New construction home - HVAC in attic?!

The key here is the attic would be conditioned, even air leaks in the duct work would be enough to satisfy this but a return could be added if there are moisture concerns.

Some folks did a study on this scenario (the link it out there but I probably won't find it if I go looking now) because some roofing companies were worried about the shingles prematurely wearing out, but the mean difference is 2-3 degrees on the shingles. The attic itself would be part of the building envelope so it would only be a few degrees different from the living space rather than 140° in the heat of summer - even with venting in place.

 

What I'm saying is there are 2 different concepts I was bundling together in my post

A roof system is either vented or unvented . ie , either there is airflow from soffit to ridge above the insulation (vented) or no airflow in the assembly (unvented)

Your AC unit , if it was in the attic above the insulation would be in unconditioned space , ie essentially outside the home

You are referring to unconditioned space as 'vented' .

What I am saying is if you move your insulation into the rafter bays , your AC unit would move into the conditioned space of the house , ie inside the building envelope .

The insulation in your rafter assembly could either consist of a vented (batt or foam over baffles allowing airflow) or unvented (spray foam , no airflow)

 

Conditioned attic with mechanicals inside the building envelope is always better , just depends how far along you are in the build as to whether it's feasible to do at this point

 

Carpenter 31 years , had my own residential home building company since 1999

 

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

 

This basically sums up what I've been saying. Good video! Now I'm wondering about a dehumidifier, but that might be a bit overkill haha

 

Attic space inside the building envelope is just another room inside the house .

 

Matt Risinger is a great resource on Youtube for building science / new tech

 

I looked at that article earlier haha!

But yeah, the plan is to get them to put insulation in the rafter bays. I just assumed that this would seal off the soffits and ridge vents.

The home is just slab right now, so it's very early and I'm hoping to give some input into the process if I can

 

If you go spray foam route then yes , they would seal soffit air flow to the attic space

 

I've watched many of his videos today!

 

I think I'm mainly confused by the insulated rafter bays + baffles + ridge venting - wouldn't some hot air still make its way into the attic this way?

 

In that scenario , the air baffles staple to the underside of the roof plywood , running from soffit to ridge , in each rafter bay . The batt (or foam) insulation goes in the rafter bays over the baffles so the air flow is contained only above the insulation and from soffit to ridge vent , it does not flow into attic area

 

Ah, I see how that would address roof moisture concerns! So I guess my main lingering question is how do I tell if the HVAC system pulls in fresh air? I have read that air quality can be an issue with unvented attics. The house will have a gas stove, not sure if that makes a difference. The furnace is a high efficiency unit so atmospheric combustion isn't a concern like with older furnaces.

 

Thats outside of my area of expertise , we never install AC here but it should not be very difficult to look up the specs of the system being installed in your house . Generally here we install an HRV system for whole home ventilation and any conditioned attic or conditioned crawlspace would be vented by a branch line from the HRV system

 

I'll have to ask the builder or look at other labels. I took a pic of the only label I could really find but google wasn't much help. I'm sure there's another tag that I'm not seeing somewhere

 

Been in construction in florida for 20 years. Plenty of a/h's in attics down here. Roof ridge vents and soffit baffles are standard and required on all homes down here, due to the high temps. It's designed to move fresh air through the attic and out of the ridge vent. If the heat is your concern you can add a radient / heat barrier and insulate the underside of your roof trusses (under side of roof decking). Even if you place the a/h system in an enclosed area, most heat loss will be from the ductwork itself. That is why it is so heavily insulated.

 

I would ask builder for full spec sheet on the unit and/or install layout

 

I'll send an email in the morning and see what I can find out. Thanks for all your help!

 

I posted in the other thread. I will add more here.

In Hot and sunny So Cal. Typical house built in 1994. Stucco walls, tile roof. No soffit or roof peak vents but large vents on each side of the attic. The pic shows two of the 4 vents on each side. It's most common to have the HVAC in the garage but they do install them in attics as well. Out here we don't have basements so ducting goes thru the attic. I installed a solar gable fan to help pull the air in the direction the wind would typically flow it to enhance it. Since it blows all day long year round it makes a huge difference in the heat buildup of the attic. Since it's solar there is no cost to run it. There are two small 30W panels on the roof. I have a second one in my garage blowing cooler air in from from the lowest point of a shaded area.

Cathedral ceilings with only 6" fiberglass batting.