A lot has been written and researched about the use of radiant barrier technologies, but primarily these technologies are applied in southern climates, to reduce cooling costs.
We live in Maine in a smallish ( 700 sq feet including loft) renovated summer cottage sitting on posts in the heartlands of a sweet island community off the coast of Maine.
We used radiant barriers in a variety of ways that are proving real value to us in this northern maritime climate, which goes from screaming winds, torrential rains, blizzards, winter lows to -20F to highs of 98F in summer.
My goal was to hold heat inside the house, and also prevent rot. There are several different ways that we lose heat: via conduction, radiation, convection(ventilation). During our renovations, we sistered rafters on the roof/ceiling and added strapping to existing walls to add insulation and radiant vapor barriers, and reduce heat lost by ventilation (stopping air movement from warm to cold), conduction (direct transferral through a heated surface contact to cold surface,) and radiation (energy waves), but in a couple of areas we had only available a 4 inch wall cavity. To reduce heat loss by conduction, we installed Rocksl in areas we could work by hand, but we also had professionals come and blow in “Insulsafe” between ceiling rafters), inside of that we installed foil faced rigid board and taped as a reflective vapor barriers. This stops thermal bridging, or the cold places in old construction where cold studs hit warm drywall and can cause moisture condensation inside the wall (we also have Lunos E3 ventilators, to control the moisture ventilation while reducing heat loss) . I did a lot of research on the impacts of radiant heat loss, and how we could use a reflective barrier and an airspace to reflect heatwave energy back into our airspace.
Radiant heat loss and thermal bridging are especially important factors in thinner walls. In other words, once you have a 1 foot thick wall, don’t bother wasting your money on radiant barrier, but where you have a thinner wall, the radiant loss is most substantial. So on the tricky renovation area where we were building on 4 inch stud walls, after filling those shallow cavities with rocksal we covered the walls and studs with 1.5 inch rigid board with reflective side facing indoors, and taped that to form a vapor barrier (it also stopped thermal bridging along the studs), then added strapping, and beadboard (sealed with a latex wash). A radiant barrier requires an airspace beside it to work, but it doesn’t have to be a big airspace, the space left by the strapping works great.
We use this sort of layering of insulation, but with much deeper wall cavities, everywhere else, with the radiant vapor barrier inside (towards the living space) on all the exterior walls and ceiling. We even installed tin ceiling in the peak of the house (just in case it helps reflect heat back). All this is baqckground info for my main topic here, which is how we were also skirting in and insulating the crawlspace. From outside to inside I used exterior plywood (+caulk), rocksall, 1 inch rigid board with reflective barrier facing in, for R28 and aiming to stop wind movement (ventilation) entirely in the crawlspace. No more windows in the crawl space! So not much heat would be lost through conduction or convection/ventilation in the basement, but that is an area where we could lose heat through heat radiation.
About ventilation: This crawlspace started with lots of moisture, think “large puddle on rocks” and so sealing off the crawlspace from the elements involved thinking about how to have better drainage and keeping the house high and dry. So we did some grading, which seems to have helped a lot, and we plan to install gutters to direct and collect rainwater off the roof. But also, the ground is naturally moist. Maybe I can’t stop that entirely, so I just wanted to seal that ground moisture out of the house. Ventilation is really not the best, most energy efficient way of protecting a house from basement moisture. Smart use of vapor barriers are the best way.
So after insulating the entire perimeter, I went around taped in a 6mm plastic floor, just draping it over the craggy maine island ground, overlapping seams by a good amount. I also went around poked some holes in low spots in the plastic, so that if water ever came in, it would have a way to get out. I found research that explains why it is fine to have lots of small holes in your crawlspace vapor barrier, but what you don’t want are any BIG “gusher” holes in your vapor barrier! I taped the plastic carefully all around every post and all around the perimeter. Then on top of that plastic, I laid a “bubblepack ” style radiant barrier all over the floor of the crawlspace, (which immediately made for much more comfortable crawling), overlapping seams and I am still finishing taping this in. So this second layer on the ground is a reflective insulated vapor barrier on top of a plain plastic vapor barrier.
This sealed the moist air (and dirt) out of our little house.Then I ran a dehumidifier in the closed crawl space to suck trapped moisture from that crawl space, and it got steadily getting dryer and dryer in there, and now I haven’t run the dehumidifier for a few weeks, but the crawlspace is staying very warm and dry. I will probably run the dehumidifier down there in spring rainy season. It’s pretty amazing. It’s as though the entire house is encased in a reflective plastic bag. It’s cozy!Our feet are happy with the comfortable floor of this little house, as the radiant/reflective barrier is turning the still air of our crawl space into a radiant heat reflector! We’re not exactly using the earth mass to store heat, but we are using the crawlspace itself to reflect heat back into the house.