Attic Cooling, pretty much a math question

[Dakling]

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.

True in colder climates, usually cooling is the primary purpose of ventilation here.

[chiang mai]

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?

All the information is in the OP, it's completely untrue that the primary purpose of attic ventilation is to prevent mildew/moisture, perhaps that is true in cooler climates such as the US but in hotter ones, the primary purpose is to vent hot air from the roof void and to create air flow. Interestingly the link you provided sets out quite clearly that ventilation and airflow in the attic are key to making a house cooler in hot summer months and that the same principles are involved

And no, air bouyancy does not require a different system from one that prevents moisture build up, the same principles are involved, all that changes is the objective and the climate, perhaps read your own link that discusses airflow.

And no again, opening a door etc etc is incorrect, were I to do that I would raise the temperature of the house since the outside air is hotter than the inside air.

Why am I ventilating:

When I bought this house there was a total of 1.20 square feet of ventilation, primarily at the gables and there was no eves/soffit vents, neither was there any insulation anywhere. The house was hotter than blazes. Subsequently I have enlarged the gables vents to provide almost 7 square feet of exhaust venting, installed soffit vents, insulation, radiant barrier (in progress) and window film. All of those things have been very effective at reducing heat in the house although two concerns remain:

1) It remains very unclear to me that the venting system in the attic is working effectively and I suspect that is a function of the central attic hatch providing a majority of the upbound cooler air flow and that was the question I asked in the OP and my sole purpose in posting.

2) Whilst the house is significantly cooler than it was at the outset there is a point in the afternoon where the "defenses" start to break down and the house begins to get hotter. This is to be expected to some degree, especially at this time of the year in Thailand but before switching on the air, I want to be certain that I've done all the appropriate things, correctly - see point one.

[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..

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.

Not entirely correct, providing there is air flow out at the gables or ridge. In my case I have quite a large area of gable vents that allow hot air top be expelled and the expelled air must be replaced from somewhere. In my house at present that "somewhere" is mostly through the attic hatch and the updraught can be felt when standing underneath it - think chimney effect because it is the same thing. And in fact, the temperature in the vicinity of the hatch is not an issue for me whereas the temperature in other rooms is, at certain times of the day

[IMHO]

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.

Right - it's thermal expansion that needs to be managed first and foremost. The only effective way to do this in a normal hip-style roof is ventilated soffits - not just a some here and a some there - vents everywhere.

[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?

All the information is in the OP, it's completely untrue that the primary purpose of attic ventilation is to prevent mildew/moisture, perhaps that is true in cooler climates such as the US but in hotter ones, the primary purpose is to vent hot air from the roof void and to create air flow. Interestingly the link you provided sets out quite clearly that ventilation and airflow in the attic are key to making a house cooler in hot summer months and that the same principles are involved

And no, air bouyancy does not require a different system from one that prevents moisture build up, the same principles are involved, all that changes is the objective and the climate, perhaps read your own link that discusses airflow.

And no again, opening a door etc etc is incorrect, were I to do that I would raise the temperature of the house since the outside air is hotter than the inside air.

Why am I ventilating:

When I bought this house there was a total of 1.20 square feet of ventilation, primarily at the gables and there was no eves/soffit vents, neither was there any insulation anywhere. The house was hotter than blazes. Subsequently I have enlarged the gables vents to provide almost 7 square feet of exhaust venting, installed soffit vents, insulation, radiant barrier (in progress) and window film. All of those things have been very effective at reducing heat in the house although two concerns remain:

1) It remains very unclear to me that the venting system in the attic is working effectively and I suspect that is a function of the central attic hatch providing a majority of the upbound cooler air flow and that was the question I asked in the OP and my sole purpose in posting.

2) Whilst the house is significantly cooler than it was at the outset there is a point in the afternoon where the "defenses" start to break down and the house begins to get hotter. This is to be expected to some degree, especially at this time of the year in Thailand but before switching on the air, I want to be certain that I've done all the appropriate things, correctly - see point one.

You obviously think you know so why post here asking the question. I am a fully qualified construction engineer and have worked with this and green energy for many years. I am in the process of building a house and have included natural air conditioning for cooling the house (no requirement for AC units, hence no increased electricity costs). To do this efficiently you need the vents in the apex of the roof and also a source of cooler air to replace the hot air. However I will bow to your superior knowledge and exit this discussion. Good Luck

[chiang mai]

OK I think the jury is back in on this one. I'm going to increase substantially the number of soffit vents and replace the attic hatch as the primary source of inbound air, I calculate that each vented soffit board allows 110 sq cms of air flow hence around 60 will need to be installed, that should keep me busy for the week

I'll report back with the outcome.

[chiang mai]

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?

All the information is in the OP, it's completely untrue that the primary purpose of attic ventilation is to prevent mildew/moisture, perhaps that is true in cooler climates such as the US but in hotter ones, the primary purpose is to vent hot air from the roof void and to create air flow. Interestingly the link you provided sets out quite clearly that ventilation and airflow in the attic are key to making a house cooler in hot summer months and that the same principles are involved

And no, air bouyancy does not require a different system from one that prevents moisture build up, the same principles are involved, all that changes is the objective and the climate, perhaps read your own link that discusses airflow.

