Green Building Forum - Why are timber floors outside the envelope anyway? Thu, 10 Sep 2026 05:28:54 +0100 https://www.greenbuildingforum.co.uk/newforum/ Lussumo Vanilla 1.0.3 Why are timber floors outside the envelope anyway? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13734&Focus=230915#Comment_230915 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13734&Focus=230915#Comment_230915 Wed, 21 Oct 2015 11:21:54 +0100 Dave_07968
I recently finished lining out my garage including 100mm of EPS on the house wall, but I'm not satisfied with the junction of the wall and the garage floor, where I've preserved the airbricks - effectively the wall of the house is "on the outside" on both sides!

I get that ventilation is needed because the timber is outside the thermal envelope, so it's cold and condensation-friendly. But this just makes me wonder - why is the subfloor outside the thermal envelope in the first place?

Could I not line out the subfloor itself with insulation and then ventilate it to the house rather than to the outside? The house air is relatively moist, but after all the first floor joists are not ventilated and do not rot away, even where they enter the inner leaf of brickwork.

How would a proper EWI retrofit deal with this, since much of the gain from EWI would surely be lost if cold air is being pumped into the house through the subfloor?]]> Why are timber floors outside the envelope anyway? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13734&Focus=230917#Comment_230917 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13734&Focus=230917#Comment_230917 Wed, 21 Oct 2015 11:48:22 +0100 tony
Problems with bringing the subfloor void into the house envelope would be smells, air leakage, gas including radon,

I would do it! No radon in my area

Floors should be inside the thermal envelope which is why I said insane.]]> Why are timber floors outside the envelope anyway? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13734&Focus=230919#Comment_230919 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13734&Focus=230919#Comment_230919 Wed, 21 Oct 2015 11:59:19 +0100 fostertom Why are timber floors outside the envelope anyway? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13734&Focus=230921#Comment_230921 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13734&Focus=230921#Comment_230921 Wed, 21 Oct 2015 12:18:59 +0100 gyrogear Posted By: Dave_07968Could I not line out the subfloor itself with insulation and then ventilate it to the house rather than to the outside?

Like Tony said, not a good idea.
However, you could try making a *conditioned crawlspace*

Posted By: Dave_07968How would a proper EWI retrofit deal with this, since much of the gain from EWI would surely be lost if cold air is being pumped into the house through the subfloor?


Same again, with EWI, the thing is to go for a conditioned crawlspace.

I.E. Condemning the CS air vents; insulating the inside of the periphery walls (with rigid foam boards); sealing the CS floor (either membrane or concrete...); heating the CS; venting the CS by admitting conditioned house air, & ejecting said conditioned air at 10 cfm per 100 square foot of CS area (which should expel any radon and other nasties).

"Heating the CS" can be done using incandescent light bulb(s): (in actual fact, an insulated CS is generally considered to "be heated" anyhow...).

CS is thus pressurized so no risk of foul air inside the breathing volume
CS is now warm, so no heat-sink through floor
CS is now ventilated, so better for timbers
(do not insulate the CS roof) (!)
(you could insulate the CS floor, if you have the inclination)

Q.F.E.: You have now brought the floor into the conditioned volume/thermal envelope...

The above assumes the CS is accessible; if it is not, then this would be the first issue to address.
gg]]> Why are timber floors outside the envelope anyway? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13734&Focus=230930#Comment_230930 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13734&Focus=230930#Comment_230930 Wed, 21 Oct 2015 14:12:48 +0100 fostertom
This newbuild one was to convince the Bldg Insp. Builder was an02ew of this forum. If you can't read this jpg, email me (click fostertom top left) and I'll send the pdf.]]> Why are timber floors outside the envelope anyway? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13734&Focus=230931#Comment_230931 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13734&Focus=230931#Comment_230931 Wed, 21 Oct 2015 14:22:07 +0100 fostertom