How to model Insulation installed in the roof deck?

[Travis Evans]

Hi Guys,
     I ran into an attic yesterday that I can not figure out how to model. Let me try to explain the situation and see if you guy can help me. We have this large 6900 sq ft home and when I climbed into the attic the have 9 inch batts stapled to the roof deck. about 5% have fallen down but overall it looks to be installed pretty well. Our training has taught us that if this is the case essentially the house is considered un-insulated because the insulation is not in contact with the air barrier. However the bills that the customer has doesn’t really reflect a home with no insulation. What would be the best way to model this so I can get a somewhat accurate idea of what is saving could be if we fix it. 

[Ryan Moore]

This is a tough one (for me, anyway). Our pro, Gamaliel, is travelling today, but I’m sure he’ll have some ideas for you, soon.

[Ryan Moore]

From Gamaliel:

It sounds like you are describing an unconditioned attic, where the
insulation was placed in the roof deck instead of the attic floor. If
there is any means of decent air exchange between the attic and the
conditioned space or any actual conditioned air delivered to the attic,
then this should be treated as a vaulted ceiling and modeled on the
Vault/Flat tab. You should model two separate areas to account for the
insulation that has fallen down. Model one area at 95% total area with the
insulation installed. Model the second area at 5% of the total area, with
no insulation. Then model the improvement (i.e. replacing the batts) to
that second area. You will also want to model the gable walls on the Walls
tab.

If the attic is really totally unconditioned then this is a trickier
situation. Are the gable walls insulated as well? Is there a floor in the
attic, creating a fairly isolated air space between the attic and
conditioned space?

[Travis Evans]

Yes the Gable walls are insulated.  I would like to give you some more information on the subject.  The house is all electric and his bills is the winter are around $400 and in the summer around $600.  For a house this size they don’t seem to be that far off.  Especially not representative of a house with no insulation.  The only exchange of air between the main body and the attic space is the large number of can lights in the house.  When I model the house with no insulation and then make an improvement it shows a savings of over $2000 a year.  I feel like this would be a large misrepresentation to present this as it stands which is why I was hoping that you guys could help with the best way to model this home.  Thanks

[Gamaliel Lodge]

I definitely wouldn’t model this as an uninsulated attic.  I think you could go a few different ways with this.

1. Model it as a conditioned attic, as I described above.  Model the roof on the Vault tab and the gable on the Walls tab.  This will probably still give you an overestimation, because the attic is still acting as a bit of a buffer space and won’t be maintained at the indoor temperature.  
2. Model it as an unconditioned attic with the roof insulation moved to the attic floor.  You would still model it as two separate sections: 95% insulated and 5% uninsulated.  This will probably give you an underestimation of savings, because the total surface insulated surface area will be underrepresented, but you will get the buffering effect of the attic.
3. You could follow option 2, but modify the attic area to be equal to the combined roof and gable area.  That may swing you back towards a slight overestimation, but is probably the best of the three options.   If you have good billing information, then you could try to find a sweet spot between the attic floor area and combined roof/gable area that yields a good calibration.

[Kirsten Shaw]

I have a related question: Is there a way to model an attic improvement of converting a vented, unconditioned attic to an enclosed conditioned attic with the roof deck as the air barrier with spray foam insulation?

It will not be an occupied space. In our climate and in the majority of our housing stock, this conversion has an added benefit of bringing the duct work and air handler into conditioned space.

[Gamaliel Lodge]

We don’t have a direct way to model changing the thermal boundary in the attic, but there is a workaround posted here: https://optimiserenergy.com/forums/topic/attic-thermal-boundary-roof-deck.

That post covers how move the location of the thermal boundary using the controls on the Attic and Vault/Flat pages. However, if the ducts are initially located in the attic, then you will have to change their location to Conditioned Space manually and include the ducts in the improvement package. This doesn’t happen automatically. The total energy savings for the procedure would then be the sum of the Attic, Vault/Flat, and Duct improvement rows.

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