Relative Humidity in Intermittently Conditioned Energy-Efficient Homes: A Preliminary Hygrothermal Assessment of Indoor Condensation and Mould Risk

Review
The paper is relevant to the journal's aims and scope, and addresses an interesting and relatively under-investigated topic at the intersection of hygrothermal simulation and occupant behaviour — specifically, how indoor environmental conditions are shaped by occupancy profiles. The discussion of vapour barriers and intermittent conditioning patterns is important, and the paper makes a meaningful contribution by attributing hygrothermal risk not only to façade quality and design strategies, but also to actual patterns of space use. However, in its current form, the development of this topic feels narrow. The assumptions underpinning intermittent conditioning are not made explicit, which is a significant gap given that this is the central focus of the paper. It is not until the methods section that any conditioning pattern is described, and even there the treatment is limited — essentially reduced to applying a standard use profile through a simulation tool, without sufficient justification or elaboration. As a result, the paper misses an opportunity to interrogate the nuances of intermittent occupancy and to establish a clearer causal relationship between those patterns and the hygrothermal outcomes reported. The following comments are offered as suggestions for strengthening the manuscript.

Major comments:
1. The paper would benefit from a clearer framing of the core problem around intermittent conditioning. Specifically, what distinguishes intermittent from continuous conditioning in terms of hygrothermal behaviour, and why does this distinction matter? This is not currently articulated, and it is fundamental to motivating the study.

2. The use of WUFI is appropriate, but hygrothermal simulation carries uncertainty. It would strengthen the paper considerably if the authors could provide a comparison with, or at least a discussion of, real measured data — either from the literature or from monitored case studies — to contextualise the simulation outputs.

3. In Section 2.3, the values presented are averages. Hygrothermal performance is a dynamic, time-dependent phenomenon, and averages may not reflect variability.

4. The concept of intermittent conditioning is central to the paper, yet it remains insufficiently defined. What precisely does "intermittent conditioning" mean in the context of this study, and how is it approached in the modelling? The current generalisation makes weak the paper's core argument and should be developed with more rigour.

5. The authors should not assume that all readers are familiar with the internal workings of WUFI. One or two sentences explaining the monitoring position within the wall assembly — and its significance for interpreting the outputs — would improve accessibility and transparency.

6. It is not clear where the 0.2 ACH value is applied — is this at a specific layer within the construction, or across the wall assembly as a whole? The authors should clarify this and provide a reference or justification for this value, as it can have non-trivial implications for the results.

7. In line 294, the percentages presented seem to refer to the 10-year simulation period, but they do not adequately communicate the nature of the exposure. Is the elevated RH condition continuous or intermittent? The percentages as presented do not provide how long, or in what pattern, the wall assembly is exposed to critical combinations of high RH and temperature. A more granular analysis — for example, duration, frequency, or temporal distribution of exceedance — would make this section considerably more informative.

8. Section 4 describes the scenarios, but the criteria used to assess mould growth risk are not sufficiently specified. ASHRAE 160 is referenced, but what is the specific threshold or performance criterion being applied? This needs to be stated explicitly.

9. Figures 6 and onwards are difficult to interpret. It would help to clarify in the caption whether the figure represents a cross-section profile of the wall assembly, and to indicate which scenario is being shown. Additionally, "ASHRAE" is misspelled and should be corrected throughout.

10. Lines 549–552 do not add meaningful content to the discussion. The authors are encouraged to either expand this remark into a substantive observation that provides analytical insight, or to remove it.

11. The term "statistically significant" appears to be used incorrectly. Statistical significance is a property of inferential tests that compare groups or distributions against a defined null hypothesis — it is not an appropriate descriptor for a comparison between two individual values without a clear sampling framework. The authors should either conduct and report a proper statistical test or rephrase this claim accordingly.

12. The claim that this constitutes a novel method is difficult to support. The approach relies on a simulation tool that generates standard intermittent use profiles — this is a legitimate and useful method, but it is not novel in itself. Furthermore, standard profiles may not adequately represent the diversity of real occupancy patterns, and the implications of this limitation for the results deserve acknowledgement.

13. The authors state: "While the latest version of ASHRAE Standard 160 (2021) acknowledges that under intermittent conditioning relative humidity should not exceed 50%." This requires clarification. Is the 50% threshold a peak instantaneous limit, a design ceiling, or a cumulative criterion assessed over a defined period? ASHRAE 160-2021 is primarily built around a time-integrated mould index rather than a simple RH threshold, and the 50% figure also needs a precise reference. The authors should clarify both the source and the temporal basis of this criterion, as it is a key interpretive anchor for the paper.

14. In general the overall style and presentation is okay, although some figures in the manuscript are difficult to read due to small font sizes and the use of abbreviations that are not consistently defined. The authors are encouraged to revise the figures to improve legibility — this includes increasing font sizes to meet journal standards and ensuring all abbreviations appearing in graphs are either spelled out in full or clearly defined in the figure captions.

15. The paper is a little bit long, and I'd suggest to shortening. Both the results and discussion sections feel a little bit long and would benefit from tightening.


Minor issues:
I'd suggest authors to check the manuscript's punctuation and phase composition for consistency - line 85 page 2 - sentence is incomplete

Note:
This review refers to round 1 of peer review.