Lets Talk Attic Venting

[Jon Bon]

So currently I have a large duplex. The third floor is finished on both sides, has its own supplemental heat/ac, and completely unvented. The roof is slate. House is located in zone 5. I assume the third floor is relatively well insulated (r19?). 

There are no ridge/eve/soffit vents.

Now I have read a fair amount about venting and I realize generally you need cooler air to enter the vents lower down on the roof to exit near the top of the roof. I have owned the house for 5 winters now and have had zero issues with ice damns.

Are older homes leaky enough to not need to be vented like a modern one would be? Does the slate roof change some of these assumptions? Does anyone have any advice or expertise on this?

[lthenderson]

Does the slate roof change some of these assumptions?

I'm not an expert by any means but I've read that shingle roofs are considered air tight because the asphalt essentially seals the interior attic air (with water vapor) from escaping and thus the need to worry about condensation and attic ventilation. With slate roofs, they are not air tight and thus they allow attic air to escape naturally and ventilation is not necessary.

[Jon Bon]

That is a pretty good answer.....

[J Boogie]

Well, I'd add a few things.

First of all, venting your attic wouldn't help. You have a conditioned attic and if you wanted to use attic ventilation to prevent ice dams you'd be talking about freezing temps in your attic.

You're probably thinking of venting the roof deck, which from the sounds of it is already occurring - but not because the slate is more vapor/air permeable than asphalt. Yes, that probably allows the slate to cool more easily than asphalt, but it doesn't matter because it's releasing its heat to the snow above just the same. In fact, you DON'T want warm air to be able to reach your roofing material. You want the roof deck to be cooled by outdoor air rather than warmed by indoor air.

With enough insulation, you don't need a vented roof deck as there won't be enough heat escaping to melt any snow. This is a TON of insulation though, and if you use rigid foam you have to use 2 layers and stagger the seams. This is known as a hot roof and you can't be careful enough to ensure you're doing it right.

It sounds like your insulation/roof ventilation combo is working for you - that's great! Wish I could say the same about my current roof assembly.

[Jon Bon]

Thanks for the response Boogie.

Yes venting the rafter bays not really the attic itself. I do however have knee walls. so that part is unconditioned attic. Same question if I had asphalt shingles (looking at a different house). It would most likely need soffit and ridge vents?

Something low and something high so air could get up and out the rafter bays? Currently the house has no vents so I was thinking a vent in the side of the house low near the knee wall, and either a ridge vent

I was going to use baffles and r19 batts. I attached a high tech sketch. A "hot roof" as you said would require a ton of expensive foam? What R-value is required for a hot roof? Basically googling says that r19 is not nearly enough. So 4+ inches of foam would be pretty expensive I feel.

[bacchi]

I was going to use baffles and r19 batts. I attached a high tech sketch. A "hot roof" as you said would require a ton of expensive foam? What R-value is required for a hot roof? Basically googling says that r19 is not nearly enough. So 4+ inches of foam would be pretty expensive I feel.

Look on craigslist. I bought some from a commercial builder who had some left over from a job.

BSC has some good articles on rigid foam roofs. Picture 4 shows what happens when the foam isn't staggered. Picture 4 is just with the spray foam under the deck.

https://www.buildingscience.com/documents/insights/bsi-063-over-roofing

[J Boogie]

Thanks for the response Boogie.

Yes venting the rafter bays not really the attic itself. I do however have knee walls. so that part is unconditioned attic. Same question if I had asphalt shingles (looking at a different house). It would most likely need soffit and ridge vents?

Something low and something high so air could get up and out the rafter bays? Currently the house has no vents so I was thinking a vent in the side of the house low near the knee wall, and either a ridge vent

I was going to use baffles and r19 batts. I attached a high tech sketch. A "hot roof" as you said would require a ton of expensive foam? What R-value is required for a hot roof? Basically googling says that r19 is not nearly enough. So 4+ inches of foam would be pretty expensive I feel.

Hmm... are you replacing your awesome slate roof anytime soon?

If not, I'm not sure how you're going to ensure any soffit venting you put in will connect to the ridge vent. Sliding baffles through the existing batting? It's probably not batting if it's giving you such good ice dam prevention performance. And since it's not batting, you won't be able to slide baffles through it is my guess.

Not sure if there's much upside to soffit venting if the air doesn't have anywhere to exit. I guess you could put a few slantback vents in, but if it ain't broke no need to fix it, especially with roof penetrations. I hate seeing a roof full of ugly vents. And roof penetrations always increase your likelihood of having a roof leak.

Kind of wondering how you've avoided roof dams for these past years. Do you have no eaves? Do you have gutters? Maybe the thermal mass of the slate HAS helped your roof resist warming up enough to melt snow. I haven't considered this, but it does seem possible in my mind when I imagine a 5" thick slab of cold stone as a roof. Perhaps if it's heavy enough, it overpowers/has enough capacity to absorb the minimal heat that an insulated building puts out? Kind of like how if you leave a window open in below zero temps the radiator in that room will freeze. Ask me how I know :)

[Jon Bon]

Hmm... are you replacing your awesome slate roof anytime soon?

If not, I'm not sure how you're going to ensure any soffit venting you put in will connect to the ridge vent. Sliding baffles through the existing batting? It's probably not batting if it's giving you such good ice dam prevention performance. And since it's not batting, you won't be able to slide baffles through it is my guess.

Not sure if there's much upside to soffit venting if the air doesn't have anywhere to exit. I guess you could put a few slantback vents in, but if it ain't broke no need to fix it, especially with roof penetrations. I hate seeing a roof full of ugly vents. And roof penetrations always increase your likelihood of having a roof leak.

