Considering a conditioned attic in Zone 5, need advice
Hi all,
Zone 5 (Massachusetts) currently with an (under) vented attic with mostly blown cellulose on attic floor. Moisture issues have forced my hand into action, and I’m leaning towards converting the attic to a conditioned space instead of increasing the ventilation. I’ve read a number of articles and threads on here about conditioned attics, but I’d like to double check that I have my facts correct.
The house was built in early 80’s, so the home is far from the tight construction you see in new builds. The rafters are 2×6. Since I’m in zone 5, and above-roof-deck rigid insulation is not an option (roof was re-shingled 2 years ago…), I’m considering an application of closed cell foam directly to underside of roof deck and the removal of all existing cellulose insulation on floor. I’ve had a few estimates done already, and I’ve received some varying information on the depth of foam needed from insulation contractors. Some say ~7 inches for R49, some said ~5.5 for R38 would be acceptable. I’m sure more is likely better, but I need to consider costs as well and I’m wondering what current code calls for as minimum. I plan on reaching out to local building inspector, but does anyone have any insight into the R requirements for conditioned attics? In 2021 IRC (R806.5) I see that I need at least R20 air-impermeable against the roof deck, but it also indicates the R requirements in N1102 are still applicable, and I’m not sure what actually applies to conditioned attics (does the Ceiling R value still apply even though we’re insulating the roof assembly and not a typical ceiling?).
I’m also curious about potential moisture issues in the conditioned attic. I’ve seen some articles on moisture problems in open-cell foamed attics, where it was recommended that air handler supply ducts be added to conditioned attics to remove moisture. It also seemed like this solution was likely against code at the time the article was written – is that still generally the case? Does my unvented attic need, dare I say, venting to exchange air and keep moisture down?
Appreciate any comments or feedback or lessons learned.
Thanks
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Replies
Why are you so set on such a foolish course of action? Just to be clear conditioning your attic will increase the surface area if your thermal envelope by 25% or so locking you into at least 25% more heat loss if in the unlikely event that your budget somehow allows you to maintain current R value.
I have to ask have you deleted all the recessed lighting?
Have you used a blower door to locate the air leaks between the conditioned space and the attic?
What is your homes ACH50 number?
How many square inches of vent do you have for each 100 square feet of attic?
Why is it so impossible to add more vent if needed?
Desperate times may require desperate measures so convince me.
Walta
So some background on my situation. Purchased the house 3 years ago with known insufficient insulation in attic. We used the local utility sponsored program to subsidize insulation improvements, which included air sealing in the attic and blower door test.
After air sealing in attic and increasing amount of blown cellulose, we started having moisture issues. Apparently by improving insulation, we exacerbated an existing ventilation issue. Cooler attic meant more condensation and eventually mold.
Our existing ventilation consists of continuous soffit vent along back (south facing) roof and ridge vent. Front (north) facing roof is a gambrel style with no eaves for any kind of soffit venting. This north side roof in particular experiences the worst moisture/mold as it has little air flow and receives less solar heat. Had I known, I could have had drip edge vents installed along north roof when the roof was redone, but I wasn’t aware of the extent of the ventilation issue at the time.
So I’m stuck with either significant roof work on a fairly new roof by installing (likely insufficient) drip edge venting, or increasing the ventilation on south facing roof by adding either more passive vents or powered vents. I’d prefer not to add *any* additional intrusions into the roof if I can help it (more holes, more chances of leaks down the road) but adding any vents to the front of house (north facing) roof is generally out of question for curb appeal reasons.
Even with the air sealing previously performed it seems I have a fairly leaky ceiling, since clearly moist air is making it into the attic. Entire top floor of house is loaded with can lights. Also HVAC registers, bathroom fans, and various electrical runs. Could I have another round of air sealing done? Sure, but wasn’t impressed with results of the first round. We have a fairly low sloped roof - perhaps part of the air sealing problem is difficulty in reaching the top plate areas?
So why conditioned attic? I have far more confidence in the robustness of air sealing provided by wholesale closed cell on the roof deck than any further attempts to account for the numerous intrusions into ceiling from living space. Conditioned attic also avoids further roof intrusions for either passive or powered vents, which may or may be enough to fix our issue of north side roof having no ventilation. Also assume at least some, albeit minor, improvement of HVAC efficiency as it no longer has to fight 130 degree attic. There’s something to be said for ease and speed of the conditioned attic solution too.
I'll start by saying I'm not a professional. I do know that due to air tightness, closed cell reaches diminishing returns at around three inches, so if you wanted to save money, you could do three inches of that straight on the roof deck, then get to R49 with whatever is the cheapest r per inch you can find. Closed cell shouldn't have moisture issues and for me personally, I haven't seen a reason to ever use open cell