Energy 10 has a table of conversions for R-values to f-values. I used the table to change f-value from 0.7 to 0.326, reflecting the slab-insulation upgrade.

I’ve regularly checked the ERV all winter – especially after we saw the thermometer reach -18°F one morning. I haven’t even seen frost, much less ice. I’ll keep monitoring, and if I see moisture-related problems, I’ll let everyone know.

A couple of questions:
- I haven’t used E-10 for a while. How did you look at the effects of the changing slab insulation? My recollection is that the only entry is an F factor.
- Have you looked at the core in the Renewaire during your coldest weather? There’s quite a range of opinion about whether these work in cold climates (even tho’ the company is in WI) and Gary Nelson mentioned to me that he has been told that the core turns into a block of ice. Fantech has a similar ERV now and they don’t want us to use it in climates as mild as Boston.

What areas did you find and fix in that air sealing process?

You’re right about the wood stove, and I have already done what you suggest. Even with the slight positive pressure from the ERV, it takes some skill to operate the stove. If other folks were to try this, I’d recommend that they seek out a stove with an outside-air kit. Unfortunately, those kits don’t seem to be available for stoves small enough to be suitable for a building as small and well-insulated as ours. It’s a dilemma.

Relative humidity has ranged between 30% and 40% all winter. We don’t have abundant moisture sources inside (mainly just two middle-aged people breathing). The interior finish is 1-inch pine, with some moisture-buffering capacity. We used a Renewaire EV70, which is used frequently in Northern climates. It passes latent energy across a special fabric core, and does not require a defrost cycle as long as outside temperatures are above -10°F and inside RH is below 40%. I agree with you that enthalpy-recovery units must be used with care in Northern climates.

Just a correction on the MemBrain VB. It changes vapor permeance (like any hygroscopic membrane) with changes in RH, not vapor pressure.

It also sounds like your woodstove is air-starved from the extreme tightness of the envelope. You might consider imbalancing the ERV to compensate for the negative pressure of the stove flue.

What is the indoor RH in the winter? Typically, with a very tight building, it’s too high and an enthalpy-recovery ventilator may not remove enough moisture to compensate. ERVs tend to be more effective in an air conditioning climate and HRVs in a heating climate for that reason. How has it worked in this application?