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Question DetailsAsked on 8/2/2016

the best way to vent a detached garage with a climate controlled storage space

Garage less than yr old with damage to roof - told it's from heat build-up. We have a space allotted for storage while we will be doing further remodeling. Working on insulation and dry wall for storage space currently. Considering the best venting for yr around control. Not a living space, at most storage and work area for art and crafting. Considering a window unit for hot months. Found out garage did not have Soffit vents. Do we need a wall fan vent? A vent panel in dividing wall? Input is welcome.

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Oops - in proofreading I noticed your question said DETACHED garage - so some of my comments regarding connections to the house probably do not apply, but I left them in so this response will be applicable for that sort of case for future readers too. Whether a craft room over the garage would be considered a "living space" would depend on local building department interpretation - yes by some, others say "living space" only if attic space is used for or connected to bedrooms because they are most concerned about fire spread while residents are asleep.

Sounds like a divided attic - part you will be using for livable space (though not connected to main house conditioned space, as I read it) - other part unused attic. Specifics for your case would have to be looked at on-site - but generally:

1) unconditioned attic space should be well ventilated with outside air - preferably with eave (and soffit if you have them) vents into the attic, and ridge vents (assuming ridged/peaked roof) to allow attic air to flow out the ridge vent as the hotter air under the roof rises, and that air is replaced by inflow from the eave vents. Generally - should have open vent area in each of those two locations roughly evenly distributed along the length of the attic and totalling intake and outlet areas each at least 1/300 the square footage of the roof - 1/100 is far more effective but that is more than normal sized eave and ridge vents can provide, but around 1/200 is better if you can achieve it. Sounds like screened vents would have to be installed into the soffits, and if no venting exists at the eaves (say has solid blocking between trusses or rafters with no air gap at the top under the roof sheathing as there should be) then the blocking would have to be drilled to provide air vents - usually several about 2" holes along the midheight line of the blocking per rafter bay to provide ventilation without destroying the lateral support of the blocking to the rafters/trusses.

2) Generally, used or "living space" in an attic should be "conditioned space" like the rest of the house - either connected to the house HVAC system, or with an independent system. If the entire attic is open and used for that, then unless in a mild climate area where open airflow from vents is not a problem, you have to totally seal off the attic, insulate as appropriate, and provide year-around heat/air conditioning to control the temperature and humidity in that space. If only using it seasonally or sporadically, then you have to consider how to prevent moisture issues when not "conditioned" - may need ventilation hoods or such or prevent air stagnation and moisture condensation, particuarly if adjacent outside air is very humid in the summer or cold in the winter - otherwise you risk turning it into a mold farm.

3) If the usable space is being built as essentially a "room within the attic" not going allthe way up to the roof, then the "room" needs to be conditioned space in most cases, otherwise you can get humidity buildup between the room and the surrounding attic space - with the surrounding attic area being vented as in 1) above, and the room insulated as needed. Sounds like this is what you are doing. You have to be careful in that case to provide the eave to ridge ventilation around and over the "room", leaving enough ventilation means so the "room" insulation does not block the airflow to the ridge vents.

4) If using open attic as storage / work space, then you have both temperature and humidity control to consider. For storage-only temperature might not matter by itself depending on what you are planning on storing up there - but seasonal humidity buildup and possible winter condensation would definitely be a concern without positive control. In nicer climes, just a large power gable fan in the endwall and open eaves or an open window near the other end would work, though airflow and cooling would not be even. Ditto with window unit - though you have to be careful, if there are fuel-fired appliances in the garage, not to end up pressurizing/depressurizing the garage because that could cause firebox flame "rollout" or poor combustion.

5) Putting a vent panel through the dividing wall between the "room" and the attic is NOT what you want in most cases - because in that case the "room" is essentailly back to being part of the attic. If you put a fan unit exhausting the room and a sealable (say window) into the attic part as ann air intake that would work but the problem is in winter that would be cold outside equivaqlent air, and in the summer would be overheated hot attic air - exactly what you don't want, so you would want your air intake to probably be either from the hosue (basically "stealing" conditioned house air which would then be wasted outside, so not energy efficient at all), or maybe from the garage which is likely tempered relative to outside air temps - again, considering not backdrafting or force-drafting any appliances in there. Also - pulling ventilation air from the garage would be illegal in most areas because of the gasoline fume/fire travel risk - garages are supposed to be isolated from the living space venitlation.

6) Consider (assuming this would be left as-is when you sell the house) not to violate the fire protection requirements between the attic and the garage - because once you put living space (as opposed to dead storage) up there or connect it to the house by doorway or ventilation system, it has to be vully isolated from the garage by fire-rated garage ceiling and walls and such. Which might mean you need to "finish" the garage if it currently has bare stud walls. - those are allowed if there is only attic over the garage rather than nliving space, except on the partywall with the house which has to be fire-rated.

7) Also - unless garage is totally fire-insulated from the attic per above, any ventilation or access door to the house has to be fire-rated - fire rated door and fire-dampers in HVAC ducting, etc.

8) A/C unit, unless you are talking just a unit to take the edge off the heat or a portable unit, could be iffy for two reasons if the temperature in the attic room is dropped significantly. First because when you run it, if the attic "room" is not totally isolated and insulated from the attic, you could easily get condensation in the room walls on the outside as humid attic air contacts the much cooler room wall and ceiling. Secondly, if insulated to avoid that problem (basically insulated like an exterior wall would be), say you have the A/C on and cool the room down. Then, once it is cool, you turn the A/C off - but have to have ventilation in the area to avoid musty smell and mildew - so that humid summer air comes into the cold room, and might condense at the walls. Not a big problem likely if temperature is not dropped more than maybe 10 degrees - but take a 90-120 degree room down to 70 say and it could be a significant issue.

9) If building a "room within an attic", be careful about potentially trapping house moisture coming up fro the house into the attic - there should be airspace under the "room" to allow moist air to escape. Also, any penetrations at that area should be well-sealed, any ceiling fixtures should be sealed at the box, and any can-type in-ceiling fixtures should be the tyhpe that is rated for insulation encapsulation - not ones requiring open air around them. Also consider any utility runs that will not be under the "room" floor - might consider relocating any wiring or ducts or such, or at least make sure they can be removed and rerun if needed for maintenance once the room floor is down.

You can find some previous questions with answers on attic ventilation and insulation issues in the Home > Roofing and Home > Insulation link in Browse Projects area at lower left.

One other thought - what is your house warranty period, if he designed it ? If 1 year, you may have time still to make a claim against the builder to have the attic ventilation issue which is being blamed for roof failure redone correctly (with ventilation), and the roof replaced - at HIS cost. If under warranty, be sure to get a written claim in within the warranty period - and if coming up soon, might file a contingent claim with the bonding company too in case the contractor does not come through with the repairs/replacement. You might need an attoreny to help with this (and attorney letters carry more weight too), and would probably need an inspection report from an Architect stipulating what code provisions were violated leading to the roof / attic issues, and some sample attic temperature readings recorded by him while still in hot part of summer.

Answered 3 years ago by LCD

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