Green Building Forum - How much underfloor ventilation is needed Sun, 24 May 2026 13:16:37 +0100 https://www.greenbuildingforum.co.uk/newforum/ Lussumo Vanilla 1.0.3 How much underfloor ventilation is needed https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14655&Focus=248952#Comment_248952 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14655&Focus=248952#Comment_248952 Thu, 27 Oct 2016 10:31:41 +0100 delprado How much underfloor ventilation is needed https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14655&Focus=248966#Comment_248966 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14655&Focus=248966#Comment_248966 Thu, 27 Oct 2016 12:25:53 +0100 CWatters
"1500mm^2/m run of external wall or 500mm^2/m^2 of floor area whichever is the greater"

Page 26 - 4.14b

https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/431943/BR_PDF_AD_C_2013.pdf

I think the NHBC recommend the vents are distributed on 2m centres.]]> How much underfloor ventilation is needed https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14655&Focus=248973#Comment_248973 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14655&Focus=248973#Comment_248973 Thu, 27 Oct 2016 14:52:53 +0100 delprado How much underfloor ventilation is needed https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14655&Focus=248991#Comment_248991 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14655&Focus=248991#Comment_248991 Thu, 27 Oct 2016 22:58:26 +0100 gyrogear (2) Where is the ventilation air coming from (and going to...).

gg]]> How much underfloor ventilation is needed https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14655&Focus=248997#Comment_248997 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14655&Focus=248997#Comment_248997 Fri, 28 Oct 2016 07:11:02 +0100 delprado
http://s10.postimg.org/kpgkttc6x/plan.jpg]]> How much underfloor ventilation is needed https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14655&Focus=249001#Comment_249001 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14655&Focus=249001#Comment_249001 Fri, 28 Oct 2016 08:24:10 +0100 Victorianeco How much underfloor ventilation is needed https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14655&Focus=249004#Comment_249004 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14655&Focus=249004#Comment_249004 Fri, 28 Oct 2016 08:53:36 +0100 Sprocket Can anyone comment on pros and cons?]]> How much underfloor ventilation is needed https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14655&Focus=249005#Comment_249005 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14655&Focus=249005#Comment_249005 Fri, 28 Oct 2016 08:59:19 +0100 Nick Parsons
Are you convinced it's *not* enough? (And if so, by what symptoms?)

Since I note your subsequent post re u/floor insulation you are definitely right to question the amount and flow of ventilation air. I (and others - viz a lot of discussion in AECB) find that under-floor insulation applications don't always behave as one might expect them to.]]> How much underfloor ventilation is needed https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14655&Focus=249009#Comment_249009 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14655&Focus=249009#Comment_249009 Fri, 28 Oct 2016 10:13:11 +0100 fostertom Posted By: Sprocketsomeone in here suggesting LecaThis one?
Not so legible as a jpg - click fostertom and tell me yr email and I'll send you the pdf.]]> How much underfloor ventilation is needed https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14655&Focus=249017#Comment_249017 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14655&Focus=249017#Comment_249017 Fri, 28 Oct 2016 11:24:08 +0100 gyrogear
Could not understand why they were there: admitting cold air in winter so UFH was working overtime to warm the crawlspace ! and warm air in the summer, rusting up my plumbing and condensing on the (muck) floor !

So insulated CS walls & house has been *much* warmer & THE CS MUCH dryer ever since !
(Am now concreting the CS floor...)

gg

edited : it is the *CS* that is much dryer (not the house!) (the house has always been dry !) :shocked:]]> How much underfloor ventilation is needed https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14655&Focus=249019#Comment_249019 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14655&Focus=249019#Comment_249019 Fri, 28 Oct 2016 11:52:44 +0100 Ed Davies How much underfloor ventilation is needed https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14655&Focus=249040#Comment_249040 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14655&Focus=249040#Comment_249040 Fri, 28 Oct 2016 23:07:55 +0100 gyrogear
For past year or more, I have had a weather station (base) in the kitchen, with temp and RH, and a slave transmitter for temp and RH, down in the CS.
I don't (yet) have a data logger, so my method is to take occasional photos, then copy these to my PC; I then file them by Month, and by Day. I have started grabbing them into an XL file, it is very laborious !

My aim is to establish a one-year record.
However, as I see it, the weak point is, I do not have the external RH of the day, against which to compare - for that, I have to visit the local weather station via internet. For example, today our local RH was 84%, and the CS is currently at 83%...

