Question

I would like some input regarding a non-vented attic space for a project in Orlando, FL.

Asked by ryan young
Orlando, FL

I am building a new home with ICF construction and plan to use a non vented attic for my mechanical equipment. I will be using R20 Icynene sprayed directly to the roof deck. 1st question - the roof assembly is 3/4" plywood roof deck with peel + stick waterproofing membrane with a 16" standing seam roof (no exposed mechanical fasteners). I have read many opinions regarding using sleepers to create an air gap between the metal roof and the peel + stick. Is this necessary? 2nd question - I like the idea of using the attic space for the mechanical systems in a semi conditioned space but not venting the attic seems counter intuitive to me. Should the attic be mechanically vented in some way as to create some sort of air movement/exchange? Any moisture issues with open cell non vented attics? Thank you.

Answer

David Toder

Answered by David Toder

Highland, NY

BOLDER Architecture, PLLC

March 22, 2012

Applying spray foam insulation to the rafters is an excellent way to build for energy efficiency.

  • There is more conditioned space, but the cooling system is not fighting the hotter-than-outside temperature of a summer attic with insulation below.
  • The zero-net energy homes I've designed in New York all have this detail (and they have ICF walls like your project).

Be sure to have sufficient depth of insulation for the type of spray foam insulation you use and consider having the insulation come 2" or so past the rafters to greatly minimize thermal bridging. This also creates a temperate location for your mechanicals, as you noted, dramatically improving their performance

With the tight envelope you are creating, you will need a heat recovery ventilator (HRV) for fresh air and to address moisture issues.

  • HRVs are very cost effective in terms of up-front cost v. operational cost savings.
  • Additionally, your home will be near dust free and very healthy with a simple filter included in the mechanical system.

Roof ventilation does not significantly lower roof temperature, it is mostly for removing moisture with fiberglass insulation.

Orient your house to true south if possible to take advantage of passive solar benefits and possible photovoltaic on the roof. There are PV lease options now that make an array very low cost, and you'll be near zero-net energy.

Way to go, you're doing it right.

Feel free to contact me directly if you have follow-up questions. Best luck with all your endeavors.

Tagged In: attic insulation, attic ventilation

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