Ventilating a Cape attic

malex

HVAC includes ventilation but it seems very hard to find companies that specialize or at least know what they are doing, when it comes ventialtion, which is strange as ventialtion can make or break a heating/cooling systems. In some areas of europe houses are built with minimal heating/colling systems and take advantage of the the elements (wind, shade, sun) to be as self sufficient as possible.



I guess it is harder to sell a hole in roof compared to something the is measured in power or output units in the american market.



Anyway, I have a 1937 Cape and I am pretty sure that the gable vents are not providing enough ventilation. Today was a pretty hot one on long island and the temperature controlled attic fan came for hours (and very loudly - so I am guessing the fan is not the most efficient one either.



Does anyone know anyone that know ventilation in old houses and can recommend the proper solution. Last thing I want is someone to recommend installing bunch of whirleybirds on the slate roof...

STEVEusaPA

Here's my opinion

Temperature controlled attic fans are waste. They are installed to cover other deficiencies.

Roofs need to be vented at the eaves, so air can flow up into the eaves and out either a ridge vent (which you obviously don't have with a slate roof), or properly sized gable vents.

If your ceiling is properly insulated, the temperature of the attic is mostly inconsequential. Using a whole house fan to pull air into your house and hopefully push it up into the attic (with not enough ventilation), to cool your house, wastes a lot of energy and makes your house dustier. Also, if you have the fan running, and any fuel burning appliance running, (boiler, how water heater, fireplace, etc), you could easily pull carbon monoxide into the living spaces.

The real remedy is to update, upgrade, increase the attic insulation, make sure the gable vents are properly sized and free of obstructions, proper eave vents, and remove the attic fan.

icesailor

My opinion too:

Attic fans are not a waste if the fan is in the attic and you have supply and exhaust air in the attic. In my opinion.

Ridge vents "suck" in more ways than one. Because they can leak.

I was turning the water on in a house last week, a house that I have done for many years. I noticed what looked like wet spots on the garage floor with a white ring around each spot. The spots were dark like they were fresh and wet, but they were dry. Always on the lookout for the unusual and being forewarned is my best policy, I had to figure out the problem. It went from the front of the garage to the back. There were at least eight spots of varying sizes, all dark, and having white rings around them. The spots were between the cars, in a perfect line. The doors are on the gable ends. I looked up and directly above, the ridge plywood was split to let the air go through the ridge vents.

The ridge vent system was parallel to the prevailing storm winds, South-East. In a heavy wind with no rain, salt would gather on the roof, or the rain would have a lot of salt in it from blowing water and ocean storms. As the rain fell on the roof, it would be blown up to the top to be sucked into the ridge vent and into the garage.

The laws of unintended consequences. No good deed goes unpunished.

If you put a fan into one side near the top, and a vent on the other side, near the top, the attic will be cool. The old New Englanders had third world AC long before Dave Lennox made the South livable. It was called roof scuttles in attics. Open the front door, and open access to the attic roof, and Mother Nature did a fine job of third world cooling. Natural convection. Even window fans work better and cheaper than AC for un-used spaces. Or bedrooms.

Mark Eatherton

You guys are forgetting one critical component of heat gain...

That being RADIANT energy, which is not significantly affected by convection. You are looking at the convective and temperature differential potentials only, and not the major driving factor in heat gain, that being radiant energy.



The BEST way to address the radiant load potential is by either creating a reflective surface on the solar absorption side (hard to do with a dark slate roof) by using a light colored, reflective roofing material, or reflect it away from the internal absorptive surface (attic insulation) by using a reflective insulation. The problem with an internal reflective insulation is that it is significantly negatively affected by the presence of dust,which IS in EVERY attic.



If it were my attic, I would place polyisocyanurate (SP?) insulation in the roof (not to be confused with ceiling rafters) rafters, sealing it tightly with reflective tape. This will reflect roughly 90% of the radiant energy back to the source, thereby significantly reducing the solar gain potential, and reducing the amount of air movement required to maintain a reasonable attic condition.



As for attic fans, if it is necessary, I would recommend the use of a solar powered attic fan to draw the warm air out during the summer months, based on a temperature differential, and NOT a fixed temperature set point. Running a fan to exhaust 90 degree F attic air with incoming air that is equal to or close to that temperature is absolutely a waste of electricity. You can't cool a warm attic with warm air.



Avoidance is the best bet. If that is impossible, then reflectance just below the source is next best. Forced ventilation using a fixed set point control is the least effective means of reducing the cooling load, unless all other potentials can not be done. Differential temperature control trumps fixed set point controls every time. You have to think outside the attic ;-)



There are also a lot of DCECM fan motors out there that can be operated on a variable speed basis based on Delta T.



Cool eh... :-)



ME

Nick W

These are the experts

You might find answers here: www.buildingscience.com.

Nick W

These are the experts

You might find answers here: www.buildingscience.com.

icesailor

Hot attics:

My comments are limited to keeping an attic cooler. My long ancestry from the Northern British Isles.

If you just want to get the temperature down in an attic, fans are the way to go. It does not matter what the outdoor temperature is, the roof surface will be hotter, and radiating into the room through the roof surcace and sheathing. The underside of the sheathing will be considerably hotter than the outdoor temperature. If you insulated the rafter bays for no other reason than to stop the passage of hest

malex

Lot of good thoughts

but there seems to be some disagreements.

FYI, I had cellulose blown in last year so should be good in terms of insulation. There are no eaves or soffit vents. There is a smallish vent placed low on each gable.

Some specific questions I have.

1. how large should the gable vents be, should they be the same size and where should they be placed (same placement on both sides?)?

2. Insulation seem to be stuffed down bays to the knee wall. Should this be cleared or does it matter since there is no venting to the kneewall from the soffit?

3. should I go to the extreme and try to add venting somehow through the vertical facia? There is no overhang, so it would essentially directly behinf the gutters.

4. Does the addition of a fan change any of the parameters or is that just a nice to have add on.

Roland_18

Ventilation

Peruse some of the info here:

http://www.buildingscience.com/search?SearchableText=roof+ventilation

It seems you will have to do some modification to your roof/attic to get the result you want.

nowwhatnapster

I would go the soffit/ridge vent solution

I'm no expert, I just read some building code.



My understanding is that an attic fan alone will not solve your problem. You said you have blown in insulation, but no soffit vents? I would invest the money and have someone install soffit vents and make sure they put in chutes or baffles so air can get past all that insulation. Likewise you should install a ridgevent with an airflow rating that is appropriately sized for the soffit vents. Also, don't let someone install a cheap ridgevent that will get clogged over time and is not easily cleanable. There is a specific ratio between the soffit/ridge vent that should be adhered to for optimal performance. The soffit vents will also help with ice damming.



Once you do that I believe you need to close up the gable vents as they will not help the natural convection. Lastly, make sure you have adequate R-value in your attic. I didn't see you mention what it was. Hopefully the installer gave you a nice even coat and it doesn't look like the sand dunes of the sahara.