Do we need to add attic ventilation?

10 years ago

Hi, I haven't posted to this forum in a long time, but I could really use some helpful insights.

We live in a 1905 hip roof foursquare with a two story addition on the back. The addition has soffit vents and a ridge vent, but the attic in the original part of the house does not have any venting. The addition is not open to the attic of the main original part of the house, so it does not provide any ventilation (the addition does not have an attic).

We have never had a problem with moisture in the attic, but we have made lots of improvements to the envelope of our house over the years, making it much tighter than it used to be. Most recently, this spring we had closed cell insulation sprayed on the basement walls and open cell insulation injected into the balloon framed walls on the first and second floors.

We are in the process of getting estimates for a new roof and I'm wondering if we need to add ventilation to our attic as part of this. As I said, we have never had moisture problems before, but we have also not gone through a winter since insulating our walls.

We could add soffit vents and a ridge vent, but I'm not sure how effective a ridge vent would be in a hip roofed house since our ridge is only about 8 feet long. Would an attic fan make much of a difference? Do we need to do anything at all?

I've spent several hours reading on the internet, but it has left me more confused. In my jaded opinion, lots of contractors try to sell you on what is easier or more convenient for them, not what is best for you.

Any help?

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Comments (7)

worthy
10 years ago
Venting keeps the underside of the roof cool in winter so as to control ice dams from melting snow and expel moisture coming from the conditioned space. In summer, venting keeps the attic a bit cooler thus reducing the cooling load.

The winter issues addressed by venting a roof are much more important than the summer issues--unless you have HVAC in the attic.

The improvements you've made will mean even less moisture and less heat goes from the conditioned space to the attic.

If you have no evidence of the winter problems--melting ice dams and high moisture in the attic, evidence by mould growth--I wouldn't bother with venting at this point--108 years old and counting.

Here is a link that might be useful: Dr. Joe: Understanding Attic Ventilation
mxyplx
10 years ago
Don't know where you live. I live where 110ÃÂºF is common so it gets mighty hot in the attic. Like 146ÃÂºF. The only way to tell if you need ventilation is to measure the temperature. For a few bucks you can get a digital thermometer that you can hang in the attic with a remote read out. Do that. Then you will know. Otherwise you are just guessing.

Believe me if it's 146ÃÂºF your nose will burn up inside. Get the remote readout. I mounted the sending unit under a hanging straw hat coated with foil to eliminate radiation from the roof. I hung it up after dark when it was cool.

I installed a fan that comes on when the attic temp reaches 115ÃÂºF. I plugged all the soffets and put a big opening in the far end of the attic with 1/2 inch screen so the air can flow thru virtually unrestricted. The opening must be sized as per the specs for the fan to ensure proper mass flow.