And no again, opening a door etc etc is incorrect, were I to do that I would raise the temperature of the house since the outside air is hotter than the inside air.

Why am I ventilating:

When I bought this house there was a total of 1.20 square feet of ventilation, primarily at the gables and there was no eves/soffit vents, neither was there any insulation anywhere. The house was hotter than blazes. Subsequently I have enlarged the gables vents to provide almost 7 square feet of exhaust venting, installed soffit vents, insulation, radiant barrier (in progress) and window film. All of those things have been very effective at reducing heat in the house although two concerns remain:

1) It remains very unclear to me that the venting system in the attic is working effectively and I suspect that is a function of the central attic hatch providing a majority of the upbound cooler air flow and that was the question I asked in the OP and my sole purpose in posting.

2) Whilst the house is significantly cooler than it was at the outset there is a point in the afternoon where the "defenses" start to break down and the house begins to get hotter. This is to be expected to some degree, especially at this time of the year in Thailand but before switching on the air, I want to be certain that I've done all the appropriate things, correctly - see point one.

You obviously think you know so why post here asking the question. I am a fully qualified construction engineer and have worked with this and green energy for many years. I am in the process of building a house and have included natural air conditioning for cooling the house (no requirement for AC units, hence no increased electricity costs). To do this efficiently you need the vents in the apex of the roof and also a source of cooler air to replace the hot air. However I will bow to your superior knowledge and exit this discussion. Good Luck

"To do this efficiently you need the vents in the apex of the roof and also a source of cooler air to replace the hot air".

I have said several times that I have vents at the apex albeit I refer to the apex as the gables. I have also said several times that I have a source of cooler air inputs!

[krisb]

Is the attic just the ceiling cavity?

What's the height to the ridge?

Problem with tiled roofs is they don't cool well unlike a tin roof. Tin roof gets hotter quicker, but cools quicker.

I'd install more whirly birds and vents on the gables, and more of those eve vents. Not much else you can do is there? Roof spaces are always gonna be hot.

[Dakling]

I think your radiant barrier and the increased sofit vents will make a big difference. I think on calm days there is still going to be a limit to how fast the air will circulate with only the density difference of the hotter air driving movement. With your ceiling insulation you should be able to have a moderatly warm attic without too much impact on the rooms below. Putting a fan in a gable with ducting back to the roof peak would be the only other thing I can see you might add if increasing the soffit vent sizes doesn't do it for you since you have already ruled out the whirly gig thingies (aka. turbine vents).

[IMHO]

"To do this efficiently you need the vents in the apex of the roof and also a source of cooler air to replace the hot air".

I have said several times that I have vents at the apex albeit I refer to the apex as the gables. I have also said several times that I have a source of cooler air inputs!

Yep, and as you've discovered (like some of us here) is that gable vents and the whole thermal buoyancy concept that everyone so believes will make a big difference, doesn't

[chiang mai]

"To do this efficiently you need the vents in the apex of the roof and also a source of cooler air to replace the hot air".

I have said several times that I have vents at the apex albeit I refer to the apex as the gables. I have also said several times that I have a source of cooler air inputs!

Yep, and as you've discovered (like some of us here) is that gable vents and the whole thermal buoyancy concept that everyone so believes will make a big difference, doesn't

I'm going to reserve judgement on that until I've balanced the volume of input air into the soffits with the volume vented at the gables. I have read in several US state documents on this subject that if the exhaust vent volume of air is greater than the soffit/air intake, the total volume of ventilation is restricted to the lower volume, eg. if gable volume is 7 sq ft and soffit volume is only 2 sq ft, the the total volume of ventilation is restricted to 2 sq ft, why that should be so is unclear and I was hoping that someone here might shed light on the why..

[IMHO]

"To do this efficiently you need the vents in the apex of the roof and also a source of cooler air to replace the hot air".

I have said several times that I have vents at the apex albeit I refer to the apex as the gables. I have also said several times that I have a source of cooler air inputs!

Yep, and as you've discovered (like some of us here) is that gable vents and the whole thermal buoyancy concept that everyone so believes will make a big difference, doesn't

I'm going to reserve judgement on that until I've balanced the volume of input air into the soffits with the volume vented at the gables. I have read in several US state documents on this subject that if the exhaust vent volume of air is greater than the soffit/air intake, the total volume of ventilation is restricted to the lower volume, eg. if gable volume is 7 sq ft and soffit volume is only 2 sq ft, the the total volume of ventilation is restricted to 2 sq ft, why that should be so is unclear and I was hoping that someone here might shed light on the why..

Even that basic statement seems flawed, because it's assuming uni-directional air flow. As hot air expands, it's going to find it's way out of the gables and the soffits - so can use all 9 sqft. As it contracts, cool air will find it's way in from both sources too.

