HVAC and venting attic design help in house

[rluckie]

I am in the process of building my house (barndominum). The metal building itself has been spray foamed with closed cell and it only vented with a ridge vent. I am framing the inside currently but am trying to get an idea of how I will be heating and cooling as well as properly venting the attic space. I live in north Texas so its gets really hot and really cold every once in a while. The family room ceiling goes all the way to the bottom of the C purlins, everywhere else in the house has attic space above. My main concern right now is that the only venting I have is the ridge vent but the C purlins from the house go to the shop so I technically have air that can come in from the shop and go along the purlins. I have no soffit vents. Should I install a gable vent?
As far as the HVAC, I plan to install the unit above the 2nd bathroom. Return air will come from the hallway. I will have supply for each room near outside walls with high/low vents to all for return to hallway. I am looking for suggestions or if there are any issues with what I have planned. Thanks.

 

[yeldogt]

What kind of roof ? I never use vents .. especially with spray foam. make the whole place conditioned .. end of all issues.

Look up Matt Risinger (sp) homes in Austin -- he has some good videos. I don't love everything he does .. but he does things better than most.

What you are asking .. is a bit confusing as typically foamed buildings are not vented.

 

[snorky18]

You can design and build a house that performs well using attic venting.

You can design and build a house that performs well that is unvented and entirely sealed air tight with closed cell foam.

But when you don't have a clear direction (vented vs unvented), you end up with a hodge podge structure that doesn't perform like it could with a little more attention to detail.

Personally, if everything else was already sealed, I'd be inclined to seal up your ridge vent with a bit more closed cell foam, and bring the attic inside the conditioned boundary of the house.

If you were going vented, I'd tell you need to have air intake area equal to or greater than your air exhaust area through the ridge vent. And I'd encourage you, if you're building a structure from scratch and you plan to own it for a long time, to take a hard look at sealing everything up with spray foam.

 

[rluckie]

What kind of roof ? I never use vents .. especially with spray foam. make the whole place conditioned .. end of all issues.

Look up Matt Risinger (sp) homes in Austin -- he has some good videos. I don't love everything he does .. but he does things better than most.

What you are asking .. is a bit confusing as typically foamed buildings are not vented.

I will look up Matt's videos. I was confused as well because as because I know a typical constructed house will be vented but since this is a barndominum things might change. I added the ridge vents because I was getting conflicting answers with my builder of the metal building and the insulator.

 

[rluckie]

You can design and build a house that performs well using attic venting.

You can design and build a house that performs well that is unvented and entirely sealed air tight with closed cell foam.

But when you don't have a clear direction (vented vs unvented), you end up with a hodge podge structure that doesn't perform like it could with a little more attention to detail.

Personally, if everything else was already sealed, I'd be inclined to seal up your ridge vent with a bit more closed cell foam, and bring the attic inside the conditioned boundary of the house.

If you were going vented, I'd tell you need to have air intake area equal to or greater than your air exhaust area through the ridge vent. And I'd encourage you, if you're building a structure from scratch and you plan to own it for a long time, to take a hard look at sealing everything up with spray foam.

I don't want to have the hodge podge and that's why I am asking. I don't want to have moisture issues down the road because the attic is unvented. If I went vented I will have to install a gable end vent to allow air in I think. If I sealed it up I am unsure where I will pull in fresh air unless I install a fresh air intake. I appreciate the feedback, just trying to figure it all out while and can and not have to go back years down the road to fix issues or correct a problem that could be prevented now.

 

[7echo]

Search for vented vs unvented attics, roofs, etc.

Building Science will have some articles...



https://buildingscience.com/documents/published-articles/pa-crash-course-in-roof-venting/view

https://buildingscience.com/documents/digests/bsd-102-understanding-attic-ventilation

 

[tcrimsonk]

If, as you say, your building has been insulated with closed-cell spray foam, then nothing should be vented.

There are significant differences between open cell and closed cell. Closed cell acts as a vapor barrier, and does not allow mold growth or moisture penetration. It is expensive, and if your building already has it, you are very fortunate.

This is assuming that the spray foam was applied to the bottom of your roof, and not on top of the ceiling. Everything inside of the spray foam envelope should be conditioned space, and therefore not vented. You shouldn't have any need to worry about mold, etc. because of the closed cell foam being a vapor barrier.

BTW, I'd never heard of a "barndominum," but I just googled it, and it looks like a great way to live.

Disclaimer: I only know what I've read, so do your own research.

 

[Chris705]

If you consider sealing the ridge vent up with foam and you create a "sealed" shell you are right to wonder where will fresh air come from. As you read and learn more from the links folks are posting look for HRV (Heat Recovery Ventilator) and see if that may be something you want to consider as you now live inside this shell. When you turn on bathroom exhaust fans you will bring in fresh outside air and if you also have a range hood this exhaust air can be ducted to pass thru the heat recovery and warm or cool the fresh outside air.

 

[tcrimsonk]

If you consider sealing the ridge vent up with foam and you create a "sealed" shell you are right to wonder where will fresh air come from.

I was assuming that OP was wondering where the fresh air would come from to ventilate his attic. (Whereas if the attic is insulated with closed cell foam, he doesn't need to ventilate it.)

If the "fresh air" question is about having fresh air in the living space, then ditto what Chris said.