Does blown in Cellulose add much R, or should I just go thicker on Exterior Insulation?
Considering possible value add of additionally blowing cellulose into stud cavities from outside before ext insulation application. Thoughts please?
Situation: 1948 Ranch, 12ACH50 (!!), starting to plan deep energy retrofit. 2×4’s w/Rockwool batts, coated paper vapor barrier stapled to interior. 1×6 T&G sheathing. Planning an exterior applied continuous air barrier (whole house) eg 3″ exterior insulation to R25?, rain screen+siding, IG triple panes. Conversion from NG/forced air to air to air heat pump, PV, R50-60 at roof but still considering how. Foundation moisture issue forcing extension of closed cell at rim joists to include basement walls behind existing studs.
Comments / advice on overall plan also very welcome. Thanks all in advance.
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You already have Rockwool batts and you want to add cellulose to the same bays?
That's the question. Once upon a time an "energy efficiency expert" contractor included this in our garage ceiling when the house above was not heating well. Found the original problem, but he insisted dense packing the insulated bays would add R. ?? Just thinking about whether it is worth the time to cost / benefit check blown in + reduced thickness of ext. continuous.
If the Rockwool batts come anywhere near filling the cavities, you won't be able to get any dense-pack into the space. If the batts were old fiberglass, sometimes they can be compressed enough to get some cellulose in as well.
To my ear the idea of blowing cellulose into a cavity filled with a properly fitted rockwool sounds impossible, ineffective and expensive. I guess properly fitted is the key.
Before you go any farther with this “deep energy retrofit” take the time make a detailed budget for your project and consider if the investment will pay off? Understanding the work is unlikely to change the market value when you sell and you are unlikely to live long enough to recover the cost in fuel savings.
Walta
Thanks guys, appreciate the quick answers that I intuitively expected when the Rockwool I pulled out of a gable end wall felt pretty dense and was well fitted.
RE budget, working up A/B plans now to have estimated. House has comfort issues that need to be fixed, decarbonizing is to me a higher priority than economic payout - I don't expect anything to be cheaper than unsustainable continued use of fossil fuels. We have a not-to-exceed $$ in mind re re-sale, leading to sacrificial decisions needed. Have already identified several value-engineering big wins, this was just a peripheral question.
Edit/Add: Planning PV and a heat pump, this is working toward a direct comparison of cost optimized air-sealing + insulation package (how much, what and where) vs more PV panels and a bigger heat pump.