Temperature/Humidity Concerns with T&G Installation

[Wayne Whitney]

Hello,

I'm about to install some T&G poplar beadboard as a ceiling on my exterior porch. Now the boards are inside @ 68 F and 30% RH, where I've primed both faces and put the first finish coat on one face. Outside we're been having an unusual cold spell for California, lows

30-35 F, highs 45-55 F, about 60% RH.

So two questions: do I want to let the board acclimate to the exterior conditions prior to installation, and/or do I need to leave a small gap between each board for future expansion? Obviously what I want to avoid is installing the boards tightly when they are at the low end of their expansion range, as then they may buckle when conditions bring them to the high end of their expansion range.

Thanks, Wayne

[Wayne Whitney]

OK, I think I have a partial answer to the question: the thermal expansion of wood is dwarfed by its expansion due to changes in moisture content with changes in relative humidity. So I definitely want to move the boards outside where the relative humidity is 60%. The only remaining question is whether it is OK to install them tightly at 60% RH, or if I need to leave a small gap to allow for expansion at 99% RH.

Cheers, Wayne

[Goedjn]

I thought that the tongue in T&G was oversized to produce the proper spacing, by force..

[Wayne Whitney]

At least in the boards I am using, the groove is slightly wider than the tongue and noticeably deeper. So the boards do butt together tightly if I wish--I'm just trying to figure out in what weather conditions (if any) it is OK to install that tightly.

Cheers, Wayne

[Dean]