Tag Archives: rock wool

Can No Longer Afford Spray Foam for a PEMB

Can no Longer Afford Spray Foam for a PEMB

Loyal reader CINDY in TYLER writes:

“I had a steel building (20×18)  built with steel frame and metal exterior. This is going to be my house. It will have a loft that is half the size of the building. Originally the builder talked me into spray foam and that’s what Ii planned to do. He said I had to use wood to frame inside the metal walls first, then run electrical and plumbing before the spray foam. That was a couple of years ago. Now that inflation has caused prices to soar, I am simply not able to afford the spray foam. My main concern is the condensation/moisture issue. i am doing the rest of the work by myself. Since I don’t have any help it’s not going to be feasible to remove wall panels to install house wrap or insulation. So I wanted to get your expert advice on how to handle this. Specifically I have a plan to run by you. So the idea is instead of building my framing inside the metal frame, move to the inside of the metal, attach wood frame to the inside edge of the metal frame. Insulate the inside of the wood frame and add a moisture barrier to the inside of the wood frame before drywall. I will lose 3.5″ of space all around the inside but i think that will take care of any moisture issues. Please tell me what you think about this plan and make any appropriate suggestions even if you don’t post this on your blog. Also I wanted to thank you for the wealth of knowledge you have readily available on your site. Can’t tell you how much help you have been.”

Mike the Pole Barn Guru responds:

Thank you for your kind words, they are greatly appreciated.

Normally (in your climate zone of 2A) I would be recommending closed cell spray foam as insulator of choice – due to a combination of heat and humidity. Your builder headed you in a correct direction.

Before we get into how to frame your interior, we need to address what is going to happen with your roof. With steel installed directly over framing (whether wood, or in your case steel), if there is no well-sealed thermal break, you are going to experience condensation issues. You are going to have to find a way to spring for two inches of closed cell spray foam sprayed directly to the underside of your roof steel. Steel frame and steel purlins should also be sprayed. If not, you are going to have condensation on them – steel is a wonderful conductor of heat and cold.

Now – on to your question at hand. For your walls, it appears most folks do exactly as you propose and build a 2×4 wood stud wall inside of their PEMB’s (pre-engineered metal building) steel wall girts. You will want to completely fill your wall cavity with insulation – I would recommend rock wool, as it is not affected by moisture (here is information on one particular product https://www.hansenpolebuildings.com/2013/03/roxul-insulation/). You want to make sure your interior vapor barrier is extremely well sealed, including outlets.

If you do not have a well-sealed vapor barrier under your slab on grade, please seal your concrete now. Your HVAC system should be designed to mechanically dehumidify, else condensation is going to haunt you forever.

Stilt Home Barndominium

Stilt Home Barndominium

For many challenging building sites (those with grade change, in flood zones or close to oceans or seas) stilt homes are a viable and practical design solution for barndominiums.

Reader DAVID in EMINENCE writes:

“We are planning to build in southern Missouri a 30′ x 36′ x10′ post frame home on a rocky slope terrain. We want it on stilts. It would be 3′ on one end and 7′ on the other end approximately. We are planning to put reflective bubble wrap on the floor joists with the subfloor on top then place down rock wool and another subfloor on top. We have 99% humidity most of the year (10 months for sure), lots of rain. We do not want a crawl space; we know the horrors of the crawl space. We may enclose the high end using a simple temporary enclosure to dry it out as needed. We are going to use a mini split heating system and composting toilets. No worries about placement of the utilities and pests. Would this be a sound construction system?

We would like to know your viewpoint on this since you are the wise guru.”

Mike the Pole Barn Guru writes:

Thank you for your kind words. Stilt houses are very easily done using post frame. I have a post frame combination garage/studio apartment/office at our home near Spokane, Washington on 14 feet of grade change and built it as a stilt building. Has been great for going on 30 years and would have been the only practical way to build on this site (for extended reading on stilt houses: https://www.hansenpolebuildings.com/2017/09/stilt-houses/).

Not sure why you are considering two layers of subfloor. I would be inclined to use either steel or an exterior rated sheathing product on the underside of my joists (with a Weather Resistant Barrier between). Rock wool is a good choice for insulation between joists as it is not affected by moisture. Place a vapor barrier on top of joists and then your subflooring. A radiant reflective barrier (bubble wrap) can be used as a vapor barrier, but will not provide any benefits you wouldn’t get from well-sealed visqueen – and would be far more expensive.