With the fan running the temp in the attic is indeed 115ÃÂºF as per the thermometer.
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OK, so you have 3 basic problems - drafts, venting and insulation. Drafts are simple to fix but sometime time consuming. Obvious areas are windows and doors that aren't sealed properly. All the insulation in the world won't help if the wind is whipping through the living space. For the attic, you should add soffit vents before insulation. That way, you can put little baffles in front of the vents and you'll avoid spaying insulation over them. You should also have roof venting. In summer, the air will come in through the soffits and out through the roof vents. For insulation, I would spend a little prep time before you start spraying cellulose. A few cans of expansion foam will do wonders to minimize air flow from the living space to the attic. You will probably have to roll back the existing insulation to see the problem areas, but you can roll it right back when you are done. Then, you can spray in cellulose over the top.
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Forced attic ventilation sounds like a great idea. That's why you see all those whirlygigs atop homes, even in our cold climate. But in practice it usually uses more energy than it conserves. As linked above, the Florida Solar Energy Center observes,"Detailed simulations suggest that the heat transfer in an attic to a residential building interior in mid-summer is dominated by radiative gains from the hot roof decking directly to the insulation surface ... This mode of heat transfer is more effectively limited by 1) increased attic insulation, 2) a truss-mounted radiant barrier or 3) a white reflective roof surface that limits solar gain to the attic structure." Photovoltaic attic ventilation is more energy efficient than equipment connected to the electrical grid, but it is more expensive to acquire. As noted by handymac, providing more intake at the eaves than exhaust at the top works better than a "balanced" system. R8 correctly points out the peculiar division of labour between the installation of roof and soffit vents. This post was edited by worthy on Sat, May 31, 14 at 8:38
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I must have been misled by the "whole house fan" reference. Yes a hood is superior to a down-draft system because the cooking plume rises to the hood where it is (when correctly designed) captured and contained and transported to the outside. One can have a blower in the hood pushing to the outside, or a roof blower pulling from the inside (either way the pressure loss is the same), or an in-line blower. A roof blower provides room in the attic for a silencer (muffler) which will significantly remove blower blade turbulence noise. Turbulence noise from the baffles will still be present, but this is usually modest. Ideally, the hood should overlap the hot pan surfaces by enough to capture the rising and expanding effluent. A minimum of three inches all around is typically recommended, but this should increase if the hood is materially farther above the countertop than three feet. Flow rate (CFM) should provide enough velocity at the baffles that all of the uprising cooking plume, upon hitting the baffles, is pulled through. This should be about 90 cfm/square foot of hood interior aperture if you do hot cooking such as grilling in a pan or on a grill surface, or wokking with typical rising hot peanut oil vapor. The rated cfm of the blower is for conditions of hanging in space, so the pressure loss of the entire path (including getting air back into the house -- make-up air) has to be accounted for and the reduced blower output compared to the necessary actual flow rate. I would take the area of the hood aperture, multiply by 90, mulitply by a 1.5 fudge factor for pressure loss, and find a blower in that ballpark. Exactitude is not needed here because there are too many unknowns to provide a refined analysis. kas
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worthy
10 years ago
As a rule, the costs of running attic ventilator fans exceeds the cooling costs savings. Photovoltaic ventilators are not parasitic. But a test in central Florida in a well-insulated attic showed only a 6% cooling cost savings over a summer season.

More effective measures are more insulation, radiant barriers and a white roof.

(It would be helpful if posters would narrow down their climate zone to something more specific than the "United States.")
arlosmom
Original Author
10 years ago
Sorry, I should have specified my location -- we are just outside of DC, so we typically have hot, humid summers and fairly cold winters. We don't get the extremes of some areas of the country though.

I'm encouraged by the comments from both of you. Worthy, thanks for the article. I understood most of it. I think we have done a pretty good job of separating the conditioned living spaces from the unconditioned attic so the underside of the roof stays cool in winter. When it snows, the snow stays on our roof much longer than it does on neighbors' roofs (I always thought this was a good sign that our insulation was working) and we've never experienced any ice dams. We never see signs of moisture or mold in the attic or on things stored there. It does get very hot in the summer. As you climb the stairs from the second floor to the attic, you can feel the temps rise.

Our basement used to be very damp, but since we had closed cell insulation sprayed on the rubble stone walls, it feels and smells much drier...if we're keeping the moisture out of the house, the attic should be drier as well, yes?

Worthy, I'm thinking and hoping that you are right and that if it has been fine for 108 years and we've improved the envelope, we should be fine. Thanks so much for weighing in.
worthy
10 years ago
if we're keeping the moisture out of the house, the attic should be drier as well, yes?

Correct. You have an unvented attic and the benefits that flow from that.
*********
The ccSPF you used on the rubble basement was the perfect measure, too, according to Building Science Corp. See the linked document for all the details on how they handled a similar project, which is/was Dr. Joe's own Massachusetts home. The picture below is from that paper.

Here is a link that might be useful: Rubble Foundations

This post was edited by worthy on Thu, Aug 15, 13 at 21:06
arlosmom
Original Author
10 years ago
Thanks worthy. It's great to get confirmation that we are moving in the right direction. I try to read a lot about each step along the way, but there is so much conflicting info out there.
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Windows on Washington Ltd
10 years ago
If you can open the soffit and ridge, do so.

Passive is much better than mechanical although the solar powered fan option that worthy mentioned works well.

Be sure the fan is equip with both a thermostat and a humidstat.

The snow remaining on your roof is sign you are headed in the right direction. You still want to vent the roof/attic assembly though.

You are going to need to open the soffits and either vent via roof top vents/ridge or via powered fan.

Joe's paper on attic ventilation lays out the options pretty good.

Was the attic air sealed as well as insulated? Sound like you did something with the balloon framing so I would guess yes.

Do we need to add attic ventilation?

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mxyplx

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Do we need to add attic ventilation?

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