My home, built in 1990, has two vents (plus a hot water tank exhaust pipe vent and various leaks to the outside) located in the roof over an attic space above the connected-to-house garage. I know these vents (I think) should be kept open during the summer months but how about temporarily closing them or at least reducing their size during the cold fairly dry winter months.   Winter-time temperature in this space can be about 30 degrees F lower than the 65 degree inside temperature of the home when the outside temperature is around 20 degrees.  I feel I am wasting a meaningful amount of $$ to heat this attic space and am obtaining little benefit. Floor of attic over the garage is insulated to R-35 at present.  I hate to spend more money on insulation to boost it to R-50 if my suggested fix would work and be less expensive.  Are their devices to accomplish this if advisable? 

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So you have an unconditioned attic with a gas water heater which sounds like it is naturally drafting & you want to close up the air vents? I am not sure what you mean by spending money on heating that place unless you are talking about heat loss into it...

Just from the water heaters standpoint, that would be a very bad idea --- with temperatures getting down that low I would recommend moving it elsewhere where the chance of water lines freezing is less

As for the venting, the only time you can eliminate the venting is with a hot roof structure - unlike a crawl space where you are supposed to close them off in the winter, the attic needs to be able to vent that warm moist air out so you don't have issues with ice-dams, condensation, etc...

The fix is simple - air seal all the penetrations, add baffles as needed & make sure you have the code minimum of insulation in there

Sean, Thank you very much for your response to my questions.   I was working on a response to yours and hit a wrong button-away it went, I think.    Let me know if you received anything from  me and if not I will draft a second response. 
Thank you very much for your time and consideration.  John

Nada

Sean, I certainly could have better described my situation for you and I apologize for that. May I try again?  

My gas hot water heater sits in my attached and un-conditioned garage right next to the walk-through door that opens into a utility room and has an exhaust pipe running from it up through the garage ceiling into un-conditioned attic space which is large in square feet than the garage sq. footage.  The exhaust pipe then runs straight up and out through the roof.  In the winter, the attic temperature runs about 50 +/- degrees when outside temps are around 25 to 40 degrees so I figure I am heating this space to some degree by heat rising . We keep our home at 65 to 66 Degrees except at night when the thermostat controlled temp is reduced to 60 degrees.  There is some heat loss from the house proper into this attic and I am seeking away to reduce this. Some cold air is coming in to the attic through the two vents plus elsewhere in the attic which lowers the attic temperature. I was hoping that there is someway to safely reduce/minimize this inward flow temporarily (during the winter months only) so that I get more out of the heat that moves into the attic from the space beneath it.  Some of the attic has blown-in insulation on the surface of the garage ceiling.  There is also blown-in R35 +/-  insulation on all portions of the attic which have heated living space below them.

I am not yet concerned about the hot water heater pipes freezing as I have a re-circulating pump which provides almost instant hot water throughout my home.  This is on a timer and runs about 12 hours a day.Should I get concerned?  This a new home to us, 20 years old, and the only other owner turned the heat in the house down to 56 degrees in the winter and then acted like a snowbird for 4 months or so for most of their time of owning this house. 

I was not sure what you meant when you stated "the only time you can eliminate venting is with a hot roof structure". I am guessing this means the roof is actively heated from below, no?  How can I measure how moist the air is in the attic?   (I have a thermometer that measures relative humidity).  It seems to me that I should be able to raise the temp in the attic by 5 to 12 degrees in the cold winter months by closing off "holes", reduce my heating costs as a result of having a warmer but uninsulated space above the garage without running afoul of ice dams and condensation.  

Rather wordy explanation and your time and consideration is appreciated.  Thank you.

John

Yes indeed as it appeared your WH was in that attic before.

So a quick search shows you generally have 32" of snow a year & based on your description I doubt any of it stays on your roof long. I know this is going to throw a few of the pros here for a loop, but yes in essence you could block off your vents and probably gain a little benefit from doing so.

With that said that is not something I would recommend as you are basically wasting money through the roof. Looking at your profile I am led to believe you are a homeowner trying to learn more &/or you sell environmental control devices and would like to learn more about building science.

So with that in mind, let me start you off with 2 links on my site which should help you out some & also point you to Building Science Corp which has a ton of more in-depth technical material.

Building Science 4 Dummies series: http://blog.sls-construction.com/tag/bs4d (From what is an ACH, ice dams, heat to pascals with numerous tips & tricks thrown in) --- the most recent article has a lot of info on the questions asked

Air Sealing & Insulating which has a ton of articles on Attics & even hot roofs; http://blog.sls-construction.com/category/tips/air-sealing-insulation   

As for the recirculating pump running 12 hours a day - that is not good as you are just wasting electricity & the water is heating the surrounding areas almost non-stop. An on-demand system would be a lot better & as for the pipes freezing, well that depends on where they are located.

