rise and the hot air will want to go somewhere with less pressure - like any light fixtures or gaps in the ceiling. IE: the hot air will go down. And it won't matter if you have a meter of insulation.

So, ventilation is a must. But fans, whirly things, and others will only help when the ambient temperature is less than the attic temp. That's probably only going to happen at night. By then, you have your A/C running and won't know the difference.

I think, other than reflective foil, any fans, whirly things, insulation, etc. are just wasted costs.

I would like to see anybody prove that their A/C bill is less b/c of this or that in the attic.

[chiang mai]

Per degree of heat added to "air" will make it expand. It will rise only if there is something cooler beneath it. Let's assume your attic has no ventilation and the radiation from the sun heats it up. The pressure will rise and the hot air will want to go somewhere with less pressure - like any light fixtures or gaps in the ceiling. IE: the hot air will go down. And it won't matter if you have a meter of insulation.

So, ventilation is a must. But fans, whirly things, and others will only help when the ambient temperature is less than the attic temp. That's probably only going to happen at night. By then, you have your A/C running and won't know the difference.

I think, other than reflective foil, any fans, whirly things, insulation, etc. are just wasted costs.

I would like to see anybody prove that their A/C bill is less b/c of this or that in the attic.

"Per degree of heat added to "air" will make it expand. It will rise only if there is something cooler beneath it. Let's assume your attic has no ventilation and the radiation from the sun heats it up. The pressure will rise and the hot air will want to go somewhere with less pressure - like any light fixtures or gaps in the ceiling. IE: the hot air will go down. And it won't matter if you have a meter of insulation".

Let's assume also that the attic DOES have insulation, as is the case here AND that the attic DOES have ventilation, as is the case here, then?

[bankruatsteve]

My point is that it really doesn't matter much what you do with your attic when you are cooling the down with A/C. OK - some things will help. But, if you get crazy about the HOT attic, it's jut crazy wasted.

[chiang mai]

My point is that it really doesn't matter much what you do with your attic when you are cooling the down with A/C. OK - some things will help. But, if you get crazy about the HOT attic, it's jut crazy wasted.

As stated earlier, aircon is a last resort for us and our preference is to make the house self sustaining and as thermally efficient as possible.

The R value associated with attic insulation measures the time and speed with which heat will penetrate the insulation barrier or delay heat penetration. The cooler the attic, the more effective is the insulation and the longer it takes to breach resistance.

It really is that simple.

[bankruatsteve]

I didn't see that "stated earlier". So, if you don't like A/C and you know what you want to do, why the post?

[ResandePohm]

chiang mai you are being told a lot of misleading information.

1. The reason for ventilation in a loft/attic is primarily to prevent damp caused by condensation. So things stored in your loft dont get mildew or mould or smell musty.You can get more information at this link:

http://www.airvent.com/pdf/literature/PAVbooklet.pdf

2. If you are looking to ventilate for other reasons such as

a. Air expansion - No additional ventilation is necessary. Enough to open a window or door.

b. Air bouyancy - Cooling the house. This requires a different kind of ventilation/different technique.

So first questions are:

Why are you ventilating?

Is there any ventilation at present and if so what is it?

[Dakling]

Per degree of heat added to "air" will make it expand. It will rise only if there is something cooler beneath it. Let's assume your attic has no ventilation and the radiation from the sun heats it up. The pressure will rise and the hot air will want to go somewhere with less pressure - like any light fixtures or gaps in the ceiling. IE: the hot air will go down. And it won't matter if you have a meter of insulation.

So, ventilation is a must. But fans, whirly things, and others will only help when the ambient temperature is less than the attic temp. That's probably only going to happen at night. By then, you have your A/C running and won't know the difference.

I think, other than reflective foil, any fans, whirly things, insulation, etc. are just wasted costs.

I would like to see anybody prove that their A/C bill is less b/c of this or that in the attic.

Unless you have an exceptionally well built and designed roof insulation and ventilation system your attic temp will be much higher than ambient from an hour or two after it gets direct sunlight to well arter the sun goes down.

[chiang mai]

I didn't see that "stated earlier". So, if you don't like A/C and you know what you want to do, why the post?

I posted a single question in the OP that asked about balance where a central attic hatch supplies a majority of the cooler up flow air, that was the reason for post. To ask the question again and in a slightly different way:

Is it important that inbound cooler air at the soffits/eves be located at the eves and distributed evenly?

[Dakling]

Also keep the hatch closed, you are just letting heat into the living area.

Heat rises, the the attic hatch serves as as source of inbound cooler air to replace the hot air that vents through the gable vents to the outside..

While it is true that hot air rises this is not some sort of a lab condition test, by your reasoning I shouldn't need a lid on my ice chest as the cool air will stay down in the chest. Unless you have a large pressure differential the air always swirls around, mixes and does its thing.

Leaving the hatch open will keep the attic cooler but at the cost of extra heat in the house.

Edited June 15, 2015 by Dakling