Kind of wondering how you've avoided roof dams for these past years. Do you have no eaves? Do you have gutters? Maybe the thermal mass of the slate HAS helped your roof resist warming up enough to melt snow. I haven't considered this, but it does seem possible in my mind when I imagine a 5" thick slab of cold stone as a roof. Perhaps if it's heavy enough, it overpowers/has enough capacity to absorb the minimal heat that an insulated building puts out? Kind of like how if you leave a window open in below zero temps the radiator in that room will freeze. Ask me how I know :)

This is the design for a second house I am looking at. Unfinished unvented attic. I am thinking about finishing it off, and what venting I need to add. The first house with the slate roof yes its not broken so lets not fix it.....

This house has traditional shingles so I could put in ridge vents, but I was just going to put in a sidewall vent at the peak. I figured that would be a good compromise. I cant really do soffit vents on this house and a ridge vent is a lot more work. So I was just thinking low/high wall vents.

Would that work or am I asking for trouble?

[Papa bear]

Hmm... are you replacing your awesome slate roof anytime soon?

If not, I'm not sure how you're going to ensure any soffit venting you put in will connect to the ridge vent. Sliding baffles through the existing batting? It's probably not batting if it's giving you such good ice dam prevention performance. And since it's not batting, you won't be able to slide baffles through it is my guess.

Not sure if there's much upside to soffit venting if the air doesn't have anywhere to exit. I guess you could put a few slantback vents in, but if it ain't broke no need to fix it, especially with roof penetrations. I hate seeing a roof full of ugly vents. And roof penetrations always increase your likelihood of having a roof leak.

Kind of wondering how you've avoided roof dams for these past years. Do you have no eaves? Do you have gutters? Maybe the thermal mass of the slate HAS helped your roof resist warming up enough to melt snow. I haven't considered this, but it does seem possible in my mind when I imagine a 5" thick slab of cold stone as a roof. Perhaps if it's heavy enough, it overpowers/has enough capacity to absorb the minimal heat that an insulated building puts out? Kind of like how if you leave a window open in below zero temps the radiator in that room will freeze. Ask me how I know :)

This is the design for a second house I am looking at. Unfinished unvented attic. I am thinking about finishing it off, and what venting I need to add. The first house with the slate roof yes its not broken so lets not fix it.....

This house has traditional shingles so I could put in ridge vents, but I was just going to put in a sidewall vent at the peak. I figured that would be a good compromise. I cant really do soffit vents on this house and a ridge vent is a lot more work. So I was just thinking low/high wall vents.

Would that work or am I asking for trouble?

I just did an attic like your second one. We have soffit vents and a ridge vent, with baffles covering the areas where we had r19 batts and drywall. So cold air can come all the way through. 


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[J Boogie]

Hmm... are you replacing your awesome slate roof anytime soon?

If not, I'm not sure how you're going to ensure any soffit venting you put in will connect to the ridge vent. Sliding baffles through the existing batting? It's probably not batting if it's giving you such good ice dam prevention performance. And since it's not batting, you won't be able to slide baffles through it is my guess.

Not sure if there's much upside to soffit venting if the air doesn't have anywhere to exit. I guess you could put a few slantback vents in, but if it ain't broke no need to fix it, especially with roof penetrations. I hate seeing a roof full of ugly vents. And roof penetrations always increase your likelihood of having a roof leak.

Kind of wondering how you've avoided roof dams for these past years. Do you have no eaves? Do you have gutters? Maybe the thermal mass of the slate HAS helped your roof resist warming up enough to melt snow. I haven't considered this, but it does seem possible in my mind when I imagine a 5" thick slab of cold stone as a roof. Perhaps if it's heavy enough, it overpowers/has enough capacity to absorb the minimal heat that an insulated building puts out? Kind of like how if you leave a window open in below zero temps the radiator in that room will freeze. Ask me how I know :)

This is the design for a second house I am looking at. Unfinished unvented attic. I am thinking about finishing it off, and what venting I need to add. The first house with the slate roof yes its not broken so lets not fix it.....

This house has traditional shingles so I could put in ridge vents, but I was just going to put in a sidewall vent at the peak. I figured that would be a good compromise. I cant really do soffit vents on this house and a ridge vent is a lot more work. So I was just thinking low/high wall vents.

Would that work or am I asking for trouble?

I haven't learned a whole lot about this particular situation, but if you don't have soffit venting potential and you're going to insulate & air seal with a conditioned attic space I don't even think I'd bother with venting the knee wall space.

I think if you put in a power vent, it would depressurize the knee wall space and would suck air from your conditioned space, which both wastes energy and risks heating the unconditioned space.

If you don't put in a power vent, and just put in a sidewall vent at the peak, I don't think there will be enough air movement to move the needle on the temperature in the knee wall section. Of course, this would not be the case if you don't insulate and air seal very well especially if you have a balloon style frame sending warm air up the stud wall cavities. Then it would make sense to offer this rising warm air a way out.

[Car Jack]

This is something that you want someone who really, really knows what they're doing to look at.

My house:  Knee wall with storage (attic) behind it.  Finished space in the rest of upstairs with cathedral ceiling and ridged foam insulation leaving 1" of space.  Here's where the problem started....and addition on the back cuts into the wall.  I can go into the attic space and see the 1" empty space.  The last time we had our roof done, the Einstein who did it thought "Oh, you need a ridge vent" and installed it.  He did not seal the attic from the 1 inch space.  So what did we get?  Well, when we get a lot of snow, the heat from our upstairs heats the attic which goes into this 1 inch space and out the ridge vent, melting the snow which then drips back INTO the ridge vent and through the insulation at the edges and through a canister light, to drip the now rusty water onto the sheet rock.

So don't let anyone put a ridge vent in without convincing you that they have properly engineered the roofing system.  Our roof is being redone in 2 weeks and the ridge vent is going away.