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Craig Dodson Posted: Jul 21, 2007 10:56 PM	Spraying Underside of Decking I have a customer who wants to spray 1" of SPF on the underside of the decking and rafters. The decking has a radiant barrier. The attic is not a closed system, it is vented. The HVAC system is located in the attic. I have received bids from local installers but they do not have much input on the outcome of the installation. I would like to find out what benifits this has. Any info would be great.
philip mullins Posted: Jul 22, 2007 06:10 PM	is there the same 1 inch in the walls? i have heard of folks puttin 1 inch for an air barrier. then insulating with glass to save money. but, a vented attic defeats that purpose.although, hear in the south east where many insurers have raised thier rates and stopped covering for wind driven rain...hmmm. you would have a stronger, better roof. and when the wind blows of ur shingles, the foam will help protect your home. and the wind driven rain wont come inside destroying your ceiling, walls, and weakassed fiberglass insulation. could be money well spent, depending on what ur trying to accomplish. i am in complete agreement with your local installers!
Craig Dodson Posted: Jul 22, 2007 07:52 PM	Thanks Troutkiller. I do think it would make the roof stronger and help on the wind and driven rain. That's a good point I did not think of. The homeowner thinks that it will help reduce temps in the attic enough to have large energy savings. The bids are in at about $6,500 but the SPF installers are saying to either put the radiant barrier (which is in place) or put the SPF but not both. The SPF Installers do not think it is a cost effective move. They say they will install but do not know the outcome. I have tried to locate people that have used this is our area but have been unsuccesful.
mason Posted: Jul 23, 2007 07:22 AM	The application you describe will not be cost effective for the building owner. They will not save a considerable amount of energy and it will not reduce the attic temperature significantly. The main benefit of the sprayfoam in saving energy in buildings is stopping air infiltration. This application will not stop air infiltration. IN order to be effective you have two choices. The most energy efficient method would be to install a sufficient thickness (around 3-5 inches, depending on your climate) to the underside of the roof deck to stop condensation and provide the required insulation. You would then want to seal the attic. Right now the attic space is supplying the HVAC units with air. When you seal the attic the air must come from another source. Get with your HVAC contractor to determine what type of vents and ductwork would be required to supply air to the units from the outside and from other parts of the building. The other option is to spray the foam to the floor of the attic thereby sealing and isolating the inhabited sections of the building from the attic. Then spray the duct work with foam to super insulate them. The attic still can get hot or cold but the sprayfoam can reduce the amount of heat transfer from the attic to the ducts. I did this in my attic in Virginia and it does save 10-15% on energy bills and makes the whole house more comfortable. It is less effective in the hottest temperatures (95 degrees or higher), because the attic temperatures still get pretty hot. In all cases check your local building codes for the correct amount of insulation to use and verify the amount of sprayfoam needed to prevent condensation

Author

Comments

Craig Dodson

Posted: Jul 21, 2007 10:56 PM

Spraying Underside of Decking

I have a customer who wants to spray 1" of SPF on the underside of the decking and rafters. The decking has a radiant barrier. The attic is not a closed system, it is vented. The HVAC system is located in the attic. I have received bids from local installers but they do not have much input on the outcome of the installation. I would like to find out what benifits this has. Any info would be great.

philip mullins

Posted: Jul 22, 2007 06:10 PM

is there the same 1 inch in the walls? i have heard of folks puttin 1 inch for an air barrier. then insulating with glass to save money. but, a vented attic defeats that purpose.although, hear in the south east where many insurers have raised thier rates and stopped covering for wind driven rain...hmmm.

you would have a stronger, better roof. and when the wind blows of ur shingles, the foam will help protect your home. and the wind driven rain wont come inside destroying your ceiling, walls, and weakassed fiberglass insulation.

could be money well spent, depending on what ur trying to accomplish. i am in complete agreement with your local installers!

Craig Dodson

Posted: Jul 22, 2007 07:52 PM

Thanks Troutkiller. I do think it would make the roof stronger and help on the wind and driven rain. That's a good point I did not think of. The homeowner thinks that it will help reduce temps in the attic enough to have large energy savings. The bids are in at about $6,500 but the SPF installers are saying to either put the radiant barrier (which is in place) or put the SPF but not both. The SPF Installers do not think it is a cost effective move. They say they will install but do not know the outcome. I have tried to locate people that have used this is our area but have been unsuccesful.

mason

Posted: Jul 23, 2007 07:22 AM

The application you describe will not be cost effective for the building owner. They will not save a considerable amount of energy and it will not reduce the attic temperature significantly.

The main benefit of the sprayfoam in saving energy in buildings is stopping air infiltration. This application will not stop air infiltration.

IN order to be effective you have two choices.

The most energy efficient method would be to install a sufficient thickness (around 3-5 inches, depending on your climate) to the underside of the roof deck to stop condensation and provide the required insulation. You would then want to seal the attic. Right now the attic space is supplying the HVAC units with air. When you seal the attic the air must come from another source. Get with your HVAC contractor to determine what type of vents and ductwork would be required to supply air to the units from the outside and from other parts of the building.

The other option is to spray the foam to the floor of the attic thereby sealing and isolating the inhabited sections of the building from the attic. Then spray the duct work with foam to super insulate them. The attic still can get hot or cold but the sprayfoam can reduce the amount of heat transfer from the attic to the ducts.
I did this in my attic in Virginia and it does save 10-15% on energy bills and makes the whole house more comfortable. It is less effective in the hottest temperatures (95 degrees or higher), because the attic temperatures still get pretty hot.

In all cases check your local building codes for the correct amount of insulation to use and verify the amount of sprayfoam needed to prevent condensation

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