Hello I am about to build my first sip house but I rather have a 12in sip at the attic floor or house ceiling because I rather heat and cool the areas where I will be living nothing extra. Also to keep sip in my budget so that I don't have to go over to 2x6. Yes I will have my hvac in a closet in the house but the duct will have to be run in the hot attic but truly how much money can that really cost you. Yes I know it isn't as good but I have no choice because I am trying to keep cost down and using sip for the roof uses more sips and you still have to have trusses. We are just going to put the prefab trusses on top to the panels. The panel supplier said it will work and they have done it many times. I they call it the shoes box design. What are your thoughts. http://www.thehousedesigners.com/plan/the-cedarbrook-8461/

Posted By adriantc5650 on 31 Dec 2014 12:56 AM
Hello I am about to build my first sip house but I rather have a 12in sip at the attic floor or house ceiling because I rather heat and cool the areas where I will be living nothing extra. Also to keep sip in my budget so that I don't have to go over to 2x6. Yes I will have my hvac in a closet in the house but the duct will have to be run in the hot attic but truly how much money can that really cost you. Yes I know it isn't as good but I have no choice because I am trying to keep cost down and using sip for the roof uses more sips and you still have to have trusses. We are just going to put the prefab trusses on top to the panels. The panel supplier said it will work and they have done it many times. I they call it the shoes box design. What are your thoughts. http://www.thehousedesigners.com/plan/the-cedarbrook-8461/

adriantc5650,
I have built one that way in Louisiana
I really don't think there is any construction savings or any energy savings, it depends on where you are building ?
the dead vaulted space with the SIP roof is conditioned only by duct leakage, on the other hand with SIPs on the ceiling, the ducts in a hot attic not only loosing efficieny , but also sucking hot humid air due to negative pressure when the system runs

Depending on the design and materials, you don't also need trusses with a SIP roof.

The question of ducts in the conditioned space vs unconditioned space has been tested quite a bit, just not with sip houses in particular. Ductwork running in unconditioned space will always cause at least a 10% energy penalty. To get it down to just 10% usually requires near perfectly sealed ducts (at least 4 times better than current code), R 10 insulation “at least” on the ducts them selves, and then burying them in lose fill insulation.

How much this costs you in real dollars depends on fuel costs and the actual heat loss of the structure. To me it does not make all that much sense to commit to sip construction than then poke any more holes in the envelope than necessary.
In your case, IF I had to put the ductwork in unconditioned space, I would probably plan on using all rigid metal ductwork, install it, seal and test it and then spray 4” of CC foam to the ducts.

If trusses are a must, then I prefer to attach a nailbase panel to the underside of the bottom cord. This option gives you all the air-sealing and thermal performance of a SIP without the cost associated with the "structural" aspect of a SIP. It is simple to screw up through the NB panel into the truss and additional thermal performance is achieved by a layer of blown insulation above. It also simplifies the framing of the walls below.

As for duct work in an unconditioned space, it's beyond ridiculous. In fact, why would you want a ducted system when minisplit systems are so affordable and efficient? Go ductless!

Do yourself a favor and hire someone like Eric (above) to get you started in the right direction and make the right choices based on sound energy modelling and proven analysis. Your project should be aided by a third party professional (HERs rater) as opposed to the subcontractors who want to do it the way they've always done it.

Al Cobb
SIPschool.org

I did think about mini splits and that will solve my hvac concerns but how will I use mini split on this floor plan please explain because I would like to know about drain lines for the minisplit in rooms like the interior bathroom etc. http://www.thehousedesigners.com/plan/the-cedarbrook-8461/ Also I was told mini splits are expensive for a whole house project I am hoping it's not because I wanted mini splits anyways. Then I have to think about how I will get fresh air into every room with a Erv.