=========
Otherwise:

Lowest CS RH observed = 74% (12 OCT 2016) (Lounge RH = 38% = a very dry & sunny autumn afternoon).

Highest RH observed = 98% (after 10 days’ absence, no heating on) at 30th Dec. 2015 @ 6 p.m. (Lounge at 16°C and 62% RH, CS at 15°C and 98% RH) (we went on holiday and the house was not heated).

Prior to blocking the vents, I have no idea what the CS RH was - it was certainly smelly, moist and very uninviting...
====================
I have to confess that my main point of interest is the CS *temperature*, not the RH...
(I don't live down there !)

gg]]> How much underfloor ventilation is needed https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14655&Focus=249057#Comment_249057 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14655&Focus=249057#Comment_249057 Sat, 29 Oct 2016 13:14:13 +0100 Ed Davies Posted By: gyrogearLowest CS RH observed = 74%That's not ideal but

Highest RH observed = 98%
that's truly scary if it lasts long.

(I don't live down there !)
Yet.

Remembering my father's leg going through the floor of my ground floor bedroom when I was about 6 or 7 due to the joists rotting away after a water leak. The flowing water wasn't directly in contact with the joists (just flowing across the ground) but must have been pushing the RH way up.]]> How much underfloor ventilation is needed https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14655&Focus=249063#Comment_249063 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14655&Focus=249063#Comment_249063 Sat, 29 Oct 2016 17:34:53 +0100 gyrogear
Actually, I have *concrete* joists !

gg]]> How much underfloor ventilation is needed https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14655&Focus=249066#Comment_249066 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14655&Focus=249066#Comment_249066 Sat, 29 Oct 2016 19:24:04 +0100 Ed Davies ]]> How much underfloor ventilation is needed https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14655&Focus=249075#Comment_249075 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14655&Focus=249075#Comment_249075 Sat, 29 Oct 2016 22:47:06 +0100 gyrogear
Severe apologies for that...
:shamed:

However, that admittedly belated parameter does not prevent the OP from consdering blocking *HIS* - depends on how humid his conditions are...

If the underfloor is an accessible crawlspace, it could be conditioned, by blocking the vents; encapsulating the floor; and insulating the periphery walls, to make a conditioned crawl...

Air could then be drawn in from the heated volume, and expelled through one of the blocked vents -

I found this on a blog: http://www.energyvanguard.com/blog-building-science-HERS-BPI/bid/70974/What-Is-the-Best-Way-to-Deal-with-Crawl-Space-Air

"continuously operated mechanical exhaust ventilation at a rate equal to 1 cfm for each 50 square feet of crawl space floor area.”
(The makeup air entering the crawl space is conditioned air from the house upstairs; since this conditioned air is drier than outdoor air, it doesn't lead to condensation problems.)

Hope this helps some, and goes some way towards correcting my earlier concrete (non-abstract...) omission :sad:

gg]]> How much underfloor ventilation is needed https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14655&Focus=249079#Comment_249079 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14655&Focus=249079#Comment_249079 Sun, 30 Oct 2016 06:55:51 +0000 delprado How much underfloor ventilation is needed https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14655&Focus=249083#Comment_249083 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14655&Focus=249083#Comment_249083 Sun, 30 Oct 2016 11:25:50 +0000 gyrogear Posted By: delpradoif there is any benefit of laying limecrete along the mud floor

well, that is what I am currently engaged in !

http://voices.yahoo.com/how-rid-moisture-crawl-space-6367824.html?cat=6

QUOTE
“For those that have concrete surface spaces, consider yourselves fortunate. These finished crawl spaces have sealed protection against unwanted water and create much appreciated surface area to stow away several items as storage".

I suspect that even lime-stabilization would be better than nothing...

gg]]> How much underfloor ventilation is needed https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14655&Focus=249092#Comment_249092 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14655&Focus=249092#Comment_249092 Sun, 30 Oct 2016 16:30:23 +0000 djh Posted By: gyrogearhttp://voices.yahoo.com/how-rid-moisture-crawl-space-6367824.html?cat=6
That redirects to a generic yahoo front page for me. So I don't have any information except what you posted.

Neither concrete nor limecrete provide a barrier against damp, so unless there's a damp proof course somewhere I doubt it will reduce humidity much.]]> How much underfloor ventilation is needed https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14655&Focus=249119#Comment_249119 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14655&Focus=249119#Comment_249119 Mon, 31 Oct 2016 12:29:29 +0000 gyrogear
http://itchyfish.com/how-to-get-rid-of-moisture-in-a-crawl-space/

gg]]>