Structurally insulated panels and heat recovery ventilator (HRVs)

Q1. There was a question during the most recent webinar and I missed your response.  The question was what do you think of SIP’s (structurally integrated panels) – what are your thoughts about SIP’s?
Q2. BTW, I was just watching a Fine Home Building webinar entitled “Principles of Residential Ventilation,” most of it over my head, but the core message was that ERV’s (energy recovery ventilators) are the bee’s knees.  Do you have any thoughts on ERV’s (sort of outside your wheelhouse, I know)?  The sub-messages are moisture management & indoor air quality). 

A 1. I think SIPs are great, but they are only one of many ways to get insulation and structure. Walls have to perform a lot of functions in addition to holding the roof up. They must block rain, block wind, block cold/heat and block moisture that is in the air as vapor or humidity. The latter is because humidity/vapor becomes liquid water (condensation or dew) when the temperature drops. You do not want liquid water in a wall – it will lead to mold, rot and likely asthma for the family. This is not theoretical. A house in our town was condemned by the board of health after the owner was taken to the ER with asthma that was caused by mold ultimately caused by vapor condensing inside the walls that did not evaporate. See my other answers on the topic of condensation. Mold and rot, caused by condensation, are probably the most insidious problems in housing today.

Blocking rain and wind are fairly easy, this is what siding is supposed to do. However, shiplap (overlapped) siding and shingles do a great job of blocking rain and a fairly poor job of blocking wind. This is why, today, most builders install an air-tight membrane under the siding. Products like Tyvek house wrap allow humidity to pass through from the inside but block both water and wind. For Tyvek to be effective as a wind barrier the Tyvek sheets need to sealed by having their seams taped from the outside. This allows a wall to dry to the outside when it gets condensation inside it. This is exactly the same idea as in a GoreTex jacket – the fabric prevents rain from getting in but still allows humidity created by sweating to escape to the outside. Condensation in walls (and inside jackets when exercising) is inevitable but mold is not inevitable – you just need to allow that condensation to evaporate. This is the job of a vapor-permeable membrane like Tyvek. Or GoreTex. Some very high-end custom builders will now install vapor tight membranes on both the inside and the outside of the wall, which keep the interior of the wall completely free of condensation because vapor cannot even enter the wall and hence cannot condense. These membranes still allow vapor to travel from the inside to the outside (just like Tyvek or GoreTex) so, if the inside of the wall gets wet for some reason other than condensation (like a water leak) it can still dry out. This is best practice in wall construction today and is so far beyond code that most builders will not be familiar with it.

And I have said nothing so far about insulation. Best practice is to have some insulation outside the air blocking layer (e.g., outside the Tyvek layer) to prevent (or reduce) thermal bridging which is where heat leaks out through less-well insulated parts of the wall like the studs – wood is a poor insulator compared to the air trapped in insulation like foam or fiberglass. Then have even more insulation in the cavities of a wood-framed wall. This would normally be the 4” cavity created by the 2”x4” wood studs (the upright planks of wood that hold up the roof). This cavity is normally filled with insulation like spray foam, fiberglass or dense-packed cellulose. When you combine this outer layer of insulation with the cavity insulation, plus the membranes to block wind and rain, plus the siding (which blocks most of the wind and rain), plus the painted drywall on the inside you have the perfect wall. 

This “perfect wall” is more difficult to build than making a wall from SIPs, but SIPs at least get you insulation and structure. Some SIPs come with the outer surface painted with a waterproof and windproof paint with the joints sealed with tape on site. This makes a very good, but probably not quite the best, wall at a reasonable cost. At the end of the day, SIP or no SIP, your wall must must block rain, block wind, block cold/heat and block moisture. Oh, and hold the roof up. How you get there is less important than that you get there.

A2: I hope this answers your first question, but it is also a lead into to the answer to your second question. When you have a very tight building envelope, (that is, less than about one air-change-per-hour at 50 Pascals, know as 1ACH50. A pascal is a unit of pressure similar to pounds per square inch. Fifty Pascals is about the pressure of a 20 mile-an-hour wind) the air leakage into the house will not be enough to evaporate all the condensation that inevitably happens in walls. You are effectively living inside a Ziplock bag. This is sometimes what happens in PassiveHouses because PH certification requires less than 0.6 ACH50 per hour. Hence, with tight building envelopes, you need great walls that can dry effectively – see above. To make a great house that is low cost to run, low carbon footprint and healthy to live in, all these separate parts (walls, windows, ventilation, insulation etc) all need to work together. You can’t fix one without fixing the others. 

So to provide the fresh air you need when you have a tight building envelope, you need ventilation. Heat recovery ventilators (HRVs) bring air into the house through a pipe rather than through gaps and cracks in the walls. The air that comes through this pipe is warmed up with the air that is leaving the house. This gives you fresh air without losing all the heat. If you have a very tight building envelope a ventilator is not really an option, you need to have it. An HRV is better than just an air pipe. I have never installed an HRV because retrofits are almost impossible to get to 1 ACH50 and my consulting work so far has been entirely on retrofits. So neither I, nor my clients, have ever needed an HRV. So, I think that HRVs make good energy sense and you really need one if you are going to have a very tight building envelope. Whether they are cost-effective is another matter and I do not know the answer to that. I would definitely ask the installer how much energy and money they will save you and then calculate the payback period. Payback periods on heat pumps, insulation, triple-glazed windows and solar panels range from  about 1 to 9 years. When I did some back of the envelope calculations on the amount of money I might save with a HRV I found that I could generate the energy I need at less money with extra solar panels than I could save by installing an HRV.