[Dakling]

I have read in several US state documents on this subject that if the exhaust vent volume of air is greater than the soffit/air intake, the total volume of ventilation is restricted to the lower volume, eg. if gable volume is 7 sq ft and soffit volume is only 2 sq ft, the the total volume of ventilation is restricted to 2 sq ft, why that should be so is unclear and I was hoping that someone here might shed light on the why..

Ventilation is pretty complicated if you want to get all technical with it because air is a compressible fluid and has many properties that affect its movement.

The documents that you are referencing are simplifying a bit but for all practical purposes without some sort of additional pressure being applied the smaller orafice will always control the flow, especially in a low pressure differential situation. Having a much larger outflow will help the efficiency of the smaller sofit opening area but the sofits would always be the restrictor unless enlarged.

[chiang mai]

I have read in several US state documents on this subject that if the exhaust vent volume of air is greater than the soffit/air intake, the total volume of ventilation is restricted to the lower volume, eg. if gable volume is 7 sq ft and soffit volume is only 2 sq ft, the the total volume of ventilation is restricted to 2 sq ft, why that should be so is unclear and I was hoping that someone here might shed light on the why..

Ventilation is pretty complicated if you want to get all technical with it because air is a compressible fluid and has many properties that affect its movement.

The documents that you are referencing are simplifying a bit but for all practical purposes without some sort of additional pressure being applied the smaller orafice will always control the flow, especially in a low pressure differential situation. Having a much larger outflow will help the efficiency of the smaller sofit opening area but the sofits would always be the restrictor unless enlarged.

"Having a much larger outflow will help the efficiency of the smaller sofit opening area but the sofits would always be the restrictor unless enlarged".

Ah, finally, a potential math answer to my OP.

Do you know how that relationship formula works, in percentage terms?

[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?

All the information is in the OP, it's completely untrue that the primary purpose of attic ventilation is to prevent mildew/moisture, perhaps that is true in cooler climates such as the US but in hotter ones, the primary purpose is to vent hot air from the roof void and to create air flow. Interestingly the link you provided sets out quite clearly that ventilation and airflow in the attic are key to making a house cooler in hot summer months and that the same principles are involved

And no, air bouyancy does not require a different system from one that prevents moisture build up, the same principles are involved, all that changes is the objective and the climate, perhaps read your own link that discusses airflow.

And no again, opening a door etc etc is incorrect, were I to do that I would raise the temperature of the house since the outside air is hotter than the inside air.

Why am I ventilating:

When I bought this house there was a total of 1.20 square feet of ventilation, primarily at the gables and there was no eves/soffit vents, neither was there any insulation anywhere. The house was hotter than blazes. Subsequently I have enlarged the gables vents to provide almost 7 square feet of exhaust venting, installed soffit vents, insulation, radiant barrier (in progress) and window film. All of those things have been very effective at reducing heat in the house although two concerns remain:

1) It remains very unclear to me that the venting system in the attic is working effectively and I suspect that is a function of the central attic hatch providing a majority of the upbound cooler air flow and that was the question I asked in the OP and my sole purpose in posting.

2) Whilst the house is significantly cooler than it was at the outset there is a point in the afternoon where the "defenses" start to break down and the house begins to get hotter. This is to be expected to some degree, especially at this time of the year in Thailand but before switching on the air, I want to be certain that I've done all the appropriate things, correctly - see point one.

What a lot of nonsense you write.

The primary purpose of loft ventilation is most certainly to prevent moisture and damp caused by condensation, especially in a country where humidity is often very high. As high as 100% on occasions. For this purpose soffit vents are adequate.

To use air bouyancy, a stupid expression, i assume that you mean convection, to cool the house you will need, as I have previously stated, a vent in the apex of the roof, not to be confused with gables which are something very different, best is in the form of a chimney painted black. The convection here will then draw out the hot air which in turn will pull in the cooler air from your cool air source. To say that soffit vents will provide this cooler air is again nonsense. Think man. You state that the air outside is hotter than the air inside. Why on earth would you want to draw in that hot air for cooling. Even if it were cooler then you would only be cooling the loft not the house. If the house provides this cooler air then this will have to be replaced. By what? What cool air source do you have? It is this source that will effectively cool the house.

However in Thailand it is more common to open up loft vents which allow the air to flow through the loft. A light breeze will give a good air flow through the loft. This stops the loft getting hotter than the ambient air outside and thus stops the house heating up more than the ambient temperature but has limited cooling effect. Insulating the roof will also help stop the loft getting too hot. However this is another subject and there are many different methods to do this but something you should consider.

You say you have large vents in your gables, this should be sufficient to take care of any air expansion caused by heating up the air in the loft. So you dont need to be concerned about that.

[chiang mai]

In post 35 you said you had exited the discussion so please remain exited and be done!

The subject is ended and I have decided a way forward, thanks to all for their input.

MODS: feel free to close the thread, before it becomes abusive.

[Naam]

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.

trying to save peanuts of electricity cost by not installing a powered attic exhaust fan is "penny wise and pound foolish"

[Sheryl]

thread closed per OP request.