I would definitely recommend you browse through those articles & linked pieces (especially this one - http://blog.sls-construction.com/2013/ice-dams-icicles-attic-conden...) which should answer a lot of your questions & we can go from there when you have a better understanding

Thank you, Sean.  I am a newbie to the colds of Oregon and ownership of a home here in Bend. I will research the links you sent me and then get back to you when I have a better understanding.

Closing your vents, even temporarily, does not solve your problem of heat loss into this space. I would take a good look at the walls between the house and the garage. I would start by pulling back the existing insulation and airsealing the top plate of the inside walls. I would also recommend dense packing with cellulose the wall connecting to the garage and installing a good weatherstrip and sweep to the door, if any, from garage to living space. If there is air movement, you can guarantee there is carbon monoxide from the garage also moving. Proper airsealing can eliminate alot of the loss you're suffering at very little cost, just labor. Look for really dirty insulation, as this is an indication of air movement. R-35 is not bad, but remember this, there is no home improvement that pays back as quickly as adding insulation.

Thank you very much for your reply to my conundrum, Robert!

All off your points will be looked into -I especially liked your last sentence and believe it with the qualification that my home is reasonably plugged up in terms of holes to the outside (and it is). In terms of R ratings, where does one draw the line? R50, R80, ???

When I follow your advice re air sealing the top plate which I will do, you are suggesting this should be accomplished with caulk and/or foam, correct?

I have replaced the door sweep and weatherstrip  for the door leading into the house from the garage.  As you can ascertain from my questions, I am still conceptually unclear as to why (to go to extremes) if I close off the vents completely, the heat loss out these vents and the cold air coming in through these vents would both be reduced. Are you thinking that these two sources of heat and cold offset one another and thus the temperature in the attic would stay pretty much the same but the air quality would be poorer? It would seem that closing the vents in the roof would reduce the inflow of cold air and would also limit the loss of rising heat that has migrated through my ceilings into my attic space.  I am willing to read some books, go to web-sites, etc. to grasp a better understanding of the dynamics involved as what seems common sense to me is probably not!  Thank you ery much. John

John,

In reply to your questions.... Where does one draw the line on R value in an attic? If I had room for an R 287 in my attic, I would put it there. When airsealing the top plates of the walls I like to use an expanding foam. Great Stuff will work and is readily available anywhere. I also meant to ask if your garage ceiling is lower than your living space ceiling. There could be an open transition wall alowing heat to escape from the living space. Common sense solutions is what I try to help my customers with. When I go up into someones attic I want it to be cold, that means that the airsealing and insulation is doing what I intend it to do. Your house is an envelope. The smaller you make that envelope, the less you pay to heat/cool it. The insulation in that envelope must touch each other in order to make the seal, so to speak. Dirty insulation and cob webs are great indicators of moving air. Search these out and you'll find the sources of movement. Don't forget the basement too. Plumbing stacks,chimney chases,pipe and wire penetrations should all be airsealed. I won't tell you not to try what you want to do, it just won't stop the heat loss into the attic.

Robert, It is an education writing to and fro with you and others and I am very happy I found this avenue to pursue.

My garage ceiling is the same height as that of the mud/laundry room which runs due west from my attached garage and is about 14 feet by 7.5 feet wide.  Then the ceiling, still moving west,  opens into a kitchen nook area to the NW which has a cathedral ceiling (no attic). (I am in the process of caulking the 2.25" wide cedar strips over the kitchen nook area  that make up my cathedral ceiling).  A thermal camera indicates I still have some moderate air leakage there.  To the SW, the kitchen proper has a flat ceiling at the same height as the garage ceiling and it seems to be fairly well insulated  but has settled over the years.  You have indicated that more insulation would be worthwhile so I shall pursue that .  In moving south from the flat ceiling in the kitchen, cathedral ceilings again take over for about 26 feet.  At that point, we are now into the bedroom wing which again is at the same level as the garage and contains two bedroom and an office/spare bedroom.. The attic space here is accessed by an opening in a bedroom's closet. The "two" attics are not contiguous to one another and the insulation is this section is about  R-28.  I suspect I should attack the weakest link first in terms of adding insulation. Today I went into both attics, separated by a cathedral ceiling in the living room, and checked for dirty insulation and cob webs. I really have very little dirty insulation and no cob-webs in the attic.  I do have some cobwebs in the garage proper but they are not significant in my opinion.  In my  basement, all ducts have been sealed and wiring penetrations, plumbing pipes were air-sealed.   My wood burning fireplace in the living room (with gas assist)  was heavily foamed and the Energy Trust of Oregon-related folks spent a lot of time, foam, and my money in sealing my house pretty well but I wish to make more improvements, keeping in mind the dollar value of my commitments.

I still have trouble comprehending why (forgetting about moisture and mold) reducing cold air inflow into my attic will not reduce my heat loss into my attic over the garage and mudroom area.  What part of this puzzle am I missing?

The only part of the puzzle that you are missing is that the attic space above the insulation is outside of the building envelope. That being said, adding more insulation would be your best bet. Transition walls between flats and cathedral ceiling also benefit from sealing. I have installed foam rigid board over the insulation in these walls with great success.

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